There are various types. Covalent bonds are formed by sharing of valence electrons rather than by transfer. In the water molecule, H 2 O, the oxygen atom, with six outer electrons, gains control of an extra two electrons supplied by the two hydrogen atoms. A particular type of covalent bond is one in which one of the atoms supplies both the electrons. Covalent or coordinate bonds in which one pair of electrons is shared are electron-pair bonds and are known as single bonds .
Atoms can also share two pairs of electrons to form double bonds or three pairs in triple bonds .
See orbital . Between these two extremes, there is a whole range of intermediate bonds , which have both ionic and covalent contributions. Thus, in hydrogen chloride, H-Cl, the bonding is predominantly covalent with one pair of electrons shared between the two atoms. The Macmillan Encyclopedia 2001 2. All rights reserved.