Atoms and isotopes

An atom has a tiny, dense, positively-charged nucleus (containing protons and neutrons) surrounded by electrons arranged in shells (energy levels) around it. The radius of an atom is about 1 × 10⁻¹⁰ m, and the radius of the nucleus is about 10,000 times smaller than the radius of the atom — the atom is almost entirely empty space.

Electrons occupy shells at fixed distances from the nucleus. Electrons can move between shells by absorbing or emitting electromagnetic radiation: absorbing radiation of the right energy moves an electron to a higher shell (further from the nucleus); an electron falling to a lower shell emits radiation.

Isotope notation shows the mass number above the atomic number, to the left of the element symbol — for example carbon-14 is written ¹⁴₆C: mass number 14, atomic number 6, so it has 6 protons, 6 electrons, and 14 − 6 = 8 neutrons.

An atom becomes an ion if it loses or gains electrons, so the number of electrons no longer equals the number of protons — this leaves the atom with an overall electric charge (positive if it loses electrons, negative if it gains them). Ionisation can happen when a substance absorbs electromagnetic radiation with enough energy to remove an electron completely.

Before the electron was discovered, atoms were thought to be tiny, solid, indivisible spheres (Dalton's model, around 1800).

1. 11897 — J. J. Thomson discovered the electron. This showed atoms were not solid indivisible spheres, and led to the Plum Pudding Model: a ball of positive charge ('pudding') with negative electrons embedded throughout it, the negative and positive charges balancing to give an overall neutral atom.
2. 21909–1911 — Rutherford, Geiger and Marsden's gold foil experiment. Alpha particles were fired at a thin sheet of gold foil. Most passed straight through undeflected, showing the atom is mostly empty space. A small number were deflected through large angles, and a very few bounced almost straight back — showing that most of the atom's mass and all of its positive charge are concentrated in a tiny, dense region: the nucleus. This replaced the Plum Pudding Model with the nuclear model.
3. 31913 — Niels Bohr proposed that electrons orbit the nucleus at fixed distances, in specific energy levels/shells. Theoretical calculations showed that if electrons weren't restricted to these fixed shells, they would quickly spiral into the nucleus and the atom would collapse — Bohr's model fixed this and matched experimental observations.
4. 4Later experimental work showed the positive charge of the nucleus could be subdivided into smaller particles, each with the same amount of positive charge — protons.
5. 5James Chadwick provided the evidence, about 20 years after the nucleus was discovered, for the existence of neutrons within the nucleus — the missing mass in the nucleus that carried no charge.