Atomic Structure, Isotopes, Electronic Structure of Atoms, Atomic Structure Experiments

Doc Brown's Chemistry KS4 science GCSE/IGCSE/O Level/AS  Revision Notes

This page describes the structure of atoms in terms of the three fundamental sub–atomic particles – protons, neutrons and electrons. Isotopes are defined with examples and nuclide notate (e.g. top right picture) is explained. Early theories and models of atomic structure are described and explained including the Bohr theory of the atom. The simplified electronic structure of atoms is explained via the rules on filling shells with electrons. Which electron arrangements are stable? and Why? and which electron arrangements are unstable giving rise to very reactive elements like the Group 1 Alkali Metals and Group 7 Halogen elements. The link between the Periodic Table and Electronic Structure is explained and described with diagrams of the periodic table and electronic structure. The important historic alpha particle scattering experiment is described, variations of atomic structure diagrams, There is a section explaining what allotropes are, don't confuse with isotopes! and, on a separate page for advanced level chemistry students, the mass spectrometer is explained with annotated diagrams and explanatory notes. These notes on atomic structure are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry, KS4 science courses and a primer for A Level chemistry courses.  These revision notes should prove useful for the new AQA, Edexcel and OCR GCSE (9-1) courses.

Index for Atomic Structure page

1. The Structure of Atoms – three fundamental particles

2. Isotopes – definition and examples

3. The Electronic Structure of Atoms – rules to be learned

4. Which electron arrangements are stable and which are not?

5. The Periodic Table and Electronic Structure – more patterns!

1. to 5. are essential for ALL KS4 Science GCSE/IGCSE/A Level science–chemistry students

Relative Atomic Mass is dealt with on a separate calculation page

Appendix 1. The Alpha Particle Scattering Experiment

Appendix 2. Atomic structure diagrams – some variations!

Appendix 3. Allotropes – don't confuse with isotopes!

Appendix 4. Mass Spectrometer (now on a separate page for advanced level students!)

KEYWORDS–phrases for this page: allotropes * Alpha particle scattering * Atomic (proton) number

Atom structureElectronElectron arrangement (examples)Electron shell rules * ionsIsotopes

Mass (nucleon) number * Mass spectrometer (advanced students only)NeutronNeutron number

Nuclide symbol notationPeriodic Table (and electron structure)

Periodic Table (its general structure)Proton * stable/unstable electron arrangements

See also Atomic structure and radioactivity

Some abbreviations used: A = mass number,    Ar = relative atomic mass,   Z = atomic number/proton number

1. The Structure of Atoms – three fundamental particles


An ATOM is the smallest particle of a substance, an element, which can have its own characteristic properties AND cannot be split into simpler substances.

However, why do we have different elements?

Is an atom the simplest particle we need to know about to understand chemistry?

In order to answer these questions we must look a bit deeper into the fundamental structure of matter, that is everything around you!

Atoms are the smallest particles of matter whose properties we study in Chemistry.

Every element or compound is comprised of atoms. All the atoms are the same in the structure of an element (ignoring isotopes) and two or more different atoms/elements must be present in a compound.

Initially, once the concept of an atom was established, it was assumed that atoms were indestructible and not divisible into smaller particles, but merely combined in different proportions to give the range of compounds we know about.

However from experiments done in the late 19th and early 20th century it was deduced that atoms are made up of three fundamental or sub–atomic particles called protons, neutrons and electrons, which are listed below with their relative masses and electrical charges.



The three fundamental particles of which atoms are composed

The table gives the relative mass and electric charge of the three sub–atomic particles known as the proton, neutron and electron

Sub–atomic particle Relative mass Electric charge Comments
Proton 1 +1 (+ positive) In the nucleus, a nucleon
Neutron 1 0 (zero) In the nucleus, a nucleon
Electron 1/1850 or 0.00055 –1 (– negative) NOT a nucleon. Electrons are arranged in energy levels or shells in orbit around the nucleus

You can think of the mass of an electron as about 1/2000th of the mass of a proton or neutron, so, a pretty small mass BUT they occupy most of the space of an atom!!! You should also realise because of the relatively small mass of the electrons most of an atom's mass is in the nucleus. You see values of 1/1836 quoted for the relative mass of an electron, but don't worry about it, there are different ways/scales on which an electron's mass has been calculated.

The actual mass of a proton or neutrons is ~1.67 x 10-31 kg and the electro's mass is ~9.11 x 10-31 kg

What can we say about 'A Portrait of an Atom'? – an image of what you can't see!

The picture of 'atomic structure, illustrated below is the result of many developing 'atomic theory' backed up by successive generations of experimental results. This is the best picture we have (at least for GCSE and A Level chemistry courses!).

(c) doc b

However this diagram, which is based on the Bohr model of atomic structure, although more realistic in terms of the real size of the nucleus compared to the atom as a whole, so it is not convenient to give a brief diagrammatic picture of the composition of an atom.

The central nucleus of protons and neutrons (most of the mass of an atom) is extremely small even compared to the size of an atom. The rest of the 'almost empty space' of an atom is occupied by the negative electrons, held by, and moving around the positive nucleus in their energy levels or 'shells'.

The electrons are also pretty tiny in mass too, compared to a proton or neutron, but the volume the electrons and their energy levels occupy, determines the size of the atom.

Bohr theorised the negative electrons can only exist in certain specific energy levels (shells) held in place by the positive nucleus (see section on the history of development of the atomic model). These are shown in the above diagram, but fully explained later on this page.

All of these theories must, and have been, backed up by repeated and varied experiments.

As each new experiment was/is done, it must support the current theory or the theory needs to be modified to take into account new discoveries.

Some of these important experiments are described further down the page.

Even new experimental findings written up in research papers should be thoroughly peer reviewed, that is checked by scientists of at least equal academic ranking to the researchers. That's how science works!

The size of an atom compared to other 'particles'

The size of an individual atom is around 0.1 nm or 1 x 10-10 m

(nm = nanometres, 1 nm = 10-9 m).

