At the centre of every atom is a nucleus, and the electrons are found in the space around it. The nucleus holds almost all of the mass of the atom but takes up only a tiny part of its volume.
An atom is about 10^{-10} m across, while its nucleus is only about 10^{-15} to 10^{-14} m across. The atom is therefore 10,000 to 100,000 times wider than its nucleus.
Most of an atom is empty space, and the electrons move through this space around the nucleus.
The nucleus is made of protons and neutrons. A proton carries a positive charge, while a neutron carries no charge at all.
An electron carries a negative charge equal in size to the charge of a proton. Its mass is only about 1/1840 of the mass of a proton, so almost all of the mass of an atom is in its nucleus.
A proton and a neutron have almost the same mass. An atom contains as many electrons as protons, so the charges cancel and the atom as a whole is neutral.
The number of protons in the nucleus is called the atomic number. It decides which element an atom belongs to: every atom with 11 protons is an atom of sodium.
The total number of protons and neutrons is called the mass number. Subtracting the atomic number from the mass number gives the number of neutrons.
The mass number is written at the upper left of the element symbol and the atomic number at the lower left. A sodium atom with mass number 23 has 11 protons, 23 - 11 = 12 neutrons and 11 electrons.
In 1803 John Dalton proposed that all matter is made of tiny atoms that cannot be divided. In 1897 J. J. Thomson discovered the electron, which showed that atoms contain even smaller particles.
In 1904 Thomson pictured the atom as a ball of positive charge with electrons scattered through it. In 1911 Ernest Rutherford showed instead that the positive charge is concentrated in a tiny nucleus at the centre.
In 1913 Niels Bohr proposed that electrons can move around the nucleus only in certain orbits, each with a fixed energy. This idea grew into the electron shells used to describe atoms today.
Under Rutherford's direction, Hans Geiger and Ernest Marsden fired positively charged alpha particles at a very thin sheet of gold foil.
Almost all of the alpha particles passed straight through the foil, but a very small number were deflected through large angles, and a few even bounced back.
Rutherford concluded that the positive charge and nearly all of the mass of an atom are packed into a tiny nucleus. Most alpha particles never come near a nucleus, and only those that pass very close to one are pushed strongly away.