AQA GCSE Physics 8463: Atomic structure – Study Guide

This guide teaches section 4.4 Atomic structure of the AQA GCSE Physics (8463) specification, Version 1.1 (30 September 2019), for teaching from September 2016 and exams from 2018 onwards. It covers every point in 4.4.1 to 4.4.4: atoms and isotopes, nuclear radiation, the hazards and uses of radioactive emissions, and nuclear fission and fusion. The topic is assessed on Paper 1 (topics 1 to 4), which is set at Foundation and Higher Tier. The one point the specification marks (HT only) is labelled Higher tier only below.

For quick recall, use the Atomic structure revision notes. To test yourself, use the Atomic structure practice questions. The course hub is AQA GCSE Physics, and the printable checklist lists every specification point.

What this unit covers

Spec point	What you must be able to do	Tier
4.4.1.1 Structure of an atom	Give the size of an atom and nucleus; describe the nucleus and electron energy levels; explain how absorbing or emitting radiation moves electrons	Both
4.4.1.2 Mass number, atomic number, isotopes	Define atomic number, mass number and isotope; read nuclear notation; describe how positive ions form	Both
4.4.1.3 Development of the atomic model	Describe the plum pudding model, the alpha scattering evidence, the nuclear model, Bohr, protons and Chadwick’s neutrons	Both
4.4.2.1 Radioactive decay	Define activity (Bq) and count-rate; describe alpha, beta, gamma and neutron emission; compare penetration, range and ionising power	Both
4.4.2.2 Nuclear equations	Balance mass and atomic numbers for single alpha and beta decays	Both
4.4.2.3 Half-life	Explain half-life and randomness; find half-life from data	Both
4.4.2.3 Half-life	Calculate the net decline, as a ratio, after a number of half-lives	Higher tier only
4.4.2.4 Contamination	Compare contamination and irradiation; explain precautions and peer review	Both
4.4.3 Hazards and uses	Background radiation sources; dose in sieverts; why half-life affects hazard; medical uses and risk	Both
4.4.4 Fission and fusion	Describe fission, chain reactions (controlled and uncontrolled) and fusion	Both

4.4.1 Atoms and isotopes

The structure of an atom

An atom has a radius of about 1 × 10⁻¹⁰ m. At its centre is a positively charged nucleus made of protons and neutrons. Negatively charged electrons surround it. The radius of the nucleus is less than 1/10 000 of the radius of the atom, yet most of the mass of the atom is in the nucleus.

Electrons sit at different distances from the nucleus, called energy levels.

• If an electron absorbs electromagnetic radiation, it moves further from the nucleus, to a higher energy level.
• If it emits electromagnetic radiation, it moves closer to the nucleus, to a lower energy level.

Worked example. An atom has a radius of 1 × 10⁻¹⁰ m. What is the largest the nucleus radius can be?

nucleus radius < atom radius / 10 000
               < 1 × 10⁻¹⁰ / 1 × 10⁴
               < 1 × 10⁻¹⁴ m

To picture it: if the atom were a stadium with a 100 m radius, the nucleus would have a radius under 1 cm.

Atomic number, mass number and isotopes

• In an atom, the number of electrons equals the number of protons, so the atom has no overall charge.
• Atomic number = number of protons. Every atom of an element has the same atomic number.
• Mass number = number of protons + number of neutrons.
• Isotopes are atoms of the same element with different numbers of neutrons.
• An atom becomes a positive ion if it loses one or more outer electrons.

In nuclear notation, the mass number is written top left and the atomic number bottom left, for example ⁷₃Li for lithium-7.

Worked example. Compare lithium-6 and lithium-7. Lithium has atomic number 3.

	⁶₃Li	⁷₃Li
Protons	3	3
Neutrons	6 − 3 = 3	7 − 3 = 4
Electrons (atom)	3	3

They are isotopes: same number of protons, different number of neutrons. A Li⁺ ion has lost one electron, so it has 2 electrons but still 3 protons.

How the model of the atom developed

New experimental evidence can change or replace a scientific model.

1. Atoms were first thought to be tiny spheres that could not be divided.
2. The discovery of the electron led to the plum pudding model: a ball of positive charge with negative electrons embedded in it.
3. The alpha particle scattering experiment showed that the mass is concentrated at the centre and that this centre is charged. The nuclear model replaced the plum pudding model.
4. Niels Bohr suggested that electrons orbit the nucleus at specific distances. His calculations agreed with experimental results.
5. Later experiments showed the positive charge of a nucleus is made of whole numbers of identical positive particles, named protons.
6. James Chadwick provided evidence for neutrons, about 20 years after the nucleus was accepted.

Why the scattering results changed the model. Most alpha particles passed straight through the gold foil, so most of the atom must be empty space. A few were deflected, and a very small number bounced back. A thin spread of positive charge, as in the plum pudding model, could not do that. The results need a tiny, dense, positively charged centre.

Plum pudding model	Nuclear model
Positive charge spread through the whole atom	Positive charge concentrated in a tiny nucleus
Mass spread throughout	Most mass in the nucleus
Electrons embedded in the positive ball	Electrons outside the nucleus; atom mostly empty space

You do not need the details of Bohr’s or Chadwick’s experiments.

4.4.2 Atoms and nuclear radiation

Radioactive decay

Some nuclei are unstable. They give out radiation to become more stable. This is a random process called radioactive decay.

