How Do We Know What Atoms Look Like?

Atoms make up everything.

All topics in chemistry come from the components of atoms and how they interact with each other.

There is nothing smaller than an atom, the fundamental building block of everything, or at least, that’s what was believed.

The first mention of atoms was over two millennia ago (~440 BCE).

 A Greek philosopher theorised that breaking matter down repeatedly would eventually lead to some fundamental, indivisible building block whose shape and size determined the material's behaviour.
The word "atom" itself comes from the Greek word meaning "indivisible".

The next milestone in atomic theory was John Dalton's Billiard Ball Model (1803).
He proposed that atoms were indivisible spheres, like billiard balls, which were identical for a given element, and that compounds were formed from combinations of these atoms.

Experiments with cathode rays, for which J. J. Thomson won a Nobel Prize, confirmed the existence of negatively charged particles that could be separated from the atom.
This led to the Plum Pudding Model (1904), consisting of negatively charged electrons embedded within a sphere of positive charge.

The famous gold foil experiment, conducted by Ernest Rutherford (1909), involved firing alpha particles at an extremely thin sheet of gold foil. Most particles passed straight through with little deviation, showing that atoms were mostly empty space. However, a small number were deflected, sometimes at large angles. This led to a new Nuclear Model (1911), where the atom comprised of electrons surrounding a small, dense, positively charged nucleus.

Niels Bohr later refined this model by limiting electrons to orbits of fixed size and energy in a new Planetary Model (1913) to explain the hydrogen emission spectrum. Hydrogen gas was heated or electrified and produced not a continuous spectrum of light, but a line spectrum where only certain colours appeared. This suggested that electrons can only exist at specific, defined energy levels.

Electron diffraction experiments showed that electrons behave both as waves and particles. This led to Schrödinger's Quantum Model (1926), in which electrons are described by wavefunctions rather than fixed paths. As a result, electrons are said to occupy orbitals, regions of space where there is a high probability of finding an electron, rather than travelling in defined orbits around the nucleus.