Types of chemical bonds

A covalent bond forms through shared electron pairs. If the atoms have the same electronegativity the bond is nonpolar (Cl₂, O₂, N₂); if they differ it is polar (HCl, H₂O, NH₃), because the shared pair shifts toward the more electronegative atom. When a typical metal meets a typical nonmetal the pair shifts completely: the metal gives electrons and becomes a cation (it is oxidised), the nonmetal takes electrons and becomes an anion (it is reduced), and an ionic bond forms between the ions, e.g. Na⁺ and Cl⁻ in NaCl. In metal crystals the valence electrons form a shared “electron gas”; a hydrogen bond is the attraction between an H atom bonded to F, O or N and such an atom in another molecule (in water, ammonia). Valency is the number of shared electron pairs an atom forms and is never negative; the oxidation state is a formal charge and may be +, – or 0. In a compound the oxidation states add up to zero; in a simple substance it is 0, H is usually +1 (–1 in hydrides), O is usually –2 (–1 in peroxides), alkali metals are +1, main-group II metals +2 and F is always –1.