Most of the mass is in the centre of the atom, that is the nucleus, which has a radius of around 1/10000th of the whole atom!

This means the radius of the nucleus is about 1 x 10-14 m (0.00001 nm), pretty small !!, but still consists of most of the mass of an atom!!!

If you look at the table of size comparison below, its not until you get to a human hair can we see clearly something with the naked eye. The width of a human hair is approximately 106 times that of an atom (a million times bigger) and 1010 times bigger than a nucleus (ten thousand million times bigger). You can of course see cells under examination with an optical microscope, but these are over 500 000 times bigger than an individual atom. You can, however, observe atoms using an electron microscope.

A comparison data table of particles sizes/dimensions

Examples of atoms, molecules, nanoparticles and other 'things'

material nucleus carbon atom sulphur atom water molecule silver atom glucose molecule typical small protein silver or titanium dioxide nano-particles typical virus e.g. cold virus typical carbon nanotube typical bacteria typical eukaryotic cell width human hair
Symbol-formula ******** C S H2O Ag C6H12O6 ******* Agn


******* Cn ******* ******* *******
Size in nm - diameter or length 0.00001 0.16 0.2 0.2 0.28 0.3 x 0.6 5-10 35-120 30-50 100 x 6 5000 50000 50000- 100000
longest length or diameter m 1 x


1.6 x


 2 x



x 10-10

 2.8 x


6 x 10-10


3 x 10-10


x 10-9


x 10-8


x 10-8


x 10-7


x 10-6


x 10-5

0.5 to 1.0

x 10-4


Atoms, sub-atomic particles, notation and diagrams

The number of protons in the nucleus of an atom decides what element that atom is.

e.g. if the atom has 3 protons in the nucleus, it cannot be anything except lithium!

Elements consist of one type of atom only determined by the proton number (atomic number).

(c) doc b

Some more concise and handy styles to show the atomic composition of the same lithium atom

  • What sub–atomic particles make up atoms? What is their mass and charge?
  • The diagram above of a 'portrait of an atom' gives some idea on the structure of an atom (sometimes called the Bohr Atomic Model), it also includes some important definitions and notation used to describe atomic structure
    • The three fundamental particles you need to know are ...
    • proton: particle mass = 1, electric charge = +1, the charged particle in the nucleus
    • neutron: particle mass = 1, charge = 0, uncharged particle in the nucleus
    • electron: particle mass = 1/1850 ~1/2000, electric charge = –1,
      • Electrons are NOT in the nucleus but exist in electronic energy levels around the nucleus (a sort of orbit, often described as a shell, see later).
    • The nucleus of protons and neutrons is tiny, even compared to the tiny atom!
      • So most of the volume of an atom is empty space, BUT it is where the tiny electrons are.
      • In fact the diameter of the nucleus of protons plus neutrons is about a ten thousandth of the diameter of the whole atom!
      • Since the nucleus is composed of positive protons and neutral neutrons, the nucleus itself must be positive.
      • A neutral atom carries no overall charge because the number of positive protons equals the number of negative electrons (both singly charged + and -), and this information is given by the atomic/proton number.
        • A neutral helium atom has 2 protons and 2 electrons and a uranium atom has 92 protons and 92 electrons. 2+ balances 2- and 92+ balances 92-, easy!
        • Since a neutron is electrically neutral, the number of neutrons in the nucleus of an atom cannot affect the total positive nuclear charge of the protons or the number of negative electrons in the atom.
  • Protons and neutrons are the 'nucleons' or 'sub–atomic' particles present in the minute positive nucleus and the negative electrons are held by the positive protons in 'orbits' called energy levels or shells.
    • Although the nucleus must be positive because of the positive protons (neutrons are neutral) an individual atom is neutral because the number of electrons equals the number of protons – so the charges 'cancel out'.
    • If electrons are removed from an atom you get a positive ion from excess positive charge, and, if electrons are added to an atom, you get a negative ion from excess negative charge.
      • An ion, by definition, cannot be neutral.
  • Some important evidence for this 'picture' is obtained from alpha particle scattering experiments (see Appendix 1).
  • The atomic number (Z) is the number of protons in the nucleus and is also known as the proton number of the particular element and it is this number that decides what element a particular atom is.
    • Each element has its own atomic number, so all the atoms of a particular element have the same atomic number.
  • It is the proton/atomic number (Z) that determines the number of electrons an element has, its specific electron structure and therefore the specific identity of a particular element in terms of its physical and chemical properties.
  • It cannot be overemphasised that it is the electronic structure that determines the chemical character of an element, hence the proton/atomic number determines everything about a particular element.
  • The mass number (A) is also known as the nucleon number, that is the number of particles in the nucleus of a particular atom–isotope (notes on isotopes – definition and examples).
  • The neutron number (N) = mass number – proton/atomic number
  • In an individual atom the number of protons (+) equals the number of electrons (–), that is the number of positive charges is equal to the number of negative charges to make the atom neutral.
  • In the example in the diagram for lithium–7, 73Li is its nuclide atomic notation ...
    • before the chemical symbol of the element Li
      • is the top left number  = nucleon/mass number = 7 (protons + neutrons)
      • and the bottom left number = proton/atomic number = 3 (protons in nucleus)
    • Similarly for ...
    • , atom of hydrogen–1, symbol H, mass 1, just 1 proton and one electron, but NO neutrons (unique, hydrogen-1 is the only atom with NO neutrons)
    • , atom of helium–4, symbol He, mass 42 protons, 4 – 2 = 2 neutrons and 2 electrons
    • , atom of sodium, symbol Na, mass 23, 11 protons, 23 – 11 = 12 neutrons and 11 electrons
    • atom of iron–56, mass 56, 26 protons, 30 neutrons (56 – 26) and 26 electrons

