Electron is a negatively charged subatomic particle. A useful model of an atom portrays it as a tiny nucleus surrounded by electrons. The electrons are at various distances from the nucleus and are arranged in energy levels called shells. Electrons occupy almost the entire volume of an atom, but electrons themselves account for only a small fraction of an atom's mass. The chemical behavior of an atom is determined largely by the number of electrons in its outermost shell. When atoms combine and form molecules, electrons in the outermost shell are either transferred from one atom to another or shared between atoms.