The properties of valence electrons are determined by the total number of electrons in an atom's outer shell. Atoms with similar numbers of valence electrons are classified into a specific group of elements, which may then combine electrons with other elements to form ionic or covalent bonds.
In an atom's outermost shell, valence electrons may be acquired or lost during chemical reactions if the atom is categorized as a main group element. Atoms of transition metals, however, may contain valence electrons within their innermost shells. These electrons are additionally shared and transferred between atoms when bonding, with halogens and metals being the most reactive during the electron exchange.