  • If the proton number and electron number are different, the atom has an overall surplus or deficiency of electrical charge, resulting in an electrically charged particle called an ion e.g.
    • If an atom loses 1/2 electrons, the protons produce an excess of 1/2 units of positive charge.
      • The excess positive charge on the positive ion is written as + or 2+ etc.
    • If an atom gains 1/2 electrons, the extra electrons produce an excess of 1/2 units of negative charge.
      • The excess negative charge on the negative ion is written as - or 2- etc.
    • reading the symbol as above, BUT, taking into account the electrical charge on the ion ..
    • the positive sodium ion Na+,11 protons, 10 electrons (11-1), 12 neutrons (23-11)
    • the positive magnesium ion Mg2+,12 protons, 10 electrons (12-2), 12 neutrons (24-12)
    • the negative chloride ion Cl,17 protons, 18 electrons (17+1), 20 neutrons (37-17)
    • the negative sulfide ion S2-,16 protons, 18 electrons (16+2), 16 neutrons (32-16)
    • for more details and examples see ionic bonding notes.
  • The electrons are arranged in specific energy levels according to a set of rules (dealt with in section 3).

  • This description of an atom consisting of the relatively minute nucleus of protons and neutrons surrounded by electrons in particular shells or energy levels is sometimes referred to as the Bohr Model of the atom, after the great Danish scientist Niels Bohr (1885–1962), one of the brilliant founders of modern atomic theory.

  • Other examples of interpreting the nuclide notation and definition reminders:

    • Top left number is the nucleon number or mass number (A = sum of protons + neutrons = nucleons)

    • Bottom left number is the atomic number or proton number (Z = protons in nucleus)

    • Electrons = protons if the atom is electrically neutral i.e. NOT an ion.

    • The neutron number N = A – Z i.e. mass/nucleon number – atomic/proton number

      • Therefore from the following 'full' atomic symbols, assuming we are dealing with electrically neutral atoms, the number of sub-atomic particles for the following atoms will be as follows ...

      • Cobalt atom (isotope cobalt–59), mass 59, 27 protons, 32 neutrons (59 – 27), 27 electrons

      • Californium atom (isotope californium–246), mass 246, 98 protons, 148 neutrons (246 – 98), 98 electrons

      • So, at this point we had better explain, slightly belatedly, what isotopes are!



  • Isotopes are atoms of the same element with different numbers of neutrons in the nucleus and therefore different masses (different nucleon number or mass number).
    • Isotopes are atoms of the same atomic number but different mass numbers.
      • Some elements have just one isotope but others may have up to eight different isotopes.
      • Most elements have one or more stable isotopes, but many other isotopes are unstable, disintegrate spontaneously (nuclear decay) and are known as radioactive.
    • This gives each isotope of a particular element a different mass or nucleon number, but, being the same element they have the same atomic number or proton number, but different nucleon number.
    • Isotopes of a particular element are also chemically identical, because they have the same number of electrons, hence the same electron structure and will therefore behave in an identical manner to each other.
    • Study the diagrams of the isotopes of carbon further down the page.
    • Relative Isotopic Mass is dealt with on a separate calculation page

  • The phrase 'heavier' or 'lighter' isotope means 'bigger' or 'smaller' mass number for a particular element.
  • There are small physical differences between the isotopes e.g. the heavier isotope has a greater density or boiling point, the lighter the isotope the faster it diffuses.
  • However, because they have the same number of protons (proton/atomic number) isotopes of a particular element have the same electronic structure and identical chemistry.
  • Examples of isotopes are illustrated and described below.
  • Caution Note: Do NOT assume the word isotope means the atom it is radioactive, this depends on the stability of the nucleus i.e. unstable atoms (radioactive) might be referred to as radioisotopes.
  • Many isotopes are extremely stable in the nuclear sense and NOT radioactive i.e. most of the atoms that make up you and the world around you!
  • (c) doc bhydrogen–1, (c) doc bhydrogen–2, and (c) doc bhydrogen–3 are the three isotopes of hydrogen with mass numbers of 1, 2 and 3, with 0, 1 and 2 neutrons respectively. All have 1 proton, since all are hydrogen! Hydrogen–1 is the most common, there is a trace of hydrogen–2 (sometimes called deuterium) naturally but hydrogen–3 (sometime called tritium) is very unstable and is used in atomic bombs – nuclear fusion weapons.
    • They are sometimes denoted more simply as 1H, 2H and 3H since the chemical symbol H means hydrogen and therefore must have only one proton (atomic number 1).
  • (c) doc b and (c) doc b or 3He and 4He, are the two isotopes of helium with mass numbers of 3 and 4, with 1 and 2 neutrons respectively but both have 2 protons. Helium–3 is formed in the Sun by the initial nuclear fusion process. Helium–4 is also formed in the Sun and as a product of radioactive alpha decay of an unstable nucleus.
    • An alpha particle is a helium nucleus (mass 4, charge +2) and if it picks up two electrons it becomes a stable atoms of the gas helium. For more details see Radioactivity Revision Notes Part 4
  • (c) doc b and (c) doc b or 23Na and 24Na, are the two isotopes of sodium with mass numbers of 23 and 24, with 12 and 13 neutrons respectively but both have 11 protons in the nucleus and 11 surrounding electrons. Sodium–23 is quite stable e.g. in common salt (NaCl, sodium chloride) but sodium–24 is a radio–isotope and is a gamma emitter used in medicine as a radioactive tracer e.g. to examine organs and the blood system.
  • and are the two nuclear symbols for the two most common and stable isotopes of the element chlorine. They both have 17 protons in the nucleus and 35–17 = 18 and 37–17 = 20 neutrons respectively (and both have 17 surrounding electrons).
  • and are the two nuclide symbols for the two most common and stable isotopes of the element bromine. They both have 35 protons in the nucleus and 79–35 = 44 neutrons and 81–35 = 46 neutrons respectively. By coincidence, there are almost exactly 50% of each isotope present in naturally occurring bromine.
  • The three known isotopes of carbon (the electron structure is fully explained in the next section)
    • isotope nuclide symbol protons neutrons electrons % abundance
      carbon–12 126C 6 6 6 98.9%, stable
      carbon–13 136C 6 7 6 1.1%, stable
      carbon–14 146C 6 8 6 trace, unstable radioactive
    • The table of information on the three isotopes of carbon is illustrated by the diagrams above it.
    • The electronic structure is also shown and is fully explained in the next section 3.
    • Two carbon isotopes are very stable, but carbon-14 slowly decays and is used by archaeologists to radiocarbon date objects. (See archaeological radiocarbon-14 dating)
    • Now is an appropriate point to introduce the concept and definition of relative atomic mass (Ar), which is required for very accurate quantitative chemistry calculations.
    • The relative atomic mass of an element is the average mass of all the isotopes present compared to 1/12th of the mass of a carbon–12 atom (12C = 12.00000 amu i.e. the standard).
      • When you average the masses of the isotopes of carbon, taking into account their relative abundance (%), you arrive at a relative atomic mass of carbon of 12.011, Ar(C) = 12.011, though at this academic level 12.0 is usually accurate enough!
      • (c) doc b See also chemical calculations on how to calculate relative atomic mass
      • I've put this calculation on its own page because there is plenty on atomic structure already on this page!
      • Anything on this page relevant to the calculation of RAM is repeated on the page.
  • EXTRA NOTE ON 'ATOMIC' NOTATION – representation of isotopes of ions

