He was the first to suggest that atoms are like miniature solar systems, except that the attractive force is not caused by gravity, but by opposing electrical charges. In the so-called Rutherford atom , electrons orbit the nucleus in circular paths. Niels Bohr revised Rutherford's theory in 1913. In the Bohr atom , the negatively charged electrons orbit the nucleus at specific median distances. These distances are represented by spheres, called shells , surrounding the nucleus.
Electrons can move from shell to shell.
When an electron absorbs enough energy, it moves to a larger, or higher, shell. When it loses a certain amount of energy, it falls to a smaller, or lower, shell. The total mass of an atom, including the protons, neutrons and electrons, is the atomic mass or atomic weight . Electrons contribute only a tiny part of this mass. For most practical purposes, the atomic weight can be thought of as the number of protons plus the number of neutrons.