Answer
a. 0 and -1 (from left to right)
b. 0
c. 0
d. 0 and 1 (from left to right)
Work Step by Step
To calculate the formal charge of an atom, use the following formula:
number of valence electrons - [number of unshared electrons + $(\frac{1}{2})$(number of shared electrons)]
a. $4 - [0+(\frac{1}{2})(8)] = 0$
$4 - [2+(\frac{1}{2})(6)] = -1$
b. $4 - [2+(\frac{1}{2})(4)] = 0$
c. $4 - [1+(\frac{1}{2})(6)] = 0$
d. $4 - [0+(\frac{1}{2})(8)] = 0$
$4 - [0+(\frac{1}{2})(6)] = 1$
