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GCSE/A Level Chemistry-Physics: Atomic structure required to understand radioactivity

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1a. The Structure of Atoms – fundamental particles

1b. A Portrait of an Atom – what is it like? - the nucleus

1c. Isotopes and radioactive decay

Mention of quarks, proton, neutron structure & radioactive decay

Doc Brown's Chemistry - KS4 science GCSE Physics Revision Notes

(1c. is an extra advanced physics section, you may need to come back to, most GCSE students do NOT need to know it)

We need to understand atomic structure in order to completely understand radioactivity and radioisotopes. The fundamental particles of an atom. The masses and charges of important particles including the positron. Extra advanced section – what are quarks? what is an up–quark? what is a down–quark? What quarks make up a proton or a neutron? These revision notes on nuclear atomic structure should help with GCSE/IGCSE physics courses and A/AS level physics courses


1a. The Structure of Atoms – Fundamental particles

  • See also history of atomic structure ideas, atomic structure and isotopes
  • Atoms, protons, neutrons and electrons are the smallest particles of matter whose properties we study in Chemistry.
  • From experiments done in the late 19th and early 20th century it was deduced that atoms were made up of three fundamental sub–atomic particles (listed below).
  • Earlier theories of atomic structure, eg the 'plum pudding' model in which 'protons' and 'electrons' were scattered or arranged evenly across the atom, were superceded by the model described in the picture below.
  • It was the only model that could explain the scattering of alpha particles by a small dense and positive atomic centre.
    • Later experiments showed 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!
    • Since then, even protons and neutrons have been shown to be made up of even more fundamental particles e.g. quarks and whole families of other particles which you do not have to worry about here!
  • It should be emphasised right from the start that radioactivity is due to energy changes in the nucleus and the surrounding electrons are not usually involved.
    • Below are listed all the particles you are most likely to come across in pre–university science courses e.g. GCSE& A Level chemistry and physics courses. They may be others in A level physics courses.

forget the quarks!

See also history of atomic structure ideas, atomic structure and isotopes

1b. A Portrait of an Atom – what is it like?

The diagram below gives some idea on the structure of an atom, it also includes some important definitions and notation used to describe atomic structure.

The atomic number (Z) is also known as the proton number. the mass number (A) is also known as the nucleon number.

The neutron number (N) = mass number (A) – atomic number (Z). Protons and neutrons are the 'nucleons' present in the nucleus and the negative electrons are held by the positive nucleus in 'orbits' called energy levels or shells.

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However when studying radioactivity and radioactive decay we are NOT interested in the electrons moving around the nucleus, but we are interested in electrons (and positrons) produced by certain types of radioactive decay AND the nuclide symbol notation e.g. like 73Li.


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 a few isotopes but others may have up to many different isotopes.
    • Isotopes of an element only differ in the number of neutrons i.e. they have the same number of protons, electrons and electronic structure.
    • 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.
  • Read atomic symbols of any isotope e.g. an isotope of sodium,11 protons, 12 neutrons, mass 23

  • 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)

  • 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)

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

    • You need to understand this notation in order to write correctly balanced nuclear equations.

Applying this notation to the three isotopes of carbon, all found naturally, two are stable and one is radioactive and disintegrates slowly over thousands of years.

isotope nuclide symbol protons neutrons % abundance
carbon–12 126C 6 6 ~98.9%, stable
carbon–13 136C 6 7 ~1.1%, stable
carbon–14 146C 6 8 trace, unstable radioactive

NOTE - atomic structure and radioactivity

  • The important point to realise right from the start is that radioactivity occurs when the UNSTABLE NUCLEUS of an atom undergoes a fundamental change (disintegration) that results in a different nucleus (of an atom) being formed and accompanied by the emission of alpha particles or beta particles or gamma radiation, which all described from section 2. onwards.

  • ALSO, remember that isotopes are atoms of the same atomic number (same element) but different mass numbers.
    • Some elements have just one isotope but others may have up to eight different isotopes.
    • Most elements have a few stable isotopes, but many other isotopes are unstable, when the nucleus disintegrates spontaneously (radioactive decay) and these atoms (isotopes) are described as radioactive, emitting ionising (nuclear) radiation e.g. alpha, beta and gamma radiation in the process.
    • In the radioactive decay process involving alpha and beta (minus and plus modes) emissions, new elements are formed - because the atomic number changes
    • All of these nuclear emissions are ionising radiations which knock electrons off atoms or molecules to create positive ions.
    • As well these ionising radiation emissions, neutrons can also be released and ALL these emissions are to do with nuclear instability when an unstable nucleus changes to a more stable nucleus.

