obtain the oxidation number for the element noted in the following: S in H2S2O3.?

Answer 1

Electrons gained must equal electrons lost.

H has an oxidation # of 1.
O has an oxidation # of 2.
H+H=2...O+O+O=6
6-2=4, 4/2=2
S has an oxidation # of 2.

Answer 2

The sum of the oxidation numbers of all atoms should be equal to the charge of your molecule/ion.

In this case the total charge is 0 (it's not an ion)

For H oxidation number is +1
For O oxidation number is -2.
So if x is the oxidation number of S in the compound you have:

2*(+1)+2x+3(-2)=0 =>
2+2x-6=0 => 2x=4 => x=+2 The sign is extremely important; never forget to write it.