1a. The Structure of Atoms – Fundamental particles

• See also history of atomic structure ideas, atomic structure and isotopes
  • You can read the above page first and then concentrate on isotopes and symbol notation here.
• Atoms, protons, neutrons and electrons are the smallest particles of matter whose properties we study in Chemistry.
  • See the Atomic structure for chemistry students page for and the use of radioactivity 'bullet' experiments to deduce the structure of an atom (Rutherford and Marsden scattering experiment).
• From experiments done in the late 19^th and early 20^th 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eg the 'plum pudding' model in which 'protons' and 'electrons' were scattered or arranged evenly across the atom, were superseded by the model described in the picture below.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.

IGCSE/GCSE physics students - forget the quarks!

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

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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, equals the number of positive protons in nucleus.

The mass number (A) is also known as the nucleon number, equals the sum of the protons and neutrons in the nucleus.

The neutron number (N) = mass number (A) – atomic number (Z), equals number of neutrons in the nucleus.

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.

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.

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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.

• How to read and write symbols used in isotope notation AND nuclear equations

• To indicate the composition of isotopes and write nuclear equations for radioactive decay, fusion or fission, we need some kind of shorthand notation - referred to as nuclide notation

• To indicate the composition of a specific atom-isotope we use the following style of notation:

  •   is an isotope of sodium, consisting of 11 protons, 11 electrons, 12 neutrons, mass 23, we know this because ...

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

  • and the bottom left number is the atomic number or proton number (Z = protons in nucleus = electrons)

  • The neutron number N = A – Z i.e. mass/nucleon number – atomic/proton number, but is NOT shown directly.

  • 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, 27 electrons, 32 neutrons (59 – 27)

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

Atomic structure and its connection with 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).

  • This 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.

  • An isotope that is unstable may be referred to as a radioisotope, meaning a radioactive atom/isotope - one that disintegrates spontaneously in a radioactive decay process.

• 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), as we have said, 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.
  • All of these nuclear processes are dealt with in detail in section 2. onwards.

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

RADIOACTIVITY IS DUE TO CHANGES IN ATOMIC STRUCTURE IN THE NUCLEUS OF AN ATOM

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1d. APPENDIX not for IGCSE/GCSE physics students

Quarks - proton and neutron structure

PLEASE NOTE

• Some 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

1e. Key Revision Points on Atomic Structure needed to understand Radioactivity

Structure of the Atom

• Protons: Positive charge, mass ≈ 1 amu
• Atomic number (Z): Equals the number of protons in the nucleus.
• Neutrons: Neutral, mass ≈ 1 amu, contribute to mass number (A).
• Mass number (A): Equals sum of protons + neutrons in the nucleus
• Electrons: Negative charge, negligible mass, orbit nucleus in shells.
• Isotopes: Same Z, different A (different neutron numbers).

Radioactivity

• Unstable nuclei emit radiation to become stable.
• Types of radiation:
  • Alpha (α): 2 protons + 2 neutrons (helium nucleus), highly ionising, low penetration.
  • Beta (β−): Neutron → proton + electron + antineutrino; moderate penetration.
  • Gamma (γ): Electromagnetic radiation, high penetration, low ionisation.

Details on Alpha, beta & gamma radiation - properties of 3 types of radioactive nuclear emission

Typical Misconceptions where atomic structure and isotope knowledge is essential

• Confusing atomic number with mass number (Z versus A).
• Thinking decay is predictable for individual atoms (it’s random).
• Forgetting gamma has no mass/charge (only energy).
• Misbalancing nuclear equations (e.g., forgetting electron in β decay).
• Assuming isotopes are unstable by default (many are stable).

Typical Exam Board Content that requires atomic structure and isotope knowledge

Atomic structure, radioactivity and nuclear physics revision notes index

Atomic structure, history, definitions, examples and explanations including isotopes

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

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

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

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

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

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

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

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

9. Nuclear Fission Reactions, nuclear power as an energy resource

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Easier Foundation Tier Radioactivity multiple choice QUIZ

Harder Higher Tier Radioactivity multiple choice QUIZ

Worksheet QUIZ Question 1 on RADIOACTIVITY - absorption of alpha, beta and gamma radiation

Worksheet QUIZ Question 2 on RADIOACTIVITY - dangers & monitoring ionising radiation levels

Worksheet QUIZ Question 3 on RADIOACTIVITY - revision of atomic structure

Worksheet QUIZ Question 4 on RADIOACTIVITY - what happens to atoms in radioactive decay?

Worksheet QUIZ Question 5 on RADIOACTIVITY - uses of radioisotope and half-life data

ANSWERS to the WORD-FILL WORKSHEET QUIZZES

Crossword puzzle on radioactivity and ANSWERS!