OxfordAQA International GCSE Physics: Atomic structure, isotopes and types of ionising radiation (9203) – Revision Notes

These notes condense sections 3.7.1 a to f and 3.7.2 a to g of the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards. The qualification is untiered. For full explanations read the study guide, then test yourself with the practice questions.

Other useful pages: the course hub, the printable checklist, the 10-minute diagnostics and the exam preparation page.

Definitions to learn word for word

Term	Definition
Atomic number (proton number)	How many protons the nucleus holds
Mass number	Protons plus neutrons, counted together
Isotopes	Atoms with equal atomic numbers but unequal mass numbers, because the neutron counts differ
Ion	An atom that has become charged by losing or gaining outer electrons
Radioactive decay	An unstable nucleus emits particles or radiation and changes to the nucleus of a different element
Random	No one can predict when an individual nucleus will decay
Background radiation	Radiation around us all of the time, from the environment, from space and from devices such as hospital X-ray machines
Ionising radiation	Radiation that can remove electrons from atoms and so make ions

The atom at a glance (3.7.1 a to e)

• Atom radius about 10⁻¹⁰ m. The nucleus is far smaller (about 10⁻¹⁵ m) and holds almost all of the mass.

• Nucleus: protons and neutrons, positive overall. Electrons surround it at various distances.

• A neutral atom has equal numbers of electrons and protons.

• Proton: charge +1, mass 1.

• Neutron: charge 0, mass 1.

• Electron: charge –1, mass very small.

Ions. Lose electrons, positive ion. Gain electrons, negative ion. Charge = protons – electrons. The nucleus does not change.

Alpha scattering in three lines (3.7.1 b)

Observation	Meaning
Most go straight on	Most of the atom is empty space
Some slightly deflected	Repelled by a positive nucleus
Very few bounce back	A tiny nucleus holds the charge and the mass

Mark-winning habit: write observation, then conclusion, as a pair.

Notation and particle counting (3.7.1 f)

Write the symbol with the mass number above and the atomic number below, as in ⁶⁰₂₇Co.

Method in steps

1. Read the protons off the bottom number.
2. Subtract the bottom number from the top number to get the neutrons.
3. A neutral atom has as many electrons as protons.
4. For an ion, change the electron count: charge = protons – electrons.

Isotopes: the same bottom number and a different top number. Their chemistry is the same because they have the same number of electrons.

Unstable nuclei and decay (3.7.2 a to c)

• Unstable nucleus emits alpha, beta, a neutron or gamma and becomes more stable. Energy is emitted by changes in the nucleus.
• Random: there is no telling when a given nucleus will go.
• Not affected by chemical or physical processes. Heat, pressure and chemical bonding make no difference.

The radiations (3.7.2 e, g)

	Alpha	Beta	Gamma
What it is	2 protons + 2 neutrons (helium nucleus)	Fast electron that leaves the nucleus (a neutron changes into a proton)	Electromagnetic radiation from the nucleus
Symbol	⁴₂He	⁰₋₁e	none needed
Charge	+2	–1	0
Ionising power	strongest	moderate	weakest
Stopped by	paper or skin	a few mm of aluminium	thick lead or concrete reduce it; not fully stopped
Range in air	a few cm	tens of cm to a few m	very long

Link to remember: strongly ionising means energy is used up quickly, so the range is short. Weakly ionising means it travels far.

Nuclear equations (3.7.2 f)

Total mass number and total atomic number must match on both sides.

Decay	Mass number	Atomic number
Alpha	–4	–2
Beta	no change	+1
Gamma	no change	no change

Method in steps

1. Write the parent numbers on the left.
2. Write the numbers of the emitted particle (alpha: 4 and 2; beta: 0 and –1).
3. Subtract the particle’s numbers from the parent’s mass number, then do the same for the atomic number, for the unknown nucleus. For beta, subtracting –1 gives +1.
4. Check the sums on each side.

Worked reminder. ²³⁰₉₀Th emits an alpha particle: A = 230 – 4 = 226, Z = 90 – 2 = 88, so the new nucleus is ²²⁶₈₈X. Now ⁶⁰₂₇Co emits a beta particle: A = 60, Z = 27 + 1 = 28, so ⁶⁰₂₈X. You do not have to name the new element.

Background and count rates (3.7.2 d)

Background count rate = counts / time with no source. Corrected count rate = count rate with source – background count rate.