  • Nuclide notation and ions

    • Ions are NOT neutral, they have an net electrical charge caused by the atom losing or gaining electrons to give a positive or negative ion.

    • This process is called ionisation and there is never any change in the number of protons or neutrons in the nucleus, just a change in the number of orbiting electrons.

    • sodium–24 isotope ion, 11 protons, 13 neutrons, 10 electrons (one electron lost to form a singly charged positive ion)

    • sodium–23 isotope ion, 11, protons, 12 neutrons, 10 electrons (one electron lost to form a singly positive ion)

    • isotope sulfur–32 in the form of the sulfide ion, 16 protons, 16 neutrons, 18 electrons (two electrons gained to form the doubly charged negative ion)

  • Knowledge of isotopes is important in modern science.
    • Radioactive isotopes are used in medicine to trace aspects of body chemistry due to their radioactive emissions, and in chemical synthesis as tracers to follow how a reaction sequence occurs.
    • Radioactive isotopes are used in radiotherapy to kill malignant cancer cells.

3. The Electronic Structure of Atomsrules to be learned

WHAT DO WE MEAN BY the electron configuration?, electronic structure of atoms?

that is what is the arrangement of electrons in the shells or energy levels?

  • The Bohr model of the atom in its more elaborate form involves the maximum numbers of electrons that each shell or energy level can hold and how the shells are progressively filled with electrons from atom to another with increase in proton/atomic number.

  • The electrons are arranged in energy levels or shells around the nucleus and with 'orbits' on average increasing in distance from the nucleus.

    • Electrons in an atom occupy the lowest available energy levels (the innermost available shells).

    • The lowest energy levels are always filled first, you can think of the lower the shell, the nearer the nucleus, and numbered 1, 2, 3 etc. as the shell gets further from the nucleus.

  • Each electron in an atom is in a particular energy level (or shell) and the electrons must occupy the lowest available energy level (or shell) available nearest the nucleus.

  • When the level is full, the next electron goes into the next highest level (shell) available.

  • There are rules to learn about the maximum number of electrons allowed in each shell and you have to be able to work out the arrangements for the first 20 elements (for GCSE students, up to at least 36 for Advanced level students).

    • The 1st shell can contain a maximum of 2 electrons (electrons 1–2)

    • The 2nd shell can contain a maximum of 8 electrons (electrons 3–10)

    • The 3rd shell also has a maximum of 8 electrons (electrons 11–18)

    • The 19th and 20th electrons go into the 4th shell, (required limit of GCSE knowledge).

    • Remember the total electrons to be arranged equals the atomic/proton number for a neutral atom.

  • If you know the atomic (proton) number, you know it equals the number of electrons in a neutral atom, you then apply the above rules to work out the electron arrangement (configuration).

  • For elements 1 to 20 the electron arrangements/configurations are written out in the following manner:
    • Note that each number represents the number of electrons in a particular shell, dots or commas are used to separate the numbers of electrons in each shell. They are written out in order of increasing average distance from the positive nucleus which holds these negative electrons in their energy levels (shells).
    • The electron configurations or electron arrangements are summarised below with reference to the periods of the periodic table and in order of increasing atomic number up to proton number 20.
    • Period 1 – elements 1 to 2 (2 elements)

      • the electron arrangement is written out simply as 1 or 2, only the 1st shell or level involved.

    • Period 2 – elements 3 to 10 (8 elements)

      • have electron arrangements of 2,1 to 2,8 (since 1st shell is full with 2 electrons i.e. the first number)

        • 2,5 would mean two electrons in the 1st level (shell) and five in the 2nd level (shell).

    • Period 3 – elements 11 to 18 (8 elements)

      • are denoted by 2,8,1 to 2,8,8 (1st & 2nd shells full with 2 & 8 electrons, ie 2.8)

        • 2,8,3 would mean two electrons in the 1st level (shell), eight in the 2nd level (full shell) and three electrons in the 3rd outer level (shell).

    • Period 4 – first two elements 19 to 20

      • are written out as 2,8,8,1 and 2,8,8,2 (1st, 2nd, 3rd full shells with 2,8,8 electrons)

        • 2,8,8,1 would mean two electrons in the 1st level (shell), eight in the 2nd level, eight electrons in the 3rd level (shell) and one electron in the 4th outer level (shell).

      • Reminder – this is as far as GCSE students need to know, after that things get more complicated, BUT only for advanced level students!

      • For example, after element 18, the 3rd shell can hold a maximum of 18 electrons!

    • The above is summarised in the diagram below

    • (c) doc b

    • The electron shell arrangements are quoted in numbers e.g. 2,4 for C (carbon) but you need to be able to draw electron diagrams showing the electronic structure of the atom.

