The model of the atom

• Early on: atoms are tiny spheres that cannot be divided.
• The electron is discovered, so the plum pudding model: a ball of positive charge with electrons scattered through it.
• Alpha scattering: positive alpha particles fired at thin gold foil. Most go straight through, a few are deflected, a very few bounce back. So most of the atom is empty space, with the mass and positive charge in a tiny nucleus.
• Bohr: electrons orbit the nucleus at fixed distances (shells).
• Later: the nucleus contains protons, and then neutrons (Chadwick).

An atom has equal numbers of protons and electrons, so it has no overall charge. Atoms are tiny: a radius of about 0.1 nm (1 × 10⁻¹⁰ m). The nucleus is about 1/10 000 of that radius, yet holds almost all the mass.

Open the periodic table and pick an element. Its atomic number is its number of protons, and so its number of electrons.