Inside an atom

If an atom were the size of a football stadium, its nucleus would be about the size of a pea in the middle.

Cut a piece of aluminium foil in half, then in half again, and keep going. Long before you run out of foil you would reach a piece so small that cutting it once more would no longer give you aluminium. That smallest piece is an atom. A single atom is about a ten-billionth of a metre across, far too small to see with any light microscope, yet every atom is built from just three kinds of particle.

Three particles

Protons and neutrons are packed into a tiny nucleus at the centre, which holds almost all of the atom’s mass. Electrons are spread out around it, and they take up almost all of the atom’s space. A nucleus is about 10,000 to 100,000 times smaller across than the whole atom. NIST gives the proton as 1836.15 times heavier than the electron, so even an atom with many electrons gets well under 1% of its mass from them.

Opposite charges attract, so the negative electrons are held near the positive nucleus. Protons push each other apart, though, and a nucleus with several of them would fly to pieces if electric force were the only force at work. A much stronger force, called the strong nuclear force, holds protons and neutrons together, but only across the tiny distances inside a nucleus.

The atomic number

The number of protons decides which element an atom is. Every atom with 6 protons is carbon, and every atom with 8 is oxygen. That number is the atomic number, and the periodic table is arranged in order of it, from hydrogen with 1 upward. In a neutral atom the number of electrons equals the number of protons, so the charges cancel. If an atom loses or gains electrons it becomes an ion: sodium that has lost one electron has 11 protons and 10 electrons, so it carries a charge of +1.

Mass number and isotopes

Protons plus neutrons give the mass number. Atoms of one element can have different numbers of neutrons. These are isotopes. Carbon-12 has 6 neutrons, and carbon-14, used to date ancient objects, has 8. The number after the name is the mass number, so the neutron count is always the mass number minus the atomic number.

How we found out

Nobody has looked at a proton through a lens. Each particle was worked out from experiments. In 1897 J. J. Thomson at the Cavendish Laboratory in Cambridge showed that cathode rays were made of tiny negative particles much lighter than any atom: electrons. Ernest Rutherford’s team fired fast alpha particles at a thin gold foil, and a few bounced almost straight back. In 1911 Rutherford explained this with a small, heavy, positive nucleus. James Chadwick proved in 1932 that the nucleus also holds particles with no charge, the neutrons, and won the 1935 Nobel Prize in Physics for it.