Flexi Says: An ion can be stable only when all atoms have a stable electronic configuration. Oxygen is most stable when having eight valence electrons. Hydroxide (OH-) is a diatomic anion. It consists of an oxygen and hydrogen atom held together by a single covalent bond. Since hydrogen cannot give it another electron through a covalent bond, it must obtain one more through an ionic bond, giving the hydroxide ion a -1 charge due to the extra electron.