Atoms

Take anything, a spoon, a glass of water, your hand, and cut it in half, then in half again, and keep going. About 30 cuts later you would reach something that could not be divided without changing what it is. The ancient Greeks guessed at this and called it the atom, from the word for uncuttable. They were right that atoms exist, and wrong that they cannot be cut. Inside an atom is a structure stranger than anyone imagined, and almost all of it is empty.

The parts

An atom has a centre, the nucleus, made of two kinds of particle: protons, which carry a positive electric charge, and neutrons, which carry none. Around the nucleus move electrons, far lighter, each carrying a negative charge. In a neutral atom the number of electrons equals the number of protons, so the charges cancel.

The sizes are hard to grasp. An atom is about a ten-millionth of a millimetre across; a sheet of paper is a million atoms thick, and a single drop of water contains more atoms than there are stars in the observable universe. Yet the nucleus is a hundred thousand times smaller than the atom as a whole. If an atom were blown up to the size of a sports stadium, the nucleus would be a pea on the centre spot and the electrons would be specks of dust somewhere in the stands. Everything in between is nothing. The solid feel of a table comes not from matter filling space but from the electric repulsion between the electron clouds of its atoms and those of your hand.

What makes one element different from another

The number of protons decides what an atom is. One proton is hydrogen; two, helium; six, carbon; eight, oxygen; 26, iron; 79, gold; 92, uranium. There are 94 naturally occurring elements and about two dozen more made artificially, each simply the next count up. Change the number of protons and you change the element, which is what happens in stars and nuclear reactors and nowhere else on Earth; chemistry, by contrast, only rearranges which atoms are bonded to which.

Atoms of the same element can have different numbers of neutrons. These variants are isotopes. Most carbon has six neutrons, but a little has eight, and that carbon-14 is slightly unstable, decaying at a steady rate that lets archaeologists date anything that was once alive.

Electrons and chemistry

The electrons do the chemistry. They arrange themselves around the nucleus in shells, and atoms are most stable when their outermost shell is full. Atoms with a nearly full shell grab electrons from others; atoms with one or two spare electrons give them away; most share. Sodium, with one spare electron, hands it to chlorine, which has room for one, and the two cling together as salt. Carbon, with four to share, can bond in chains and rings almost without limit, which is why life is built on it. Every reaction, from rusting to breathing to burning, is electrons moving between atoms.

The familiar picture of electrons orbiting the nucleus like planets is wrong, though. Electrons do not have paths. Quantum mechanics describes each as a cloud of probability, a region where it is likely to be found, with a definite energy but no definite position until it is measured. The shells are not tracks but allowed energies, and when an electron drops from a higher one to a lower one it releases the difference as a flash of light of a specific colour. Those colours are the fingerprint of each element, and they are how astronomers know what distant stars are made of.

Inside the nucleus

Protons all carry positive charge and should fly apart. They do not, because a far stronger force, the strong nuclear force, binds protons and neutrons together at very short range. In large nuclei the balance is delicate, and some, such as uranium, are unstable and occasionally break apart, releasing energy. That release, in a chain reaction, is nuclear fission, the basis of reactors and the first atomic bombs. The reverse, forcing small nuclei together to make larger ones, is fusion, which powers the Sun and every star.

Protons and neutrons are not the end of the story either. Each is made of three smaller particles called quarks, held together by the strong force. Quarks and electrons are, as far as anyone knows, truly fundamental.

How we know

Nobody had seen an atom until the 1980s, when microscopes that feel surfaces with a needle a single atom wide began producing images of them, row on row. Long before that, the evidence was indirect but overwhelming: the fixed ratios in which elements combine, the jittering of dust grains knocked by invisible molecules, the scattering of particles fired at gold foil that revealed the tiny dense nucleus in 1911. Today atoms are routinely imaged, moved one at a time and spelled into words.

Why it matters

Atoms are the alphabet of the material world. Ninety-odd kinds of them, combined in different ways, make every substance that exists. Understanding them gave us chemistry, materials, medicine, electronics and nuclear power. And they are a reminder of how odd reality is beneath the surface: you, and everything around you, are mostly empty space held in shape by electric charge.