      • In the above table, check out the atomic number, the lower subscript on the element symbol, and apply the rules, and hopefully it makes sense.

      • Some examples of electronic diagrams are given below and GCSE/IGCSE/O level students need to be able to work and draw the electronic structures of the first 20 elements.

      • You should notice that the number of shells used equals the period number of the element in the periodic table.

      • They can be all worked by the 'shell filling' rules described above.

    • For the rest of Period 4 and other Periods you need a more advanced electron configuration system up to at least Z=36 using s, p, d and f orbital notation BUT for advanced level chemistry students only!

Examples: diagram, symbol or name of element (Atomic Number = number of protons and the number of electrons in a neutral atom), shorthand electron arrangement and a diagram to help you follow the numbers.

Filling 1st shell, electron level 1 (c) doc b (c) doc b2 elements only, Period 1 of the Periodic Table

Filling 2nd shell, electron level 2 (c) doc b to (c) doc b to (c) doc b 3 of the 8 elements of Period 2

Filling 3rd shell, electron level 3 (c) doc b to (c) doc b (c) doc b  3 of the 8 elements of Period 3

The first 2 elements of the 4th shell (c) doc b (c) doc b to Kr [], start of Period 4

Only the first 2 of the 18 elements of Period 4 are shown above, the rule for 3rd shell changes from element 21 Sc onwards (studied at Advanced level, so GCSE students don't worry!)

A few more 'snappy' examples – given atomic number, work out electron configuration (abbreviated to e.c.)

Z = 3 e.c. 2,1  or   Z = 7 e.c. = 2,5  or  Z = 14 e.c. = 2,8,4 or Z = 19 e.c. = 2,8,8,1 etc. up to Z = 20

4. Which electron arrangements are stable and which are not?

Both atoms and ions are considered




  • When an atom has its outer level full to the maximum number of electrons allowed, the atom is particularly stable electronically and very unreactive.

    • This is the situation with the Noble Gases: He is [2], neon is [2,8] and argon is [2,8,8] etc.

    • There atoms are the most reluctant to lose, share or gain electrons in any sort of chemical interaction because they are so electronically stable.

    • For all elements most of their chemistry is about what outer electrons do or don't!

    • [2], [2,8] and [2,8,8] etc. are known as the 'stable Noble Gas arrangements', and the atoms of other elements try to attain this sort of electron structure when reacting to become more stable.

    • More details on Electron configuration notes for Advanced Level Chemistry Students

  • The most reactive metals have just one outer electron.

    • These are the Group 1 Alkali Metals, lithium [2,1], sodium [2,8,1], potassium [2,8,8,1]

    • With one outer shell electron, they have one more electron than a stable Noble Gas electron structure.

    • So, they readily lose the outer electron when they chemically react to try to form (if possible) one of the stable Noble Gas electron arrangements – which is why atoms react in the first place!

    • When Group 1 Alkali Metal atoms lose an electron they form a positive ion because the positive proton number doesn't change, but with one negative electron lost, there is a surplus of one + charge e.g.

      • sodium atom ==> sodium ion

      • Na ==> Na+

      • is [2.8.1] ==> [2.8] electronically

      • in fundamental particles [11p + 11e] ==> [11p + 10e]

      • IONS are atoms or group of atoms which carry an overall electrical charge i.e. not electrically neutral.

  • The most reactive non–metals are just one electron short of a full outer shell.

    • These are the Group 7 Halogens, namely fluorine [2,7], chlorine [2,8,7] etc.

    • These atoms are one electron short of a stable full outer shell and seek an 8th outer electron to become electronically stable – yet again, this is why atoms react!

    • They readily gain an outer electron, when they chemically react, to form one of the stable Noble Gas electron arrangements either by sharing electrons (in a covalent bond) or by electron transfer forming a singly charged negative ion (ionic bonding) e.g.

      • chlorine atom ==> chloride ion

      • Cl ==> Cl

      • is [2.8.7] ==> [2.8.8] electronically

      • in fundamental particles [17p + 17e] ==> [17p + 18e]

      • the positive proton number of Cl doesn't change but the chloride ion carries one extra negative electron to give the surplus charge of a single – on the ion.

  • For more on electron structure and chemical changes and compound formation see ...

  • and for more on metal and non–metal reactivity see

5. The Periodic Table and Electronic Structure – more patterns!

Selected Elements of the Periodic Table are shown below with atomic number and chemical symbol.


(c) doc b

  • The elements are laid out in order of Atomic Number – that is the number of protons in the nucleus.

  • It is important to realise that the 'chemical structure' of the periodic Table (shown above), that is the chemical similarity of vertical groups 'like' elements (apart from the Noble Gases), was known well before the electronic structure of atoms was understood.

    • In other words the elements are laid out in vertical columns (groups) and horizontal rows (periods) so that chemically (usually) VERY similar elements appear under each other – and there is a very good electronic structure reason for this!

    • However, it wasn't understood why they behaved in the same way chemically e.g. similar compound formulae and reactions etc. nor was it understood at first why Noble Gases were so unreactive towards other elements.

    • BUT, once the electronic structure of atoms was understood, 'electronic' theories could then be applied to explain the chemical similarity of elements in a vertical Group of the Periodic Table.

  • Originally they were laid out in order of 'atomic weight' (now called relative atomic mass). This is not correct for some elements now that we know their detailed atomic structure in terms of protons, neutrons and electrons, and of course, their chemical and physical properties in more accurate and extensive detail.

  • For example: Argon (at. no. 18, electrons 2,8,8) has a relative atomic mass of  40. Potassium (at. no. 19, electrons 2,8,8,1) has a relative atomic mass of 39. BUT Argon, in terms of its physical, chemical and electronic properties is clearly a Noble Gas in Group 0. Likewise, potassium is clearly an Alkali Metal in Group 1.

  • Hydrogen, 1, H, does not readily fit into any group

  • A Group is a vertical column of chemically and physically similar elements e.g.

