Doc Brown's chemistry notes on atomic and electron structure

4. Isotopes - definition and examples explained, use of nuclide symbol notation to distinguish isotope nucleus compositions

• Isotopes are atoms of the same element with different numbers of neutrons in the nucleus and therefore atoms of the same element with 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.
    • Isotopes of an element only differ in the number of neutrons in the nucleus.
    • i.e. all the isotopic atoms of an element have the same number of protons, electrons and electronic structure - so isotopes of an element are chemically identical - same reactivity, same formula compounds etc. etc. !
    • 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 mass 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

    • (and see also mass spectrometry)

• 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 isotopes have the same number of protons (proton/atomic number) isotopes of a particular element, they also 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!
• hydrogen–1, hydrogen–2, and hydrogen–3 are the three nuclide symbols for the three isotopes of hydrogen with mass numbers of 1, 2 and 3, with 0, 1 and 2 neutrons respectively. All have 1 proton and 1 electron, 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).
•  and or ³He and ⁴He, are the two nuclide notations for the two isotopes of helium with mass numbers of 3 and 4, with 1 and 2 neutrons respectively but both have 2 protons and 2 electrons.
  • 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
•  and or ²³Na and ²⁴Na, represent 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).
  • They both have 17 electrons surrounding the nucleus in the arrangement 2.8.7 (see section 3. on electron arrangement)
• 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.
  • Both isotopes will have 35 electrons arranged in the appropriate energy levels around the nucleus.
• Note: You can have two different isotopes with the same mass number BUT they must be of different elements!
  • e.g. is the nuclear symbol for the isotope magnesium-24 (12 protons, 12 neutrons, 12 electrons)
  • and is the nuclear symbol for the isotope sodium-24 (11 protons, 13 neutrons, 11 electrons).
  • Apart from the obviously different chemical symbol, the atomic numbers are different too!
• The three known isotopes of carbon (the electron structure is fully explained in the next section 3.)
• 
• 

  • 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 5.
  • 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 (A_r), 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, A_r(C) = 12.011, though at this academic level 12.0 is usually accurate enough!
    • 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.

    • See also Relative Isotopic Mass

• EXTRA NOTE ON 'ATOMIC' NOTATION – representation of isotopes of ions

• Nuclide notation and ions - need to a bit more careful in interpreting nuclear symbols of ions.

  • Ions are NOT neutral, they have an overall net electrical charge caused by the atom losing or gaining electrons to give a positive or negative ion. In other words the number of protons no longer equals the number of electrons.

  • 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.

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

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

  • symbol for 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.
    • For lots more details see the RADIOACTIVITY NOTES
• DO NOT CONFUSE ISOTOPES and ALLOTROPES – see section 8

  More atomic structure diagrams – some variations in style!

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

  

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