OCR GCSE Physics A: Nuclear structure, isotopes and decay equations (J249) – Revision Notes

These revision notes condense nuclear structure, isotopes and decay equations for OCR GCSE Physics A (Gateway Science, J249), specification Version 5.0 (July 2026), learning outcomes P6.1a to P6.1g. All of these statements are assessed in both tiers. For full explanations and worked examples, read the study guide first. Course links: OCR GCSE Physics hub, printable checklist and free 10-minute diagnostics.

The nucleus (P6.1a)

Particle	Charge	Mass
Proton	+1	1
Neutron	0	1
Electron	-1	negligible (about 1/1800)

• The nucleus contains protons and neutrons. Electrons are outside it.
• The nucleus is always positive, because only protons carry charge.
• Atomic number Z = number of protons. It is characteristic of the element.
• Mass number A = protons + neutrons.
• Neutrons: N = A - Z.
• An uncharged atom has Z electrons.

Isotopes (P6.1b)

• Same element, same Z, different number of neutrons, so different A.
• Same nuclear charge, different nuclear mass.
• Same mass number with different Z means different elements, not isotopes.

Isotope pair	Protons	Neutrons
Copper-63	29	34
Copper-65	29	36

Nuclear notation (P6.1c)

Write as ᴬ_Z with the symbol: top number is A, bottom number is Z, as in ⁶³₂₉Cu.

Particle	Symbol	A	Charge
Proton	¹₁p	1	+1
Neutron	¹₀n	1	0
Alpha	⁴₂He or α	4	+2
Beta	⁰₋₁e or β	0	-1
Gamma	γ	0	0

To compare isotopes using symbols, state three things: same element (same bottom number), same nuclear charge, different mass (different top number).

The four emissions (P6.1d)

• Alpha: 2 protons and 2 neutrons, a helium nucleus.
• Beta: a fast electron from the nucleus, made when a neutron becomes a proton.
• Neutron: a single neutron.
• Gamma: electromagnetic radiation, no mass, no charge.

Effect on the nucleus (P6.1e)

Emission	A	Z	Element changes?
Alpha	-4	-2	Yes
Beta	0	+1	Yes
Neutron	-1	0	No (new isotope)
Gamma	0	0	No

Method: balancing a decay equation (P6.1f, P6.1g)

1. Write the parent nucleus and the emitted particle with both numbers.
2. Work out the daughter’s top number: parent A minus particle A.
3. Work out the daughter’s bottom number: parent Z minus particle Z. Subtracting the beta charge of -1 adds 1.
4. Name the element from its Z.
5. Check: top numbers add up on both sides, and bottom numbers add up on both sides.

Reminders.

• Alpha: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He (238 - 4 = 234, 92 - 2 = 90).
• Beta: ³²₁₅P → ³²₁₆S + ⁰₋₁e (32 stays, 15 + 1 = 16).
• Gamma: ²⁰⁸₈₂Pb (excited) → ²⁰⁸₈₂Pb + γ.
• Neutron: ⁵₂He → ⁴₂He + ¹₀n.
• Two-step chain: ²¹²₈₂Pb → ²¹²₈₃Bi + ⁰₋₁e, then ²¹²₈₃Bi → ²¹²₈₄Po + ⁰₋₁e.

Worked reminders

Reading a symbol. For ¹³⁷₅₆Ba: protons 56, neutrons 137 - 56 = 81, nuclear charge +56, mass 137. A neutral atom has 56 electrons.

Building a symbol. A nucleus with 82 protons and 125 neutrons has A = 207 and Z = 82. Z = 82 is lead, so the symbol is ²⁰⁷₈₂Pb.

Comparing isotopes in words. For ⁶³₂₉Cu and ⁶⁵₂₉Cu: “Both are copper because both have 29 protons. The nuclear charge is the same, +29. Copper-65 has two more neutrons (36 against 34), so its nuclear mass is greater.”

Finding the particle. If ²¹⁰₈₃Bi turns into ²¹⁰₈₄Po, the top number is unchanged and the bottom number rises by 1. That fits beta emission, so the missing particle is ⁰₋₁e.

Finding the parent. Work backwards by adding. If the daughter is ²⁰⁶₈₂Pb after an alpha emission, the parent has A = 206 + 4 = 210 and Z = 82 + 2 = 84. It is ²¹⁰₈₄Po.

Show that an equation is balanced. Write a check line for each side.

• Top numbers: 238 on the left. On the right 234 + 4 = 238.
• Bottom numbers: 92 on the left. On the right 90 + 2 = 92.
• Both agree, so the equation is balanced.

Sentence templates for written answers

• Nucleus: “The nucleus contains protons and neutrons. It is positive because the protons are positive and the neutrons are neutral.”
• Isotopes: “Isotopes are atoms of one element that have equal protons and unequal neutrons.”
• Beta: “A neutron in the nucleus changes into a proton and a fast electron, which is emitted as the beta particle.”
• Gamma: “Gamma is electromagnetic radiation emitted by the nucleus. The nucleus keeps the same mass number and charge, but loses energy.”
• Alpha: “An alpha particle is two protons and two neutrons, so the nucleus loses 4 from its mass number and 2 from its atomic number.”

