Atomic Models & the Nucleus

What is inside an atom? It took a century of clever experiments, ending with alpha particles bouncing back off a sheet of gold, to find out that an atom is almost entirely empty space around a tiny, heavy nucleus.

From atomism to electrons#

The idea of atoms is ancient. With Robert Boyle (1627–1691), who studied how air compresses and expands, it moved from reasoning to experiment: compressible air must be tiny particles with empty space between them. An atom is the smallest unit of an element that carries all its properties. Discharge tubes and Thomson’s e/m measurement showed atoms contain negative electrons, so, since matter is neutral, there must be positive charge too. The question was how it is arranged.

Three models#

Thomson (1898)

A solid sphere of positive charge with electrons embedded in it, like cherries in a cake. It explained why heating a metal ejects only negative particles, and how ions and ionic compounds form.

Lenard (1903)

Cathode rays pass through thin foils almost undeviated, so atoms can’t be solid: they must be mostly empty. Lenard proposed atoms made of electrons and similar tiny positive particles. But then why would heating never eject the positive ones?

Rutherford (1906–1911)

Rutherford fired alpha particles at a thin gold foil. Most went straight through or were barely deflected, as expected for a heavy alpha brushing past tiny particles. But about one in 8000 was turned back by more than 90°.

• An alpha particle is about 7350 times heavier than an electron, so neither electrons nor electron-sized positive particles could throw it back. It must have hit something with a mass comparable to the whole atom.
• Since most alphas pass undisturbed, that heavy object must be tiny.

The scattering pattern showed this nucleus is positive and holds nearly all the atom’s mass, with a size of about 10 fm (1 fm = 10⁻¹⁵ m): some 10⁻⁵ of the atom’s diameter and 10⁻¹⁵ of its volume.

Rutherford’s nuclear atom: a tiny, massive, positive nucleus at the centre, with electrons moving round it at a distance; the space between is empty and sets the atom’s size; the nuclear charge equals the electrons’ total charge. It explains neutrality, the empty space, and why only electrons are easily ejected (the heavy nucleus barely notices heating). The electrons must move: at rest they would fall into the nucleus, but in circular orbits the Coulomb force just bends their path. That motion, as the next lessons show, brought its own trouble.