I need help assigning oxadation numbers.
I know that A is H1 I-1.
And I know that for PBr3, it is X+3(-1)=0. What I don't understand is how you know that Br is a -1 to begin with.
Also, I'm having trouble knowing how to start the process if there are no rules for the elelments in the compound I'm tryig to find oxidation numbers for.
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Oxidation numbers are used to follow where the electrons are going in oxidation-reduction reactions.
There are some basic rules you can use to assign oxidation numbers:
1. The oxidation number of every element is zero (0)
2. The oxidation number for monoatomic ions = the charge of the ion.
3. Group 1 metal ions are always +1
4. Group 2 metal ions are always +2
5. H is +1 when bonded to a nonmetal; -1 when bonded to a metal.
6. F is always -1
7. Oxygen is almost always -2
8. The sum of all oxidation numbers in a compound is always zero
9. The sum of the oxidation numbers in a polyatomic ions is equal to the charge of the ion.
Now to your examples:
HI: H = +1 (rule 5) so I has to be -1 (rule 8)
PBr3: The ionic charge of halogens is -1 (rule 2) and you have 3 Br so the total negative charge is -3; therefore by rule 8 P has to be +3
GeS2: Sulfur in group 16 has a charge of -2 (rule 2) so 2 sulfur = a total of -4. Therefore Ge is +4 (rule 8)
KH: H is -1 when bonded to a metal (rule 5) and K is +1 (rules 2 &3)
As2O5: O is usually -2 (rule 7) so a total negative charge of -10; therefore 2 As = +10 and each As is +5.
H3PO4: H is +1 (rule 5); O is -2 (rule 7). So you have +3 - 8 = -5 when you add the H & the O charges. Since total charge is zero (rule 8) the P has to be +5.
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