[1c2h3e4m]

I need some help with determining oxidation numbers for each atom of a compound.  I was reading the book and it didn't help much.  For example could someone explain the oxidation number for each atom of PBr5 and PBr3 and how you got it?

[1c2h3e4m]

any help plz?

[agrobert]

Lets call phosphorous A (example) and Bromine X (halogen).  Halogens (X) when covalently bound to A have an oxidation state of -1.  The overall charge of AX_n is neutral.  So oxidation states must balance. 

If you have n = 3 in the instance of AX₃ you have n * X (oxidation state) so 3 * -1 = -3. 

To balance this you determine the oxidation state of A so that A (oxidation state) + (n * X (oxidation state)) = 0

So for AX₃ A (oxidation state) = ?

and AX₅ A (oxidation state) = ?

This is a generally understood topic which you should review in your book which has a lot to do with periodic trends.

[1c2h3e4m]

Im not that great in chemistry and all those variables just confused me.  Like you said the one + the one must  = 0 (neutral)  i understand why they must be nuetral but how can u change an oxidation number to make it nuetral i thought they already had a number considering how many valence electrons they had how can you change it, i dunno if what im saying even makes sense but you can try to answer it.

[agrobert]

Phosphorous and Sulfur can have expanded octets or hypervalency (the third period and below elements can be stabilized by accessing higher orbitals and orbital hybrids).  So the oxidation state for PBr₃ P is +3 and for PBr₅ P is +5.  If you are not satisfied with this answer read

http://en.wikipedia.org/wiki/Hypervalency

 

[Padfoot]

And additionally,

If you are not satisfied with this answer read

http://www.molecularsoft.com/help/oxinum.htm

[Borek]

And not forget:

http://www.chembuddy.com/?left=balancing-stoichiometry&right=oxidation-numbers-method

Some elements have ON assigned once and forever, some can have more then one ON.