[CenturionMonkey]

Okay, I'm supposed to name arsenic (III) oxide. And I figured it would be As₂O₃. But when I thought about it I figured, shouldn't As oxidation state be +3 if it's (III) cause it would go over to Kr. But if As is +ve, then O oxidation state must be -ve, meaning it is -6. Cause one has to be +ve and one has to be -ve. So wouldn't it be As₂O? I'm really confused, please help.

Also, I'm trying to name PH3. I don't know whether I'm supposed to say phosphorous trihydride or phosphorous (III) hydride.

Please help, I have a test tomorrow.

[enahs]

It is +3 (though your reasoning is not correct).

Oxygen is usually -2 (except in cases like superoxides). Thus if you have three -2's, and 2 X's, to make it zero (it is a neutral molecule) you need 2 * +3

(2*3) + (3 * -2) = 0

[CenturionMonkey]

Thanks, yeah I meant to say:

"if As is going to have an oxidation state of 3, it would be -3. To get to a full shell similar to Kr. If it was +3 it would be like Ca which isn't a full shell. So wouldn't O have to be +6. Cause we can't have 2 negatives. One of the atoms has to have a positive oxidation and wouldn't that have to be O?"

sorry for the confusion.

[cliverlong]

Ah ... the wonderfully confusing and almost useless concept of the "full outer shell"

You probably know that sulphuric acid has formula H₂SO₄

Try and explain the bonding between the sulphur and oxygen atoms in terms of "full outer shells" - but don't try too hard you will just waste your time

Similarly you may know that iron can form Fe²⁺ or Fe³⁺ ions. Looking at iron's place in the periodic table if I count to the left until I get a "full outer shell", iron should have a charge of +8. Have a look at Zinc and apply the same logic.

To sort this out one needs to look at bonding slightly differently and the role played by s, p and d orbitals and what constitutes a stable configuration that does not have a "full outer shell".

I recommend Jim Clarke's Chem Guide

http://www.chemguide.co.uk/inorganic/redox/definitions.html#top

http://www.chemguide.co.uk/inorganic/redox/oxidnstates.html

Then have another stab at answering the question.


Clive