Niels Bohr is known both as one of the 20th. The most outstanding theoretical physicists of the century and its philosophical interpretation of quantum mechanics, the so-called Copenhagen interpretation. The lecture will focus on Bohr as a physicist, and especially on his pioneering atomic physics theory from 1913 and its development to quantum mechanics in the 1920s. This theory, which Bohr in 1922 was awarded the Nobel Prize for, has long since been abandoned, but it was an essential prerequisite for modern quantum mechanics, as developed 1925-26 by Heisenberg, Schrödinger, and the "old" quantum theory of the atom is a beautiful example of a Physical theory, and it contains many aspects that are not universally known, including an ambitious attempt to understand the chemistry on a nuclear physical basis.