Chapter summary
Atomic Structure, at a glance
Use particles, orbitals and ionisation data to explain the behaviour of atoms and ions.
A-Level 8873, revised syllabus (2026-2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Proton (atomic) number | The number of protons in the nucleus of an atom. |
| Nucleon (mass) number | The total number of protons and neutrons in the nucleus of an atom. |
| Isotopes | Atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same number of electrons (the same electronic configuration). |
| Orbital | A region of space around the nucleus where there is a high probability of finding an electron. Each orbital holds at most two electrons, with opposite spins. |
| First ionisation energy | The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous singly charged positive ions: M(g) → M+(g) + e-. |
| Question | First move |
|---|---|
| How many neutrons/electrons? | Neutrons = A - Z; electrons = Z - signed charge. |
| Which way does a beam bend? | Positive to negative plate; negative to positive; neutral straight. |
| Which orbital fills? | 1s 2s 2p 3s 3p 4s 3d 4p; two per orbital, singly before pairing. |
| Which electron leaves an ion? | Remove outer-shell electrons; 4s before 3d for transition cations. |
| Why a different ionisation energy? | Compare charge, shielding, distance, subshell and pairing. |
| What does the large jump reveal? | Count electrons removed before reaching the inner shell. |
Quick recall
How are beams of protons, neutrons and electrons deflected in an electric field?