Nucleus composition and relative charges
The nucleus contains protons and neutrons, together called nucleons; the electrons are outside it and surround the nucleus. Relative charges: proton +1, neutron 0, electron -1. Relative masses: proton 1, neutron 1 (neutral, but not massless), electron negligible, about 1/1840 of a proton. A neutral atom has equal numbers of protons and electrons, so the +1 and -1 charges cancel exactly and the atom carries no overall charge.
Proton number Z, nucleon number A, isotopes
Proton number (atomic number) Z counts the protons and fixes which element the atom is. Nucleon number (mass number) A counts protons plus neutrons. The neutron number comes by subtraction: N = A - Z. In nuclide notation the TOP number is A, the BOTTOM is Z. Isotopes are atoms of the same element (same Z) with different neutron numbers, so different A: carbon-12, -13 and -14 each have 6 protons but 6, 7 and 8 neutrons. Some, like carbon-14, are radioactive.
(Extended) Nuclear charge, fission and fusion
Relative nuclear charge = +Z. The charge comes only from the protons (each +1); neutrons add none, so a nucleus with 8 protons has relative charge +8. Nuclear fission is a large nucleus SPLITTING into smaller nuclei; nuclear fusion is two small nuclei JOINING into a larger one. Fusion of hydrogen into helium powers the Sun; fission is used in power stations. Both alter the nucleus itself, so they are nuclear changes, not chemical.
Drawn from real examiner reports.
(Extended) Fission and fusion swapped
Fission and fusion are stated the wrong way round more often than not. Fis-sion = splitting: one large nucleus divides into smaller nuclei. Fu-sion = fusing: two small nuclei join into a larger one. Fission must be described as splitting -- "exploding", or a vague "breaking down", is not credited.
Electrons are not in the nucleus
Only protons and neutrons sit inside the nucleus. The electrons surround it, moving in the space around it. Writing that electrons are "in the nucleus", or drawing them there on an atom diagram, loses the mark even when the rest of the answer is right. Treat the location as part of each particle's description, not an optional extra.
Digest citation -- electrons SURROUND the nucleus (protons + neutrons are IN the nucleus) (Nov 2023 P11 Q16, P32 Q2bii).
Neutron: charge 0 but relative mass 1
A neutron has relative charge 0, yet its relative mass is 1 -- the same as a proton. Candidates who reason "no charge, so no mass" write mass 0; others give the neutron a charge of +1 or -1. Charge and mass are independent properties. Only the electron has negligible mass, about 1/1840 of a proton.
Digest citation -- relative mass of a neutron = 1 (not 0) (Nov 2022 P42 Q2b).
Isotope definition needs both halves
A full isotope answer states BOTH ideas: same proton number (same number of protons), and different numbers of neutrons. Answers giving only "atoms with different masses", or only "atoms of the same element", score partially or not at all. The definition is frequently left incomplete -- write both clauses every time.
Digest citation -- isotopes = same protons, different neutrons; candidates give incomplete isotope answers (Jun 2022 P42 Q8bi; Jun 2023 P42 Q8b; Nov 2023 P42 Q2aiii).
Nucleon number, not "nucleus number"
A is the nucleon number -- the total of protons and neutrons. "Nucleus number" is not an accepted term and scores nothing. Nucleon is the collective name for a proton or a neutron, which is exactly why their total carries that name. "Mass number" is an accepted alternative; "nucleus number" never is.
Digest citation -- "nucleon number" (not "nucleus number") = protons + neutrons (Jun 2022 P12 Q15).
Reading a neutron number off the symbol
Nucleon number, proton number and neutron number are three different quantities and are routinely mixed up. Neither number printed on the nuclide symbol is the neutron number -- you must subtract, N = A - Z. Giving A (or Z) when the neutron number was asked, or adding A and Z together, is the standard slip here.
Digest citation -- distinguish mass number, atomic number and neutron number (Nov 2023 P11 Q40).
(Extended) The Sun runs on fusion
Energy is released in the Sun by nuclear fusion -- hydrogen nuclei joining to form helium. Answers naming fission, combustion (burning) or radiation are all wrong. Fission is the power-station process, not the stellar one. Keep the two settings apart: stars fuse, reactors split.
Digest citation -- energy is released in the Sun by nuclear fusion, not fission/combustion/radiation (Jun 2022 P23 Q31; Nov 2022 P41 Q6c).
Decode the symbol before anything else
In nuclide notation the TOP number is the nucleon number A and the BOTTOM number is the proton number Z. Write both down, labelled, before you start calculating -- most errors in this topic come from reading the two the wrong way round.
Subtract for neutrons, and label the count
Neutrons = A - Z, always by subtraction. In a neutral atom the number of electrons equals Z. Show the subtraction as working, and label what your number counts (neutrons, protons, nucleons) rather than leaving a bare figure on the line.
Match the command word
"State" the relative charge of a neutron wants one value (0). "Define" proton number wants "the number of protons in the nucleus". "Describe" fission or fusion (Extended) wants splitting of a nucleus, or joining of two nuclei into a larger one.
Test for isotopes with Z, not A
Two nuclei are isotopes of the same element when their proton numbers are EQUAL and their nucleon numbers DIFFER. In a data table, pair the nuclei by matching Z; pairing by matching A finds nothing, because isotopes never share a nucleon number.
Cambridge 0654 spec reference: Section P5 "Nuclear physics", sub-topic P5.1 (Core + Extended). This leaf covers the composition of the nucleus (protons and neutrons), the relative charges of the proton, neutron and electron, the proton number and nucleon number , calculating the neutron number, nuclide notation , isotopes, and (Extended) the relationship between proton number and relative nuclear charge and the meaning of nuclear fission and fusion.
Scope note: the alpha-scattering nuclear-model experiment, radioactive decay/half-life, and mass-defect/binding-energy are NOT in this leaf. P5 is qualitative in 2025-27.
| Quantity | Relationship | Notes |
|---|---|---|
| Neutron number | nucleon number minus proton number | |
| Nucleon number | protons + neutrons | |
| Proton number | protons only (fixes the element) | |
| Relative nuclear charge (Ext) | equals the proton number | |
| Electrons (neutral atom) | electrons | protons and electrons balance |
Particle facts (mark-scheme form):
| Particle | Location | Relative charge | Relative mass |
|---|---|---|---|
| Proton | nucleus | ||
| Neutron | nucleus | ||
| Electron | around the nucleus | negligible |
(Core) Define the proton number (atomic number), .
(Core) A carbon nucleus is written .
Calculate the number of neutrons in this nucleus. Show your working. (2 marks)
