oxidation numbers?

Question

when finding oxidation numbers, i know you have to balance the equation. for example

PbO2 , where Pb = x

x + 2(-2) = 0
x + 4 = 0
x = -4

how do i know that this equation is supposed to balance at 0. Some of them i do it is -1, some it is -2? Help pleeease!

Answer 1

Its very easy----
When you u calculate the oxidation number of a neutral species(means that has no charge) then it is equated = 0
But if the species is anionic(means it carry a -ve charge) then it is equated = the amount of charge.
for example HSO4^-2 where S= x
1+ x +(-2)*4 = -2
Similarly if it is cation(carry +ve charge) then it is equated = the amount of +ve charge.
for example
NH3^+ where N= x
x +1*3 = +1
got it......!

Answer 2

Firstly:

PbO2 , where Pb = x

x + 2(-2) = 0
x - 4 = 0
x = +4, oxidation no. of Pb is +4.

Ok, for a chemical compound that has no net overall charge like above, they must add up to zero.

For simple ions like Na+, the oxidation no. is equal to the charge (i.e. in this case +1).

For complex ions like CO3^2-, the total has to equal to the overall charge, in this case for C, it has to be +4, as 4 +(-6) = -2.

As you probabaly know already, for elements like N2 etc, its always 0.

Answer 3

If an ion has a -1 charge, then it has to balance at -1, and so on. It is not always 0.

Answer 4

you have to multiply so you can get zero. so multiply -1 by 2 and -2 by -1