See also history of atomic structure ideas, atomic structure and isotopes


APPENDIX not for GCSE students

Quarks - proton & neutron structure

PLEASE NOTE Some GCSE sciences courses require some basic knowledge of the 'quark structure' of protons and neutrons i.e. the composition of the nuclear protons and neutrons in terms of up–quarks and down–quarks and their relative electric charge. So this is an extra section I've added under the heading 'Atomic Structure'. Section 1c. also discusses beta+ and beta– radioactive decay in the context of quark changes in the nucleus, so it is only appropriate to study section 1c. when you have already studied the nuclear equations for the two modes of beta radioactive decay in section 7.

  • Quarks are elementary particles even more fundamental than the 'fundamental' sub–atomic particles we know as the proton and neutron. There mass must be smaller than a proton or a neutron.

    • I think of them as sub–sub–atomic particles and the proton and neutron are the most stable particles known and are formed when quarks combine.

    • The quarks have some properties you would associate with any elementary fundamental particle e.g. mass and electric charge (the latter you must know about).

    • Some textbooks quote the relative mass of up–quarks and down–quarks as 1/3 but this not strictly true, its more complicated than that (BUT don't worry about it, as 3 x 1/3 = 1),

    • AND as described below, protons and neutrons are each composed of three quarks and as a result both their relative masses are 1.

  • The electric charge carried by quarks.

    • Up–quarks are positive with a relative electric charge of +2/3.

    • Down–quarks are negative with an relative electric charge of 1/3.

    • By adding up the electric charges of the quarks in protons and neutrons you get their overall electric charge.

    • Note: You are used to integer electric charges like +, +1, 2+, +2, –, –1, –2 or 2– in chemistry (ions, protons, electrons etc.), but I'm afraid in nuclear physics things are not always that simple!

  • Quark composition of a proton

    • A proton consists of two up–quarks and one down–quark

    • Therefore the electric charge on a proton = (+2/3) + (+2/3) + (–1/3) = +1

      • (which is what you knew in the first place!)

  • Quark composition of a neutron

    • A neutron consists of one up–quark and two down–quarks.

    • Therefore the electric charge on a neutron = (+2/3) + (–1/3) + (–1/3) = 0

      • (zero, which is also what you knew in the first place!)

  • See section 7. for quarks and radioactive decay

  • -




Atomic structure, radioactivity and nuclear physics revision notes index

Atomic structure, history, definitions, examples and explanations including isotopes gcse chemistry notes

1. Atomic structure and fundamental particle knowledge needed to understand radioactivity gcse physics revision

2. What is Radioactivity? Why does it happen? Three types of atomic-nuclear-ionising radiation gcse physics notes

3. Detection of radioactivity, its measurement and radiation dose units, ionising radiation sources - radioactive materials, background radiation gcse physics revision notes

4. Alpha, beta & gamma radiation - properties of 3 types of radioactive nuclear emission & symbols ,dangers of radioactive emissions - health and safety issues and ionising radiation gcse physics revision

5. Uses of radioactive isotopes emitting alpha, beta (+/–) or gamma radiation in industry and medicine gcse notes

6. The half-life of a radioisotope - how long does material remain radioactive? implications!, uses of decay data and half-life values - archaeological radiocarbon dating, dating ancient rocks gcse physics revision

7. What actually happens to the nucleus in alpha and beta radioactive decay and why? nuclear equations!, the production of radioisotopes - artificial sources of radioactive-isotopes, cyclotron gcse physics revision notes

8. Nuclear fusion reactions and the formation of 'heavy elements' by bombardment techniques gcse physics notes

9. Nuclear Fission Reactions, nuclear power as an energy resource gcse physics revision notes

(c) doc bRADIOACTIVITY multiple choice QUIZZES and WORKSHEETS

Easier–Foundation Radioactivity Quiz

or Harder–Higher Radioactivity Quiz

 (c) doc b five word–fills on radioactivity * Q2 * Q3 * Q4 * Q5and ANSWERS!

crossword puzzle on radioactivity and ANSWERS!


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