ἄτομος

Átomos in Greek means 'uncuttable': a- (not) + tomós (cut, from témnein, to cut). Leucippus and his student Democritus, working in the fifth century BCE, proposed that all matter is composed of indivisible particles — atoms — separated by void. They had no experimental evidence. It was a philosophical argument: if you keep cutting a piece of matter in half, you must eventually reach a particle that cannot be cut further. Otherwise, matter could be divided into nothing, which seemed absurd.

The atomic theory was controversial in ancient Greece. Aristotle rejected it — he believed matter was continuous and infinitely divisible. Since Aristotle's authority dominated European thought for nearly two thousand years, atomism remained a minority position. Epicurus adopted and modified it. Lucretius wrote De Rerum Natura (On the Nature of Things) around 50 BCE, preserving Democritean atomism in Latin verse. The poem was lost for centuries and rediscovered in a German monastery in 1417.

John Dalton revived atomic theory in 1803 with experimental backing: he showed that chemical elements combine in fixed, whole-number ratios, which made sense only if matter came in discrete units. He called these units atoms, using the Greek word. The name was a philosophical commitment: atoms were the uncuttable foundation of chemistry. Dalton's atoms had no internal structure.

In 1897, J.J. Thomson discovered the electron — a particle inside the atom. In 1911, Ernest Rutherford discovered the nucleus. In 1932, James Chadwick discovered the neutron. By the mid-twentieth century, protons and neutrons had been split into quarks. The uncuttable had been cut, and cut again, and cut again. The word atom is now one of the most famous misnomers in science. The thing that cannot be cut has been divided into dozens of subatomic particles.