Z, A and the isotope definition
Atomic (proton) number Z = number of protons — it alone fixes the element. Mass (nucleon) number A = protons + neutrons. Neutrons = A − Z, and a neutral atom has Z electrons. Isotopes are atoms of the same element — same Z — but with different numbers of neutrons, so a different A. Definition trap: an isotope is not simply an atom with a different mass; you must state same protons, different neutrons for full credit.
Alpha, beta, gamma: nature and charge
Alpha (α) is a helium nucleus — 2 protons, 2 neutrons, charge +2: most ionising, least penetrating (stopped by paper). Beta-minus (β⁻) is a fast electron from the nucleus, charge −1: moderately ionising, stopped by a few mm of aluminium. Gamma (γ) is a high-frequency electromagnetic wave with no charge or mass: least ionising, most penetrating — only reduced by thick lead or concrete. Ionising means knocking electrons off atoms to form ions.
Balancing nuclear equations
Balance a decay equation by conserving two totals separately: the mass numbers (top) must balance across the arrow, as must the atomic numbers (bottom). Alpha emission loses a helium nucleus, so A falls by 4 and Z falls by 2. Beta-minus turns a neutron into a proton and emits an electron, so A is unchanged and Z rises by 1. Gamma emission removes energy only — neither A nor Z changes. Never guess the element by name before the columns add up.
Drawn from real examiner reports.
Activity is in becquerel; subtract background
The unit of activity is the becquerel (Bq), where 1 Bq = 1 decay per second — quoting an activity with no unit, or writing "counts", loses the mark. A detector's count rate also includes background: the corrected count rate due to the source = measured count rate − background count rate. Forgetting to subtract it overstates the source.
Nov 2024 1P Q4(a): nearly three-quarters recalled the unit of activity (the becquerel); lost marks were for missing or incorrect units.
Use ionising/penetrating, not stronger
When comparing alpha, beta and gamma the examiner credits only two technical ideas: penetrating power (how far it travels before being stopped) and ionising power (how strongly it knocks electrons off atoms). Vague words — stronger, more powerful, more energy — score nothing. State it precisely, e.g. gamma penetrates most but ionises least.
Jun 2024 1PR Q8(b)(ii): only technical language — a reference to ionising power or penetrating power — was accepted; vague comparatives scored nothing.
Ionising vs penetrating: opposite rankings
These two rankings run in opposite directions. Alpha is the most ionising but the least penetrating; gamma is the least ionising but the most penetrating; beta is intermediate for both. The reason: a strongly ionising radiation loses its energy over a short distance, so it is stopped quickly. Do not assume the most penetrating radiation is also the most ionising.
Isotope is not an ion
An isotope is an atom with the same number of protons but a different number of neutrons — a difference in the nucleus. An ion is an atom that has gained or lost electrons and so carries a charge — a difference in the electrons. They are completely different ideas: isotopes are about neutrons, ions are about electrons and charge.
Mass number vs relative atomic mass
The mass number A is a whole number of nucleons in one nucleus. The relative atomic mass is an average over an element's isotopes and is usually not a whole number — copper is 63.5 on the Edexcel data sheet, not 63 or 64. Do not round the relative atomic mass to a mass number, and do not treat 63.5 as the nucleon count of any single copper atom.
Explaining scatter in count-rate data
Background radiation is the low-level ionising radiation always around us. Asked why readings scatter about a decay curve, "there is background radiation" scores zero. Say the background varies / is random, or that radioactive decay itself is random and spontaneous — unaffected by temperature, pressure or chemical state — so you cannot predict which nucleus decays.
Jun 2023 1P Q1(c)(iv): stating background radiation was present was insufficient; the mark needed that the background varies (is random) or that decay is random.
Balance equations column by column
Balance a decay equation in two steps: add up the top (mass) numbers, then the bottom (atomic) numbers. Use α = helium-4 and β⁻ = an electron (bottom number −1). Alpha: A−4, Z−2; beta-minus: A same, Z+1. Name the element from its final Z.
Do the nucleon bookkeeping first
For any nuclide, write protons = Z, neutrons = A − Z, electrons = Z before answering. Read the symbol as top = mass number, bottom = atomic number; isotopes share the bottom number. For count rates, subtract the background first; quote activity in becquerel (Bq).
Compare radiations with precise terms
When comparing radiations, answer only in terms of ionising power and penetrating power, and name a stopping material: paper stops alpha, aluminium stops beta, thick lead/concrete only reduces gamma. Never write "stronger"; let the mark tariff set how many differences to give.
This topic mixes a little number work (counting nucleons and balancing decay equations) with a lot of precise description (the nature, ionising power and penetration of the three radiations). Marks are won on exact definitions and column-balanced equations, and lost on vague words like "stronger".
Nucleon counting (from the nuclear symbol ):
where = mass (nucleon) number (protons + neutrons) and = atomic (proton) number (protons). In a neutral atom, number of electrons .
Decay changes to the nucleus:
| Emission | Symbol | Change in mass number | Change in atomic number |
|---|---|---|---|
| Alpha (α) | |||
| Beta-minus (β⁻) | (unchanged) | ||
| Gamma (γ) |
Corrected count rate:
Activity is measured in the becquerel (Bq): decay per second.
Define the atomic (proton) number of an atom.
An atom of sodium has 11 protons and 12 neutrons in its nucleus.
State the mass (nucleon) number of this sodium atom.