Worked reminder. 2130 counts in 3 minutes with the source: 2130 / 3 = 710 per minute. Background is 30 per minute. Corrected rate = 710 – 30 = 680 per minute.

Sources of background: radioactive substances in the environment (for example radon from rocks), radiation from space, and devices such as hospital X-ray machines.

Identifying a source with absorbers

Place paper, then thin aluminium, then thick lead between a source and a detector, and subtract the background each time.

Result	Conclusion
Count rate drops to background with paper	Alpha only
Paper makes little difference, aluminium cuts the rate sharply	Beta present
Aluminium makes little difference, thick lead cuts the rate a lot	Gamma
Rate falls at each step	A mixture

Small changes of a few counts are within random variation, so do not read them as absorption. Compare changes with the size of the corrected count rate.

Worked reminder (ions). ⁵⁶₂₆Fe has 26 protons and 56 – 26 = 30 neutrons. If it has 24 electrons, charge = 26 – 24 = +2.

Answer templates

• Scattering: “Most went straight on, so most of an atom is empty space. A few were turned back, so a small, dense, positive nucleus must exist.”
• Random: “The moment any one nucleus decays cannot be predicted.”
• Unaffected: “Decay comes from changes in the nucleus, so chemical and physical conditions do not alter it.”
• Penetration: name the material that stops the radiation, rather than writing “it is blocked”.
• Isotopes: “Equal proton numbers, unequal neutron numbers.”

Must-know distinctions

Often confused	Difference
Atom vs nucleus	The atom is about 10⁻¹⁰ m across and is mostly empty space. The nucleus is far smaller and holds almost all the mass
Atomic number vs mass number	Protons only, against protons plus neutrons
Beta particle vs an electron in the atom	Beta is made in the nucleus when a neutron changes into a proton
Gamma vs alpha and beta	Gamma is a wave with no mass or charge. Alpha and beta are particles
Atom vs ion	Same protons, different number of electrons
Isotope vs ion	An isotope has a different neutron count. An ion has a different electron count

Quick self-test

1. Roughly how large is the radius of an atom?
2. Give the particle counts (protons, neutrons, electrons) for a neutral ⁶⁰₂₇Co atom.
3. An atom of ²⁷₁₃Al has 10 electrons. What is its overall charge?
4. What does a large deflection of an alpha particle tell you about the nucleus?
5. Complete the mass number and atomic number: ²³⁰₉₀Th → ᴬ_Z X + ⁴₂He.
6. Complete the mass number and atomic number: ⁶⁰₂₇Co → ᴬ_Z X + ⁰₋₁e.
7. Which of alpha, beta and gamma is the most ionising, and which has the longest range in air?
8. Explain why heating a radioactive source does not change how quickly it emits radiation.
9. A detector records 2130 counts in 3 minutes with a source, and the background is 30 counts per minute. Find the corrected count rate.
10. Explain why two atoms with different mass numbers can still be the same element.

Answers

1. About 10⁻¹⁰ m.
2. 27 protons, 60 – 27 = 33 neutrons, 27 electrons.
3. 13 protons and 10 electrons, so 13 – 10 = +3.
4. The nucleus is positive, and it is tiny and dense: it packs the charge and nearly all the mass into a very small volume.
5. A = 226, Z = 88.
6. A = 60, Z = 28.
7. Alpha is the most ionising. Gamma has the longest range in air.
8. Decay depends on the nucleus, and physical processes such as heating do not affect the nucleus, so the random decay carries on at the same rate.
9. 2130 / 3 = 710 per minute, then 710 – 30 = 680 counts per minute.
10. They have the same number of protons (same atomic number), and differ only in neutrons.

Where marks are usually lost

• Writing “mostly empty space” about the nucleus instead of the atom.
• Giving the conclusion of the scattering experiment without the observation it comes from.
• Calculating neutrons from the atomic number alone.
• Calling the beta particle “an electron from the atom” without saying it comes from the nucleus.
• Getting the beta atomic number change the wrong way round (it goes up by one).
• Forgetting to subtract background, or subtracting it before converting counts to a rate.
• Saying lead “stops” gamma completely.
• Writing that heating or chemical reactions change the decay of a nucleus.
• Mixing up penetration and ionising power: the one with the most ionisation penetrates least.

Official syllabus

OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards, published by OxfordAQA. Sections 3.7.1 a to f and 3.7.2 a to g.