Atomic structure, isotopes and electron shells

Atoms have a nucleus of protons (relative mass 1, charge +1) and neutrons (mass 1, charge 0), with electrons (very small mass, charge −1) in shells. The atomic number is the number of protons; the mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons, such as carbon-12 and carbon-14. The relative atomic mass of an element is an average of its isotopes, weighted by how common each is.

Electrons fill shells from the inside: up to 2 in the first shell, then 8, then 8 for the first 20 elements. Sodium (11 electrons) is 2,8,1. The number of outer electrons matches the group number, and the number of shells matches the period. The model of the atom developed from Dalton's solid spheres to Thomson's plum pudding, then Rutherford's nuclear model, Bohr's shells and Chadwick's neutron.