Atomic structure. Artwork depicting the classical model of the structure of the atom. In the centre is the nucleus, made up of protons and neutrons. Orbiting this are several electrons, three in a small inner shell and six in a larger outer shell. This model is known as the Bohr Model, after the Danish physicist Neils Bohr. Modern physics regards this model as too simplistic - the quantum interpretation regards atomic electrons as inhabiting fuzzy clouds determined by probability wave functions rather than discrete particles in regulated shells.