What is Formal charge?

Example 3.5 (Nitric acid)

$\ce{HNO3}$: $N = 1 + 5 + 3 \times 6 = 24$ 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 $\ce{N=O}$ bond. Formal charges: nitrogen $5 - 0 - 4 = +1$; the singly bonded oxygen without hydrogen $6 - 6 - 1 = -1$; the others 0. The total is 0, as Proposition 3.3 requires.

Example 3.9 (Ammonia and boron trifluoride)

$\ce{NH3 + BF3 -> H3NBF3}$. In the adduct, boron completes its octet; nitrogen, which now shares its lone pair, carries the formal charge $+1$ and boron $-1$. Once formed, the $\ce{N-B}$ bond is an ordinary covalent bond.