    • Group 1 The Alkali Metals (Li, Na, K etc.) with one outer electron (one more than a Noble Gas structure),

    • Group 7 The Halogens (F, Cl, Br, I etc.) with seven outer electrons (one short of a Noble Gas arrangement)

    • and Group 0 The Noble Gases (He, Ne, Ar etc.). The group number equals the number of electrons in the outer shell (e.g. chlorine's electron arrangement is 2.8.7, the second element down Group 7 on period 3).

  • A Period is a horizontal row of elements with a variety of properties (left to right goes from metallic to non–metallic elements. All the elements use the same number of electron shells which equals the period number (e.g. sodium's electron arrangement 2.8.1, the first element in Period 3).

  • The ten elements Sc to Zn are called the Transition Metals Series and form part of a period between Group 2 and Group 3 from Period 4 onwards.

  • Below are the electron arrangements for elements 1 to 20 set out in Periodic Table format (Hydrogen and The Transition metals etc. have been omitted). When you move down to the next period you start to fill in the next shell according to the maximum electrons in a shell rule (see previous section).

  • NOTE: In the most modern periodic table notation Groups 3–7 and 0 are numbered Groups 3 to 18.

(c) doc b


APPENDIX 1. The history of the atom concept and the alpha particle scattering experiment

(some of the theory ideas described here goes above GCSE/IGCSE level!)

The development of the 'atomic model' is an excellent example of how new experiment evidence initiates the need to change an existing scientific model or even come up with a different theoretical model, in this case for atomic structure.
You should appreciate that knowledge and understanding of atomic structure has evolved over time.

The Greeks Leucippus and Democritus ~400 BC wondered what was the result of continually dividing a substance i.e. what was the end product or smallest bit i.e. what was left that was indivisible – the word atomic is from Greek adjective meaning 'not divisible'.

The Greeks idea was not forgotten and later revived by Boyle and Newton but with little progress.

However, in 1808 Dalton at the beginning of the 19th century, proposed his atomic theory that all matter was made up of tiny hard particles/spheres called atoms.

Dalton also proposed (correctly) the theory that different types of atoms (elements) combined together to give all the different substances of the physical world (all which of course is true, except for the 'hard solid indivisible spheres'!).

He also produced the first list of 'atomic weights' (we now call relative atomic masses) on a scale based on hydrogen – given the arbitrary value of 1 since it was lightest element known, and, as it happens, correctly so.

He was incorrect by stating that atoms were indivisible, but we now know that atoms consist of electrons, protons and neutrons and that atoms can be 'taken apart' by ionisation or nuclear changes  e.g. radioactivity.

Until the discovery of the electron, atoms were thought of as hard indivisible spheres, but brilliant 'JJ' changed all that. New experimental evidence may lead to a scientific model being changed or replaced and with the discovery of the electron, recognised as the first known 'sub-atomic particle', change was on the way.

J J Thomson Around 1897 proposed his 'plum pudding model' theory based on the growing evidence that atoms were themselves composed of even smaller more fundamental particles like the electron i.e. atoms were not hard indivisible spheres.

His experiments had shown that atoms contained small negatively charged particle called electrons which could be removed from atoms using a vacuum tube and applying a high potential difference (voltage) to a very low pressure gas. He showed that the mass of an electron was much less than the mass of an atom. Therefore the 'hard indivisible sphere' model of an atom was wrong.

From his experiments Thomson envisaged a plumb pudding atom consisting of a positively charged 'pudding' (a sort of ball of positive charge) with just enough lighter negatively charged electrons embedded in it to produce a neutral atom. Note that both the positive charges and negative charges are evenly distributed through the sphere of the atom (shown later by Rutherford and Bohr etc. to be completely wrong).

The idea of positive particles balancing the negative particles was correct but the relative size and nature of the nucleus and distribution of electrons were not, BUT it was a more advanced model.

Ernest Rutherford, Hans Geiger and Ernest Marsden (the latter two were students of Rutherford at Cambridge University) conducted alpha particle scattering experiments (1902–1910, and described in detail below).

These experiments established

(i) minute nature of the nucleus even compared to the size of an atom

(ii) the nucleus was positive and the positive charge varied from element to element.

(c) doc bDiagram of the Rutherford and Geiger–Marsden scattering experiment

When positive alpha particle beams are fired on very thin layers of metals (e.g. very fine gold leaf) some rather surprising results were made by scientists of the early 20th century.

By using a 360o charged particle detection system it was found that ...

3. most particles passed through un–deflected (as if there was nothing there!), this was expected.

2. a small proportion were deflected slightly (so there was something there!), this again was not unexpected.

1. about 1 in 20,000 were 'bounced' back through an angle of over 90o, in other words were reflected backwards, a totally unexpected result and quite shocked the experimenters - not what they were expecting. This was because the JJ Thompson model predicted the positive charge was spread out and not sufficiently concentrated to cause, for some alpha particles a 180o deflection! So, whatever was there, was substantial and positively charged to cause the repulsion 'bounce' of the positive alpha particles, BUT what it was (the 'nucleus') it wasn't very big!

These results made it quite plain the JJ Thompson plumb pudding model was in some way wrong ie the positive charge was NOT spread throughout the volume of an atom.

From a detailed mathematical analysis of the scattering experiment results, the only 'atomic model' which could account for the pattern was an atom consisting of ...

1. mainly empty space (which is why most alpha particles passed through undeflected), thus completely contradicting JJ Thompson's 'plum pudding' model. Other experiments showed that the electrons were orbiting in energy levels around the nucleus, but occupying virtually no significant volume in themselves as particles.

2a. a relatively minute positive centre (the nucleus) causing deflection (like charges repel, alpha particles are positively charged and so were being repelled by the 'later to be discovered' positive protons in the nucleus), we now know the nucleus is positive due to protons,

2b. a tiny dense centre of similar or greater charge or mass to an alpha particle (which we now call the nucleus), so most of the mass of an atom was in the central nucleus, we know the mass is made of protons and neutrons.

