i mean when rutherford did his experiments years and years ago he produced his planetary model of the nucleus, of the atom where the nucleus plays the role of the sun and the electrons play the role of the planets, and people said well why don't they just spiral in an erm rutherford had actually no answer to this, but the answer to this was produced by the danish physicist nils bore, who said ‘well they don't spiral in because erm electrons cannot just take up any orbit, they can take up certain specified orbits which he called stationary states, and there is a lowest one of these, and when the electron gets down there it cannot go any further.