A new model was developed from previous ideas but with changes based on the contributions of several scientists over the years. The nucleus is where most of the mass of an atom is found and contains the protons and neutrons. The number of protons in the nucleus determines what element the atom is. The electrons exist in very definite areas around the nucleus. The movement of electrons are difficult to show on a diagram as they take up an area of space, and don't have a set orbit like the Bohr.