By 1910 a picture of an atom had emerged as a tiny nucleus containing most of the mass of the atom and bearing a positive charge, surrounded by a swarm of electrons with enough negative charge to make the atom electrically neutral. The only problem was: What were these electrons doing? If they were motionless, then electrostatic attraction between the positive nucleus and the negative electrons should instantly cause the atom to collapse. This obviously was wrong.

The electrons could be moving in circular orbits with just enough centrifugal force to balance the pull of the nucleus, like planets around the sun (right). This was the basis of an atomic theory proposed by the Danish physicist Niels Bohr in 1913.