Sodium atom model, c 1912-1926.
© Science Museum / Science & Society Picture Library
A model of a sodium atom made according to the Bohr theory of the structure of atoms. Danish physicist Niels Bohr (1885-1962) constructed atom models with fixed orbits - the nucleus is shown at the centre with electrons (the hoops of metal) orbitting around it. Bohr's is a convenient and useful model of an atom but not the only view - electrons also behave as if they are waves of energy and are sometimes best considered as being spread around the nucleus in a sort of cloud.