Atomic models: the story of the atom

The first idea: "atomos"

Two Greek philosophers argued that if you kept cutting a piece of matter in half, you would eventually reach a piece so small it could not be cut any further. They called it átomos — "uncuttable". Everything, they said, was made of these tiny, solid, indestructible particles moving through empty space.

It was pure philosophy — there was no experiment behind it — but the core idea, that matter is not endlessly divisible, turned out to be right and waited 2,200 years for proof.

Dalton's solid sphere

Dalton turned the old philosophy into testable chemistry. From the way elements combine in fixed, whole-number ratios by mass, he argued that each element is made of identical tiny atoms — pictured as solid, indivisible spheres, like microscopic billiard balls.

• All matter is made of tiny indivisible particles called atoms.
• All atoms of a given element are identical in mass and properties.
• Compounds form when atoms of different elements join in fixed whole-number ratios.
• A chemical reaction only rearranges atoms — it never creates or destroys them.

Thomson's plum pudding

Studying cathode rays, Thomson found tiny negatively charged particles — electrons — thousands of times lighter than an atom. So the atom was not indivisible after all. Since atoms are neutral, he pictured these electrons scattered like plums (or seeds) through a sphere of spread-out positive charge — the "plum pudding" model.

• The atom contains small, negative electrons — the first subatomic particle found.
• The rest of the atom is a diffuse ball of positive charge that balances them.
• Overall the atom is electrically neutral.

Rutherford's nuclear atom

Rutherford (with Geiger and Marsden) fired positively charged alpha particles at a sheet of gold foil only a few atoms thick. Most sailed straight through, but a tiny fraction were deflected sharply — and a very few bounced almost straight back. Rutherford said it was "as if you fired a shell at tissue paper and it came back to hit you."

• An atom is mostly empty space — which is why most particles passed through.
• Its positive charge and nearly all its mass sit in a tiny, dense central nucleus.
• The light electrons move in the space around the nucleus.

Bohr's energy shells

Bohr rescued Rutherford's atom with a bold rule borrowed from the new quantum idea: electrons may only travel in certain fixed orbits — energy shells — and while an electron stays in its shell it does not radiate energy, so the atom never collapses. An electron only gains or loses energy when it jumps between shells, absorbing or emitting a packet of light of an exact energy, E = hν.

• Electrons orbit only in allowed shells of fixed energy (n = 1, 2, 3 …).
• No energy is radiated while an electron stays in a shell — so atoms are stable.
• Jumping down a shell emits a photon; jumping up absorbs one — this explains the sharp line spectrum of hydrogen exactly.

The quantum (electron-cloud) model

The modern model treats the electron as a wave as much as a particle. We can no longer say exactly where an electron is and where it is going at the same time (Heisenberg's uncertainty principle). Instead, Schrödinger's equation gives the probability of finding an electron in a region of space. Those regions are the orbitals — the fuzzy "electron cloud" — with the familiar s, p, d and f shapes. Chadwick then completed the nucleus by discovering the neutron in 1932.

• Electrons behave as waves; they occupy orbitals — regions of high probability, not fixed orbits.
• Each electron is described by four quantum numbers (shell, sub-shell shape, orientation, spin).
• The nucleus holds protons and neutrons (Chadwick, 1932); electrons fill the orbitals around it.