CoachA covalent bond is a pair of electrons shared by two atoms, the bonding pair; two or three pairs make a double or a triple bond. A pair of valence electrons that belongs to a single atom is a lone pair. A Lewis structure shows every valence electron of a molecule or ion, as bonds (lines between atoms) and lone pairs (lines or pairs of dots on an atom). By the octet rule, the atoms of period 2 (C, N, O, F) are surrounded in a stable structure by four pairs, eight electrons; by the duet rule, hydrogen by one pair.
Examples
Example 3.5 (Nitric acid)
: electrons, 12 pairs. Skeleton: nitrogen central, bonded to three oxygens, one of which carries the hydrogen. Completing octets with single bonds leaves nitrogen with six electrons; one oxygen lone pair becomes an bond. Formal charges: nitrogen ; the singly bonded oxygen without hydrogen ; the others 0. The total is 0, as Proposition 3.3 requires.
Example 3.9 (Ammonia and boron trifluoride)
. In the adduct, boron completes its octet; nitrogen, which now shares its lone pair, carries the formal charge and boron . Once formed, the bond is an ordinary covalent bond.