how to identify how many elements are in a compound

[lboogy]

Hi everyone,

 

I'm struggling to know how to realise how many atoms are in a compound if you are provided with the word of the chemical.

 

For example, for Silver Nitrate I know this is a combination of Ag, N & O. I know the electronic config of Ag is 4f1. I would use this as a basis to work out how many O & N atoms I need to fill the 4th shell of Ag. So Ag needs 13 electrons from O & N. Because N & O are bonded together - does this have a different implication for sharing electrons? Or is this totally wrong and in fact it is ionic bonding? I know when s block & p block bond together they do so through ionic bonding, and 2 p block atoms bond together via covalent bonding - but what about d block atoms with either s or p block atoms?

 

I know the answer is AgNO3 - but I want to know how to get there! If I didn't know that was the answer I would have used as many N & O atoms to fill up the outershell of Ag, but obviously knowing the answer I know this is wrong!

 

thanks for your help

 

Gav

[thedarkshade]

I think what you need to know are the oxidation numbers, and there are some rules there:

 

1.The sum of oxidation number in a molecule should be 0

2.The oxidation number of elements in elementary conditions is 0 (oxidation nr of [ce]O2[/ce] is 0)

3.Hydrogen's oxidation number is always +1 except in hydrures (ignore the writing) when it's oxidation number is -1

4.Oxygen's oxidation number is always -2, except peroxides where it's oxidation number is -1, and some other cases like [ce]KO2[/ce](superoxide) when it's oxidation number is 1/2.

 

Examples:

[ce]NaNO2[/ce] --Na is +1, N is +3 and O is -2. As there are two atoms of O then we have -4 for oxygen. If the add them together we get: +1 +3 -4 = 0. And there we get 0 (look rule 1)

 

[ce]H2SO4[/ce] --H is +1, S is 6+ and O is -2. As we have two atoms of H then its oxidation nr here us +2 and as we have four atoms of O, its oxidation nr here is -8. We add them together as we get: +2+6-8=0. See!

 

[ce]Ca(OH)2[/ce] you might ask why brackets and why 2 after. This is because OH itself is -1 and Ca is +2, so we'd get a +1 molecule here. So we put OH in brackets and put 2 after so that 2 multiplies the -1 on top to get -2. And then we'd have -2+2=0.

 

If there is anything you need more just let me know.

 

Cheers,

Shade