3. Most of the atom is mainly empty space with a cloud of negative electrons moving around the positive nucleus.

Putting these three points together formed the basis of the modern picture of the 'nuclear atom', in other words the nuclear atomic model.

Other experiments have shown that electrons are arranged in energy levels, sort orbits around the nucleus, ideas first proposed by scientists such as Bohr.

So an atom is quite well represented by the Bohr model of the atom (picture below) which moves the Rutherford nuclear model another step forward.

Bohr's suggestion that the negative electrons can only exist in certain specific energy levels (shells) held in place by the positive nucleus complimented the Rutherford model of the atom to gives a reasonably complete picture of an atom (at least for this academic level!).

Bohr envisaged the electrons orbiting the nucleus in specific energy levels (or fixed shells) at specific distances from the central nucleus with nothing in between. The negative charge of the electrons was balanced by the positive charge (protons) of the nucleus.

Diagram of a 'Bohr' lithium atom

AND, most importantly, experimental results matched a theoretical mathematical model of simple 'electronic' atoms like hydrogen.

So by now, earlier theories of atomic structure, e.g. the 'plum pudding' model in which 'protons' and 'electrons' were scattered or arranged evenly across the atom, were superseded by the nuclear models of Rutherford and Bohr.

It was the only model that could explain the scattering of the high speed alpha particles by a small dense and positive atomic centre AND the behaviour of electrons.

Experiments had shown that the outer bits could be knocked off atoms and these had a very tiny mass and a negative charge, in other words the electron!

In 1913 Moseley studied the X–rays emitted by highly energised–ionised atoms and from the X–ray spectra of elements (the K alpha line, Kα) he was able to deduce the electric charge of the nucleus which we now know is equal to the atomic number of protons in the nucleus.

Moseley showed that when atoms were bombarded with cathode rays (electrons) X–rays where produced which he investigated with an X-ray spectrometer.

It was found that the square root of the highest energy emission line (called the K alpha line, Kα) gave a linear plot with the apparent atomic number,

Z = constant x √Kα

but the plot of √Kα against atomic weight (relative atomic mass) gave a zig–zag plot, suggesting this 'atomic number' was far more important the 'atomic weight' of an element in terms of the atom's fundamental structure.


(i) The K alpha line, Kα is due to an electronic transition of the inner most electron nearest the nucleus.

(ii) Sadly, Moseley was killed in action during the First World War at Gallipoli in 1915, a great loss to science as well as his family and friends.

However, there was still the problem of why the atomic mass and atomic number where different i.e. in the case of the lighter elements, the atomic weight was often about twice the atomic number.

In 1919 Aston developed a cathode ray tube i.e. like those used by Wien and Thompson etc. into a 'mass spectrograph', which we now know as a mass spectrometer GCSE–AS atomic structure notes.

This showed that atoms of the same element had different masses but there was no experimental evidence that they had different atomic numbers (which of course they didn't). These different atoms of the same element were called isotopes.

In 1920 Rutherford suggested there might be a 'missing' neutral particle and in 1932 Chadwick discovered the neutron by bombarding beryllium atoms with alpha particles which produced a beam of neutrons.

These were shown to have a relative mass of 1 (same as a proton) and were electrically neutral and quite penetrating into matter. This penetration and lack of charge had made them difficult to detect.

Prior to this, Rutherford and others had conducted experiments to show that the smallest particle in an atom was equivalent to a hydrogen atom without its electron, that is the proton.

It was not until 1932 that the nature of the neutron was finally deduced by Chadwick and this completely explained the nature of isotopes and backed up the ideas from Moseley's work that the fundamentally important number that characterises an element is its atomic number and NOT the atomic mass. The neutron discovery, ~20 years after the discovery of the nucleus, completed the 'modern' picture and theory of the composition of an atom in terms of the three principal sub-atomic particles - which is sufficient for the needs of us chemists!

Advanced level note on the discovery of neutrons:

Chadwick bombarded a thin metal foil of beryllium atoms (94Be) with alpha particles (42He) and this produced a highly penetrating radiation that was unaffected by electrical or magnetic fields.

94Be  +  42He  ====>  126C  +  10n

This 'neutral' penetrating radiation was eventually identified as a beam of neutrons!

Note the carbon-12 atom produced balances the nuclear equation in terms of mass (9 + 4 = 12 + 1) and positive proton nuclear charge (4+ + 2+ = 6+).

See section 2. Radioactivity Notes page on other experiments with mixed particle beams and their separation.

See also Atomic structure and radioactivity

Appendix 2. Atomic structure diagrams – some variations!

e.g. for the element lithium 73Li consisting of three protons and four neutrons

(c) doc b

Appendix 3. Allotropes – don't confuse with isotopes!


As explained above, Isotopes are atoms of the same element with different masses due to different numbers of neutrons in the nucleus. Same protons and electrons. e.g. atomic number 6 = 6 protons = carbon, but there can be 6, 7 or 8 neutrons giving isotopes of carbon–12, 13 or 14.

Oxygen atoms usually form 'stable' O2 oxygen molecules (also called dioxygen), BUT they can form a very reactive unstable molecule O3 ozone (also called trioxygen). The mass of the oxygen atoms in each of the molecules is mainly 16 (99.8%), and about 0.2% of two other stable isotopes of masses 17 and 18. Whatever isotope or isotopes make up the molecule, it doesn't affect the molecular structure or the respective chemistry of the O2 or O3 molecules.

However, what sometimes confuses the issue is the fact that oxygen O2 and ozone O3 are examples of allotropes.

Allotropes are defined as different forms of the same element in the same physical state.

The different physical allotropic forms arise from different arrangements of the atoms and molecules of the element and in the case of solids, different crystalline allotropes.

They are usually chemically similar but always physically different in some way e.g.

O2 (oxygen, dioxygen) and O3 (ozone, trioxygen) are both gases but have different densities, boiling points etc.

