GCSE Physics Atoms & isotopes revision

An atom is about 1 × 10⁻¹⁰ m across — but almost all of that is empty space around a nucleus less than 1/10 000 of the atom’s radius. If an atom were a cathedral, the nucleus would be a fly on the centre spot, carrying nearly all the mass. The atomic number (Z) counts protons and names the element; the mass number (A) counts protons plus neutrons. Nuclide notation writes them as ᴬ_Z X — so uranium-238 is ²³⁸₉₂U, with 92 protons and 238 − 92 = 146 neutrons.

1. 1Dalton (early 1800s)

   Atoms are tiny solid spheres that cannot be divided — good enough to explain chemical combining by mass.

2. 2Thomson (1897)

   Discovers the electron; the atom must have internal structure. His plum pudding model: negative electrons dotted through a sphere of positive charge.

3. 3Rutherford, Geiger and Marsden (1909–1911)

   Fire alpha particles at thin gold foil. Most pass straight through, some are deflected through large angles, and roughly 1 in 8 000 bounces back. Conclusion: the positive charge and mass are concentrated in a tiny nucleus, and the atom is mostly empty space — the plum pudding is dead.

4. 4Bohr (1913)

   Electrons occupy fixed shells with set energies; light is emitted or absorbed at specific frequencies as electrons jump between shells — explaining line spectra.

5. 5Chadwick (1932)

   Proves the neutron — explaining why atoms are heavier than their proton count alone suggests, and completing the modern picture.

The scattering experiment deserves its own attention because it shows how evidence replaces a model. If the plum pudding were right, alpha particles (positive, fast) would be deflected only slightly by the smeared-out positive charge. The observed large-angle deflections were, in Rutherford’s words, as surprising as a shell bouncing off tissue paper. The model had to change — a nuclear atom with a tiny, dense, positive centre.

Isotopes of an element have the same number of protons but different numbers of neutrons. Hydrogen’s three isotopes make the idea concrete: protium (¹₁H, no neutron), deuterium (²₁H, one neutron) and tritium (³₁H, two neutrons). Because chemistry is governed by electrons, isotopes react identically — but nuclear stability depends on the neutron count, which is why some isotopes (carbon-14, cobalt-60, uranium-235) are radioactive while their siblings are not. An ion forms when an atom gains electrons (negative ion) or loses electrons (positive ion); the proton count — and therefore the element — never changes in ordinary chemistry.

Nuclear fission: a large, unstable nucleus — usually uranium-235 or plutonium-239 — absorbs a neutron, becomes briefly even more unstable, and splits into two smaller nuclei plus two or three further neutrons and a large burst of energy. Those released neutrons can hit other uranium-235 nuclei and split them too — a chain reaction, which reactors control (with absorber rods) to release energy steadily and power stations to generate electricity. Nuclear fusion: two light nuclei (in the Sun, hydrogen isotopes) are forced close enough to join into a heavier nucleus, releasing enormous energy. Fusion powers stars, because at their cores the temperature and pressure overcome the electrical repulsion between positively charged nuclei. Fusion releases far more energy per kilogram than fission and leaves less problematic waste — but sustained, controlled fusion on Earth remains an engineering frontier.