Today is the birthday of Niels Bohr, a Danish physicist born in 1885, who elucidated the structure of the atom and explained the process of nuclear fission. Working with Ernest Rutherford to investigate the structure of the atom, Bohr developed a model of the atom in which electrons existed in shells around the nucleus. He then added Max Planck's quantum theory--that radiation is emitted or absorbed by atoms in discrete units or quanta of energy--to the picture.
In 1913, Bohr proposed that an atom can only exist in a certain number of stable states, each with a particular amount of energy, and that an atom will absorb or emit energy when an electron moves from one shell--or energy state--to another. By reconciling Rutherford's and Planck's views, Bohr produced a new and insightful model of the atom. Bohr's vision of the atomic nucleus, as particles weakly held together, led to his explanation of why some elements are prone to break apart by fission. Bohr received the Nobel Prize in physics in 1922.