Philip Ball looks at how a little-known paper by Niels Bohr demonstrates the turmoil in physics on the brink of quantum ...

Thus when an atom absorbs or gives off energy (as in ... of light emitted by real hydrogen atoms. Bohr's theory that electrons existed in set orbits around the nucleus was the key to the periodic ...

Bohr began to work on the problem of the atom's structure. Ernest Rutherford had recently suggested the atom had a miniature, dense nucleus surrounded by a cloud of nearly weightless electrons.