Atoms, isotopes and atomic evidence

Atomic number Z counts protons and fixes element identity. Mass number A counts protons plus neutrons, so neutron number is A−Z. A neutral atom has Z electrons; a positive ion has lost electrons and a negative ion has gained them. Isotopes have the same proton number but different neutron numbers. Their chemical behaviour is similar because their electron arrangements are similar.

Large-angle deflections in alpha-particle scattering support a small, dense, positively charged nucleus. Atomic emission and absorption lines support discrete electron energy levels: a photon energy equals the difference between the involved levels. Different elements have characteristic line patterns, allowing identification from supplied spectra.

Watch out: Element identity is fixed by proton number, not neutron number or mass number.