The part of an atom that has a negative charge is the electron. Electrons are the subatomic particles that orbit the nucleus of an atom.
There are two types of electrons: core and valence. While core electrons occupy the innermost, lowest energy levels of an atom, valence electrons are found the highest energy level of an atom in the outermost orbits. The latter are responsible for the bonding of adjacent atoms. The number of valence electrons determines the bonds an atom can form and the formal charge of an atom. The main group number of an elemental atom on the periodic table determines how many of valence electrons it has. Atoms with either fewer or more electrons than normal are known as ions.