Graphite, diamond and buckminsterfullerene are all solid allotropes of the element carbon and have significantly different physical and in some ways chemical properties! (details on bonding page)

Rhombic and monoclinic sulphur have different geometrical crystal structures, that is different ways of packing the sulphur atoms (which are actually both made up of different packing arrangements of S8 ring molecules). They have different solubilities and melting points. There is also a 3rd unstable allotrope of sulfur called plastic sulphur made by pouring boiling molten sulphur into cold water which forms a black plastic material consisting of chains of sulphur atoms –S–S–S–S–S– etc..

It doesn't matter which isotopes make up the structure of any of an element's allotropes described above, so to summarise by one example ...

oxygen–16, 17 or 18 are isotopes of oxygen with different nuclear structures due to different numbers of neutrons, but they behave chemically in an identical manner.

BUT O2 and O3 are different molecular structures of the same element in the same physical state and are called allotropes irrespective of the isotopes that make up the molecules. Allotropes can have different physical and chemical properties.

Appendix 4. Mass Spectrometer (now on a separate page for advanced level students!)

See also Atomic structure and radioactivity

other associated web pages

FT GCSE/IGCSE Foundation Atomic Structure multiple choice QUIZ

HT GCSE/IGCSE Higher Atomic Structure multiple choice QUIZ

(c) doc b GCSE/IGCSE Atomic Structure Crossword Puzzle * ANSWERS!

(c) doc b GCSE/IGCSE multi–word gap–fill worksheet on atomic structure

Word-fill quiz "Atomic structure and elements"

2nd Word-fill quiz "Atomic Structure"

and definitely NOT GCSE/IGCSE pages on atomic structure

Advanced Level Chemistry notes on electronic structure – s, p, d orbitals etc.

A Level notes on electron configurations of elements & the periodic table

A Level Notes on mass spectrometers, mass spectrometry and relative atomic mass

KEYWORDS–phrases for this page: allotropes * Alpha particle scattering * Atomic (proton) number * Atom structure * Electron * Electron arrangement (examples) * Electron shell rules * ions * Isotopes * Mass (nucleon) number * Neutron * Neutron number * Nuclide symbol notation * Periodic Table (and electron structure)  * Proton * stable/unstable electron arrangements * mass electric charge proton neutron electron fundamental sub–atomic particles *1. The Structure of Atoms – three fundamental particles 2. Isotopes – definition and examples 3. The Electronic Structure of Atoms – rules to be learned 4. Which electron arrangements are stable and which are not? 5. The Periodic Table and Electronic Structure – more patterns! 1. to 5. are essential for ALL GCSE/IGCSE science–chemistry students Appendix 1. The Alpha Particle Scattering Experiment * 2. Atomic structure diagrams – some variations! 3. Allotropes – don't confuse with isotopes! 4. The Mass Spectrometer (separate page for advanced level students!) AND Atomic Structure Crossword Puzzle * ANSWERS! * multi–word gap–fill worksheet on atomic structure GCSE/IGCSE Foundation Atomic Structure lower  tier multiple choice quiz or GCSE/IGCSE Higher Atomic Structure higher tier multiple choice quiz What does a nucleus of an atom consist of? what are isotopes? A Level GCSE IGCSE revision notes on atomic structure information on atomic structure KS4 Science atomic structure GCSE chemistry guide notes on atomic structure for schools colleges academies science course tutors images pictures diagrams of apparatus for atomic structure investigations word balanced symbol equations of atomic structure science chemistry revision notes on atomic structure revising the chemistry of atomic structure help in chemical understanding of atomic structure description of atomic structure experiments for chemistry courses university courses in chemistry careers in chemistry jobs in the chemical industry laboratory assistant apprenticeships in chemistry technical internship in chemistry IGCSE chemistry atomic structure USA US grade 8 grade 9 grade10 atomic structure chemistry explanations of atomic structure chemistry revision notes atomic nucleus AQA science GCSE chemistry proton particle Edexcel science GCSE chemistry neutron particle OCR 21st century science GCSE chemistry masses of sub-atomic particles OCR Gateway science GCSE chemistry electric charges of sub-atomic particles WJEC gcse science chemistry atomic models CCEA/CEA gcse science chemistry  history of developments in atomic structure notes for revising models of atoms atomic structure of the elements gcse chemistry revision free detailed notes on structure of an atom to help revise igcse chemistry igcse chemistry revision notes on structure of an atom O level chemistry revision free detailed notes on structure of an atom to help revise gcse chemistry free detailed notes on structure of an atom to help revise O level chemistry free online website to help revise structure of an atom for gcse chemistry  free online website to help revise structure of an atom for igcse chemistry free online website to help revise O level structure of an atom chemistry how to succeed in questions on structure of an atom for gcse chemistry how to succeed at igcse chemistry how to succeed at O level chemistry a good website for free questions on structure of an atom to help to pass gcse chemistry questions on structure of an atom a good website for free help to pass igcse chemistry with revision notes on structure of an atom a good website for free help to pass O level chemistry what is the structure of an atom? what is the mass and charge of a proton? what is the mass and charge of a neutron? what is the mass and charge of an electron? what are isotopes? definition of proton number, what are isotopes? what are sub-atomic particles? diagram of an atom, atomic structure diagrams, diagrams of isotopes, give examples of isotopes, definition of mass number, definition of atomic number, atomic notation for elements, nuclide notation for isotopes, isotopes of hydrogen, isotopes of helium, atomic notation for ions, isotopes of bromine, isotopes of chlorine, isotopes of sodium, diagrams of the isotopes of carbon, electron arrangements of isotopes, rules on electron arrangements, history of atomic models, Dalton's theory, experiments and theories of JJ Thompson, explanation of Rutherford's alpha particle experiment, nuclear atom theory, electrons in orbits, explaining what we mean by allotropes, allotropes of oxygen, allotropes of sulfur, what is the relative size of an atom? how small is an electron? how small is a proton? how small is a neutron? what is the relative size of the nucleus

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See also Atomic structure and radioactivity

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