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1a. The Structure of Atoms – fundamental particles

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

1c. 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 students do NOT need 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eg the 'plum pudding' model in which 'protons' and 'electrons' were scattered or arranged evenly across the atom, were superceded by the model described in the picture below.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.

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


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

  • ALSO, be aware 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 one or more stable isotopes, but many other isotopes are unstable, disintegrate spontaneously (nuclear decay) and are known as radioactive, emitting ionising (nuclear) radiation e.g. alpha, beta and gamma radiation.
    • 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.
    • Some isotopes are completely stable, others may be highly unstable, disintegrate with the resulting radioactivity.

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

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



1c. 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

  • -




1. Atomic structure, fundamental particles and radioactivity

2. What is radioactivity? Why does it happen? What radiations are emitted?

3. Detection of radioactivity, measurement, dose units, ionising radiation sources, background radiation

4. The properties and dangers of alpha, beta & gamma radioactive emission

 5. The uses of radioactive Isotopes emitting alpha, beta or gamma radiation

6. Half–life of radioisotopes, how long does material remain radioactive? Uses of decay data & half–life values

7. Nucleus changes in radioactive decay? how to write nuclear equations? Production of Radioisotopes

 8. Nuclear fusion reactions and the formation of 'heavy elements'

 9. Nuclear Fission Reactions, nuclear power energy resources

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

(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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