Doc Brown's chemistry notes on atomic and electron structure

5. The electronic structure of atoms – configuration rules and the connection between an element's electron arrangement and its position in the periodic table of elements

• 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 volume or space occupied by the electronic energy levels determines the radius of an atom - referred to as the atomic radius. The space occupied by an energy level or shell is known as an orbital - but this is for more advanced A level chemistry courses!

• 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),

• BUT up to at least 36 for Advanced A-level students using a more advanced notation).

  • So, for GCSE level students

  • The 1^st 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 19^th and 20^th electrons go into the 4^th shell, (required limit of GCSE chemistry knowledge).

  • Examples and remember the (atomic number) = electron number of a neutral atom

  • ,  , ,

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

    Z = 3 e.c. 2,1  ,   Z = 7 e.c. = 2,5  ,  Z = 14 e.c. = 2,8,4  ,  Z = 19 e.c. = 2,8,8,1

• 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 connection between electron arrangement and an element's position in the periodic table.

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, i.e. 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

• 

• The first 20 elements of the periodic table in terms of their electronic structure in shells and numbers.

• I've repeated the same 20 elements of the periodic table showing simple diagrams of their electronic structure and group numbers with full diagrams of the shells of electrons.

• Group 1 are the alkali metals, Group 7 are the halogens and group 0/8 are the noble gases.

• You should note that the group number of the element equals the number of electrons in the outer shell (1-8).

• 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 this is for advanced A 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 2 elements only, Period 1 of the Periodic Table

Filling 2nd shell, electron level 2 to to 3 of the 8 elements of Period 2

Filling 3rd shell, electron level 3 to   3 of the 8 elements of Period 3

The first 2 elements using the 4th shell to Kr [2.8.18.8], 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!)

• 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 the elements 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) in the outer shell.

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

  • and Group 0 The Noble Gases (He, Ne, Ar etc.). These elements have the maximum possible number of electrons permitted in the outer shell.

• A Period is a horizontal row of elements with a variety of properties, with 'observable trends'

  • (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: For advanced level pre-university/college level students, in the most modern periodic table notation Groups 3–7 and 0 are numbered Groups 3 to 18.

• The first element in a period has one outer electron (e.g. sodium Na 2.8.1), and the last element has a full outer shell (e.g. argon Ar 2.8.8)

• Apart from hydrogen (H, 1) and helium (He, 2) the last electron number is the group number (in the old notation) and the number of shells used is equal to the Period number.

• The periodicity of elements i.e. the repetition of very chemically similar elements in a group is due to the repetition of a the same outer electron structure – check out the last number from element 3 onwards.

• More GCSE/IGCSE notes on the Periodic Table

• and the electronic explanations of chemical bonding–formulae

• Advanced Level Chemistry – electron configurations and the Periodic Table

Quizzes - practice exam questions

• Some basic practice exam questions on atomic structure (at end of index page)

• GCSE level Foundation (easier) Atomic Structure multiple choice QUIZ

• GCSE level Higher (harder) Atomic Structure multiple choice QUIZ

• gap-fill worksheet on Atomic Structure

• matching pair quiz on Atomic and electronic structure 1. fundamental particles

• matching pair quiz on Atomic and electronic structure 2. periodic table