Atomic structure — the nucleus, shells and the two numbers
The nucleus at the centre holds protons and neutrons, together called nucleons; electrons occupy shells around it. Proton (atomic) number = protons in the nucleus. It identifies the element, and in a neutral atom equals the electron count. Mass (nucleon) number = protons + neutrons; as electrons are negligibly light it is essentially the atom's mass relative to 1/12 the mass of a carbon-12 atom. So neutrons = mass number - proton number.
Relative charge and relative mass of the three particles
Relative charges: proton +1, neutron 0, electron -1. Relative masses: proton 1, neutron 1, electron very small (about 1/2000 of a proton, usually treated as negligible). Because protons and neutrons carry effectively all the mass, almost all of an atom's mass is concentrated in the nucleus, even though the nucleus occupies only a tiny fraction of the atom's volume. Protons and electrons carry equal and opposite charge, so a neutral atom always has equal numbers of the two.
Electronic configuration and its Periodic Table links
For proton numbers 1-20, electrons fill from the nucleus outwards: shell 1 up to 2, shell 2 up to 8, shell 3 up to 8, then a fourth shell begins. Write one number per occupied shell, innermost first (aluminium 2,8,3; potassium 2,8,8,1); the digits must sum to the proton number. This shapes the Periodic Table: Group VIII atoms have a full outer shell, hence their unreactivity; for Groups I-VII the outer-shell count equals the group number; occupied shells equal the period number.
Drawn from real examiner reports.
Electrons surround the nucleus
Protons and neutrons sit in the nucleus at the centre of the atom; electrons occupy shells around it. Candidates write that electrons are "in the nucleus with the protons", or reverse the whole picture so that the nucleus orbits the electrons. In a describe-the-structure answer, name both parts and state their positions explicitly.
Flagged Nov 2023 P11 Q16, P32 Q2bii
Neutron: charge 0, but mass 1
A neutron is uncharged, so its relative charge is 0 — but its relative mass is 1, exactly the same as a proton. Candidates who correctly recall the zero charge then carry that 0 across into the mass column of the table. Only the electron has a relative mass small enough to call negligible; a neutron's is a full 1.
Flagged Nov 2022 P42 Q2b
2,8,1 is a list, not one number
An electronic configuration gives the electrons in each successive shell, read from the nucleus outwards. Candidates misread 2,8,1 as a single value — treating "281" as a proton number, or quoting only the final digit as the total electron count. Add the digits instead: 2+8+1 = 11, matching the proton number of sodium.
Flagged Jun 2022 P31 Q11ai
Nucleon number, not "nucleus number"
The total of protons and neutrons in the nucleus is the nucleon number (or mass number). Writing "nucleus number" loses the definition mark even when the numerical idea behind it is right. Use the exact mark-scheme terms every time: proton (atomic) number and mass (nucleon) number, never an informal paraphrase.
Flagged Jun 2022 P12 Q15
Group number = outer-shell electrons only
For sulfur, 2,8,6, the group is VI — read from the 6 outer-shell electrons, not from 16, the total electron count. The mirror error reads the period from the last digit instead: the period number is how many shells are occupied at all (3 here), not how many electrons sit in the outermost one.
Neutrons: subtract, never add
Neutrons = mass (nucleon) number - proton (atomic) number. For a chlorine atom with mass number 35 and proton number 17, that is 35 - 17 = 18 neutrons. Candidates add the two numbers instead, giving 52, or subtract the wrong way round. Only the mass number counts neutrons at all, so the proton number must always come off it.
Shell 3 stops at 8 — start a fourth
For proton number 20 the third shell fills to 8 and the last two electrons begin a fourth shell, giving 2,8,8,2. Candidates force all 20 into three shells (writing 2,8,10, which breaks the shell-3 maximum) or misjudge an earlier shell (2,7,4). Either way the digits then fail to sum to the proton number.
Sum-check every configuration
Fill shell 1 with 2, shell 2 with up to 8, shell 3 with up to 8, then start shell 4 if electrons remain. Now add every digit: the total must equal the proton number. Phosphorus, proton number 15, gives 2,8,5 and 2+8+5 = 15, so the configuration checks out.
Read group and period off the configuration
The last digit is the group number for Groups I-VII; a full outer shell (2 if there is only one shell, otherwise 8) means Group VIII. The number of terms in the configuration is the period number. So 2,8,3 is Group III, Period 3, and 2,8,8 is Group VIII, Period 3.
Match the command word
"Determine" or "deduce" a configuration means give every shell number in order, not just the outer-shell count. "State" the proton or mass number means one value with the correct term. Explain questions on group similarity want the shared outer-shell number, not a group trend.
Drawing electron-shell diagrams
Draw one small central nucleus, labelled with its protons and neutrons, then evenly spaced concentric shells around it. Fill each shell to its maximum before starting the next, mark the electrons clearly on each shell, and check the electron total equals the proton number.
Cambridge 0654 spec reference: Section C2 "Atoms, elements and compounds", sub-topic C2.2 (Core only -- this leaf has no Extended/Supplement content). It covers atomic structure, the relative charges and masses of subatomic particles, proton (atomic) number and mass (nucleon) number, electronic configuration for proton numbers 1-20, and how electronic configuration links to a group and period in the Periodic Table.
| Term | Mark-scheme-precise meaning |
|---|---|
| Atom | The smallest particle of an element that can exist; has a nucleus surrounded by electrons in shells |
| Nucleus | The small, dense central part of an atom, containing protons and neutrons |
| Proton (atomic) number | The number of protons in the nucleus of an atom |
| Mass (nucleon) number | The total number of protons and neutrons in the nucleus of an atom |
| Electronic configuration | The arrangement of electrons in shells around the nucleus, written as numbers per shell (e.g. 2,8,3) |
Relative charges and masses:
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | ||
| Neutron | ||
| Electron | very small (about of a proton; often treated as negligible) |
Key relationship:
In a neutral atom, the number of electrons equals the number of protons (equal and opposite charge, so the atom overall has zero charge).
Electronic configuration rules (proton numbers 1-20): electrons fill the shell nearest the nucleus first. Shell 1 holds up to 2 electrons; shell 2 holds up to 8; shell 3 holds up to 8 (for these proton numbers); a 4th shell then begins (proton numbers 19-20). The digits of a correct configuration always sum to the proton number.
Periodic Table links:
Define proton (atomic) number.
An atom of aluminium has a proton (atomic) number of 13. Determine its electronic configuration, and show that the electron total agrees with the proton number. (2 marks)
