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.