state of any element:
(1) The total charge of a stable compound is always equal to zero (meaning no charge).
For example, the
H
2
O
molecule exists as a neutrally charged substance.
(2) If the substance is an ion (either there is a positive or negative charge) the total oxidation state of the ion is the charge (i.e. oxidation state of
N
O
-1
3
ion is -1).
(3) All elements from Group 1A has an oxidation state of +1 (e.g.
N
a
+1
,
L
i
+1
). All Group 2A and 3A elements have an oxidation state of +2 and +3, respectively. (e.g.
C
a
2+
,
M
g
2+
,
A
l
3+
)
(4) Oxygen always have a charge -2 except for peroxide ion (
O
2-
2
) which has a charge of -1.
(5) Hydrogen always have a charge of +1 if it is bonded with a non-metal (as in the case of
H
C
l
) and always have a -1 charge if it is bonded with a metal (as in
A
l
H
3
).
So for your problem, according to rule 2, the overall oxidation state of
N
O
−
3
is -1.
N
O
−
3
= -1
According to rule 4, the
O
atom should have a -2 charge. But since the oxygen has a subscript in the formula, we need to multiply the charge by the subscript.
x
+ [(-2) (3)] = -1 where
x
is the oxidation state on
N
atom
solving for
x
,
x
+ (-6) = -1
x
= -1 + (+6)
x
= +5
Therefore, answer is
N
5+