Niels Bohr explained the structure of the atom in 1913

Person: Niels Bohr
Lived: 1885–1962
Field: Physics
From: Danish

The story

At the start of the twentieth century physicists knew that atoms contained electrons, and in 1911 Ernest Rutherford showed that they also had a tiny, heavy nucleus at the centre. But classical physics predicted that electrons circling a nucleus should spiral into it almost instantly. There was another puzzle too: since 1885 Johann Balmer's formula had described the colours of light given off by hydrogen, yet for nearly thirty years nobody could explain why it worked.

Niels Bohr, a young Danish physicist, travelled to England in 1911 on a Carlsberg Foundation fellowship, first to Cambridge and then to Rutherford's laboratory in Manchester. Back in Copenhagen, he combined Rutherford's nucleus with Max Planck's quantum theory. In 1913 he published three papers in the Philosophical Magazine, in July, September and November, that became known as the trilogy.

Bohr proposed that electrons can only occupy certain fixed orbits, or energy levels, where they are stable. When an electron drops from a higher level to a lower one, it gives off a single quantum of light with a precise energy, and so a precise colour. From this idea he derived Balmer's formula for hydrogen. When the astronomer Alfred Fowler challenged him over some spectral lines that did not fit, Bohr showed that they came from helium atoms that had lost one electron, and the model worked for those too.

Older physicists such as J. J. Thomson and Lord Rayleigh disliked the theory, but younger scientists including Rutherford, Albert Einstein, Enrico Fermi, Max Born and Arnold Sommerfeld hailed it as a breakthrough. Einstein called it the highest form of musicality in the sphere of thought. It won acceptance because it explained results other models could not, and predicted new ones that experiments confirmed.

Bohr's model was later replaced by fuller quantum mechanics, much of it developed by physicists who worked with him in Copenhagen. Its central ideas, discrete energy levels and quantum jumps, remain valid, and it is still the picture of the atom that most students meet first. When its 50th anniversary came in 1963, Denmark issued a stamp showing Bohr, the hydrogen atom and the formula linking light to the jump between two energy levels.