• Activity is the rate at which a source of unstable nuclei decays, measured in becquerel (Bq).
• Count-rate is the number of decays recorded each second by a detector, such as a Geiger-Muller tube.

Emission	What it is	Penetration	Range in air	Ionising power
Alpha (α)	2 protons + 2 neutrons (a helium nucleus)	Stopped by a sheet of paper	A few centimetres	Strongly ionising
Beta (β)	High-speed electron ejected from the nucleus as a neutron turns into a proton	Stopped by a few millimetres of aluminium	Further than alpha	Moderately ionising
Gamma (γ)	Electromagnetic radiation from the nucleus	Reduced by thick lead or concrete	Very far	Weakly ionising

A nucleus can also emit a neutron (n). You must be able to use these properties to choose the best source for a given use.

Nuclear equations

In nuclear equations, an alpha particle is written ⁴₂He and a beta particle ⁰₋₁e. The top numbers (mass numbers) must balance, and so must the bottom numbers (atomic numbers).

• Alpha decay: mass number falls by 4 and atomic number falls by 2. Mass and charge both decrease.
• Beta decay: mass number unchanged; atomic number rises by 1. Charge increases.
• Gamma emission: no change to mass or charge.

Worked example. Polonium-210 (atomic number 84) decays by alpha emission to lead. Strontium-90 (atomic number 38) decays by beta emission to yttrium.

²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂He      check: 210 = 206 + 4;  84 = 82 + 2
⁹⁰₃₈Sr  → ⁹⁰₃₉Y  + ⁰₋₁e       check: 90 = 90 + 0;    38 = 39 + (−1)

You will be given the names of the daughter elements; you only balance the numbers.

Half-life and randomness

You cannot predict when a particular nucleus will decay. But with a very large number of nuclei, the fraction that decays in a given time is predictable. That is why half-life is constant for an isotope.

The half-life is the time for the number of nuclei of the isotope in a sample to halve, or the time for the count-rate (or activity) to fall to half its initial level.

Worked example. A sample’s count-rate is 960 counts per minute. It is 480 after 15 minutes and 240 after 30 minutes. Find the half-life.

960 → 480 takes 15 min;  480 → 240 takes another 15 min
half-life = 15 minutes

Higher tier only: net decline as a ratio. After 60 minutes (four half-lives):

960 → 480 → 240 → 120 → 60 counts per minute
net decline = 60 : 960 = 1 : 16   (the count-rate is 1/16 of its first value)

After n half-lives, the remaining fraction is 1/2ⁿ.

Contamination and irradiation

• Radioactive contamination is the unwanted presence of radioactive atoms on or in other materials. The hazard comes from the decay of those atoms, and the type of radiation affects how hazardous it is.
• Irradiation is exposing an object to nuclear radiation. The object does not become radioactive.

Contamination lasts as long as the contaminating atoms keep decaying. Irradiation stops when the source is removed or shielded. Precautions such as shielding, keeping your distance, limiting time and handling sources with tongs protect against the source. Findings on the effects of radiation on humans must be published and shared so other scientists can check them by peer review.

4.4.3 Hazards and uses of radioactive emissions

Background radiation

Background radiation is around us all the time. It comes from:

• natural sources, such as rocks and cosmic rays from space
• man-made sources, such as fallout from nuclear weapons testing and nuclear accidents.

Your dose can depend on your occupation and location. Radiation dose is measured in sieverts (Sv): 1000 mSv = 1 Sv, so 1.5 mSv = 0.0015 Sv. You do not need to recall the unit.

Different half-lives

Half-lives range from fractions of a second to billions of years. A short half-life means high activity at first, but the hazard soon falls. A long half-life means lower activity, but the material stays hazardous for a very long time.

Uses in medicine

Nuclear radiation is used to explore internal organs (a tracer is taken into the body and detected from outside) and to control or destroy unwanted tissue, such as a tumour. For a tracer, choose a gamma emitter (it passes out of the body and ionises little) with a half-life long enough for the test but short enough that activity falls quickly afterwards. You must weigh the benefits against the risks using the data given.

4.4.4 Nuclear fission and fusion

Fission

Nuclear fission is the splitting of a large, unstable nucleus, such as uranium or plutonium. Spontaneous fission is rare; usually the nucleus must first absorb a neutron. It then splits into two smaller nuclei of roughly equal size, and emits two or three neutrons plus gamma rays. Energy is released, and all the fission products have kinetic energy.

The emitted neutrons may cause further fissions: a chain reaction. In a nuclear reactor the chain reaction is controlled to control the energy released. A nuclear weapon’s explosion comes from an uncontrolled chain reaction. You should be able to draw and interpret diagrams of these steps.

Fusion

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. Some of the mass may be converted into the energy of radiation.

Common errors

• Writing that the nucleus is “about 1/10 000 the size” when the specification says less than.
• Saying mass number is the number of neutrons.
• Calling a beta particle an electron from the shells; it comes from the nucleus.
• Balancing only the mass numbers in a decay equation.
• Saying an irradiated object becomes radioactive.
• Confusing activity (decays per second of the source) with count-rate (what the detector records).
• Mixing up fission (splitting heavy nuclei) and fusion (joining light nuclei).

Next steps

Move on to the revision notes and then the practice questions. To find gaps across the whole course, try the free diagnostics.

Official syllabus

AQA GCSE Physics (8463) specification, Version 1.1, 30 September 2019, for teaching from September 2016 and exams from 2018 onwards (AQA), section 4.4 Atomic structure.