A quick routine for exam questions

1. Decide which two numbers you need: top (A), bottom (Z), or both.
2. Write the known nuclei with both numbers before you start.
3. Subtract the emitted particle’s numbers from the parent’s numbers.
4. Name the daughter using Z.
5. Add the products back up and compare with the parent.

Elements by atomic number

Naming the daughter nucleus needs the periodic table, so practise the early part of it. In the examples on these pages the elements are:

Z	Element	Z	Element
2	Helium	16	Sulfur
6	Carbon	17	Chlorine
8	Oxygen	20	Calcium
11	Sodium	29	Copper
12	Magnesium	82	Lead
15	Phosphorus	86	Radon

Beta emission moves you one place along the periodic table to the right. Alpha emission moves you two places to the left.

Linking the ideas

• Nuclear charge decides the element, so any change in Z changes the element.
• Mass number changes only when a particle with mass leaves: alpha (4) or neutron (1).
• Gamma is the only emission here that leaves both numbers alone.
• The sum of top numbers and the sum of bottom numbers are both conserved in every equation.

Must-know distinctions

• Isotope vs element: isotopes of one element differ in neutrons only.
• Mass number vs atomic number: top is protons plus neutrons, bottom is protons.
• Beta particle vs orbiting electron: a beta particle is made in and emitted by the nucleus.
• Alpha vs beta in the table: alpha changes A and Z, beta changes only Z.
• Neutron emission vs gamma: neutron emission lowers A by 1 and leaves the element the same, while gamma changes neither number.

Question types to practise

• Recall: name the particles in the nucleus, or state what an isotope is. Give the whole definition, including “same number of protons”.
• Use a symbol: read off protons and neutrons, or build a symbol from a description. Write A - Z as a line of working.
• Compare: two isotopes, or an atom and its decay product. Mention charge, mass and element each time.
• Complete an equation: find one or two missing numbers or names. The numbers must both balance.
• Explain: why a beta emission changes the element but gamma emission does not. Link the change in protons to the change in element.

Quick self-test

1. How many protons, neutrons and electrons are in a neutral atom of ⁶⁵₃₀Zn?
2. A nucleus has relative charge +9 and relative mass 19. How many neutrons?
3. Zinc-64 and zinc-66: what is the same and what is different?
4. Why are ¹⁴₆C and ¹⁴₇N not isotopes of each other?
5. Write the symbol for a nucleus with 38 protons and 50 neutrons.
6. State the mass number and charge of a beta particle.
7. By how much do A and Z change in alpha emission?
8. Balance: ²²²₈₆Rn → ?₈₄Po + ⁴₂He.
9. Balance: ³⁵₁₆S → ?₁₇Cl + ⁰₋₁e.
10. An unstable nucleus with A = 27 and Z = 12 emits a neutron. What nucleus is left?
11. What happens to A and Z when a nucleus emits only gamma?
12. A nucleus ²¹⁰₈₄Po decays by alpha emission. Give the daughter nucleus and explain how the nuclear charge changes.

Answers

1. 30 protons, 65 - 30 = 35 neutrons, 30 electrons.
2. 9 protons, so 19 - 9 = 10 neutrons.
3. Same: 30 protons, same element, same nuclear charge. Different: neutrons (34 and 36) and so mass number.
4. They have different numbers of protons (6 and 7), so they are different elements. Equal mass numbers do not make isotopes.
5. A = 88, Z = 38, strontium: ⁸⁸₃₈Sr.
6. Mass number 0, charge -1.
7. A falls by 4 and Z falls by 2.
8. Top number 222 - 4 = 218. Check the bottom: 84 + 2 = 86. The product is ²¹⁸₈₄Po.
9. Top stays 35, bottom 16 + 1 = 17 (check: 17 + (-1) = 16). The daughter is ³⁵₁₇Cl.
10. A = 26, Z = 12: ²⁶₁₂Mg, a different isotope of magnesium.
11. Neither changes. The nucleus loses energy only.
12. ²⁰⁶₈₂Pb (A = 210 - 4 = 206, Z = 84 - 2 = 82). The nuclear charge falls from +84 to +82 because two protons leave in the alpha particle.

Where marks are usually lost

• Writing the top and bottom numbers the wrong way round in the symbol.
• Giving neutrons as A, or Z + A, instead of A - Z.
• Saying isotopes have different atomic numbers or different numbers of electrons.
• Calling two nuclei with the same mass number isotopes.
• Using +1 or a mass of 1 for the beta particle.
• Forgetting to name the new element after finding Z.
• Writing an equation where the top numbers balance but the charges do not.
• Giving the alpha particle as a hydrogen nucleus or as two protons only.
• Not showing the check of each side in a “show that the equation is balanced” question.

Next: apply these notes in the practice questions and review gaps with the diagnostics.

Official syllabus

OCR GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), for first assessment in 2018, published by OCR. These notes cover learning outcomes P6.1a to P6.1g.