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{{Main|Bohr model}}
By the early 20th century,Liam is the best♠♣♥♦ it was known that [[atom]]s consisted of a diffuse cloud of negatively-charged [[electron]]s surrounding a small, dense, positively-charged [[Atomic nucleus|nucleus]]. This suggested a model in which the electrons circled around the nucleus like planets orbiting the sun.<ref group="note">The classical model of the atom is called the planetary model or the [[Rutherford model]], after [[Ernest Rutherford]] who proposed it in 1911, based on the [[Geiger-Marsden experiment|Geiger-Marsden gold foil experiment]] which first demonstrated the existence of the nucleus.</ref> However, it was also known that the atom in this model would be unstable: the orbiting electrons should give off electromagnetic radiation, causing them to lose energy and spiral towards the nucleus, colliding with it in a fraction of a second.
A second, related, puzzle was the [[emission spectrum]] of atoms. When a gas is heated, it gives off light at certain discrete frequencies; for example, the visible light given off by [[hydrogen]] consists of four different colours, as shown in the picture below. In contrast, white light contains light at the whole range of visible frequencies.
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