Atom = tiny nucleus + electron shells
Nucleus (tiny, dense, central; positive charge, almost all the mass): protons (charge , relative mass 1) and neutrons (charge 0, relative mass 1). Diameter m, about of the atom. Electron shells hold electrons (charge , relative mass of a proton). A neutral atom has electrons = protons, so net charge is zero and the atom is mostly empty space.
Z, A and neutrons: N = A − Z
Atomic (proton) number = number of protons; it defines the element. Mass (nucleon) number = protons + neutrons. Neutrons . Nuclide notation ( top-left, bottom-left). Example: → 6 protons, neutrons. Isotopes = same , different (e.g. and , both carbon); they have identical chemical properties (same electron arrangement).
Plum-pudding → Rutherford nuclear model
Plum-pudding (Thomson): a sphere of positive charge with electrons embedded; no central nucleus. Rutherford scattering: alpha particles fired at thin gold foil gave three observations — (1) most passed straight through → atom is mostly empty space; (2) some deflected by large angles → a concentrated positive charge; (3) a very few bounced almost straight back → almost all the mass sits in a tiny, dense, positive nucleus. This nuclear model replaced plum-pudding.
Drawn from real examiner reports.
Mass number ≠ neutron count
Mass number counts nucleons (protons + neutrons), not neutrons alone. Use . Trap: for the neutron count is , not 14. Reading the top number off as the neutron count scores zero on "how many neutrons?" The same slip is why some know an alpha particle has 4 nucleons but cannot split it into 2 protons + 2 neutrons.
November 2024 Paper 2P Q1 — many candidates recalled that an alpha particle consists of 4 nucleons but could not state the correct composition (2 protons + 2 neutrons), confusing the total nucleon count with the proton/neutron split.
Say proton/neutron, not "particle"
Nuclear questions demand precise names. Writing "particle", "molecule" or "ion" instead of proton, neutron, electron or nucleon loses marks even when the idea is right. State the exact sub-particle and where it sits: protons and neutrons in the nucleus, electrons in shells. Precision of vocabulary is mandatory across this whole topic.
June 2024 Paper 2PR Q2, Q3(a) and Q8(a)(i) — candidates who used vague terms ("particle", "molecule", "ion") instead of precise nuclear vocabulary ("proton", "neutron", "nucleon") lost marks.
Isotopes: you must name the neutrons
"Same element, different mass" scores only the first mark. The mark scheme wants two elements: (1) same number of protons (same ); (2) different number of neutrons (different mass number ). "Different mass" alone gives no nuclear reason. Remember isotopes share chemical properties (same electron arrangement) but differ in physical mass.
The nucleus is positive, not negative
The nucleus contains protons (and neutrons), so its charge is positive — it repels the positive alpha particles in scattering. Electrons (negative) orbit in shells; they are NOT inside the nucleus. Calling the nucleus negative, or drawing electrons embedded in it, merges the two models into one wrong picture and loses marks.
Nuclide notation: A on top, Z below
In the mass number goes top-left and the atomic number bottom-left. Swapping them (writing on top) is penalised. Quick check: the larger number is almost always and sits on top; the smaller is underneath. Example: sodium-23 is , not .
Plum-pudding vs Rutherford model
When asked what scattering showed, describe the Rutherford nuclear conclusions, not plum-pudding predictions, and link each observation to its conclusion. Plum-pudding spreads positive charge out and predicts only small deflections; it cannot explain large-angle or back-scattering. A conclusion with no matching observation scores no marks.
Scattering: observation then conclusion
Use Observation → Conclusion: most pass straight through → mostly empty space; some deflected → a concentrated positive charge; a few bounce almost straight back () → a tiny, dense, positive nucleus. State the observation first, then the conclusion it supports.
Match the mark count
A 3-mark scattering question needs all three observations plus three matching conclusions. A 2-mark question usually wants one observation + one conclusion, or two observations. Count the marks available and supply exactly that many linked points — no more, no fewer.
Read counts from the notation
From : protons ; neutrons ; electrons for a neutral atom (but not for an ion). Worked: → 11 protons, neutrons, 11 electrons.
Use the examiner's key words
Scattering answers should include "tiny", "dense", "positively charged", "nucleus" and "mostly empty space". Each missing key word can cost the mark for that point, even when the idea is broadly right. Learn the exact adjectives and use them.
There are no calculation formulas in this topic, but the following relationships and notation are essential:
where = mass (nucleon) number, = atomic (proton) number, = number of neutrons.
Nuclide notation: — mass number top-left, atomic number bottom-left, symbol centre.
Example: (sodium-23) has 11 protons, neutrons, and 11 electrons (neutral atom).
Define atomic (proton) number, .
An atom of magnesium has the notation .
(a) State the number of protons in this atom. (1 mark)
(b) Calculate the number of neutrons in this atom. (1 mark)
(c) State the number of electrons in a neutral atom of this element. (1 mark)
