[jackfiore]

This may well be a ridiculously easy question but I just can't get my head around it and I can't find anything on the internet or in any of my books about it. I understand that when elements combine to form compounds, all electrons must be taken into consideration so that each atom receives a noble gas structure. Therefore when Lithium and Oxygen react, for instance, for oxygen to get a full outer shell, 2 lithium atoms must be used so that they can each donate an electron for the outer shell of oxygen to become full. But when say Aluminium and Oxygen combine, how do you work it out like that for them? And why does swapping the charge or the valency equate to the ratio? I'm really confused... 

[Borek]

I understand that when elements combine to form compounds, all electrons must be taken into consideration so that each atom receives a noble gas structure.

That's not necessary. It can be this way, but it doesn't have to.

But when say Aluminium and Oxygen combine, how do you work it out like that for them? And why does swapping the charge or the valency equate to the ratio? I'm really confused... 

Molecules are neutral, that explains the ratio.

In the case of aluminum oxide you can apply the same approach - how many electrons oxygen needs to get noble gas structure? How many electrons Al has to loose? How many of each do you need to combine to make a neutral molecule?

[jackfiore]

Thanks a lot for the reply. But why is it that swapping the valences work?

[jackfiore]

I'm sorry, I'm probably sounding very stupid. It's just that ok, chemical compounds are neutral so the sum of their charges=0. Therefore in the case of Al and O surely an expression would be X[+3]+Y[2-]=0. Doesn't that mean that there could be multiple values in AlXOY for X and Y?

[DevaDevil]

you take the smallest numbers without common denominator possible.

yes, you could have Al₄O₆, but that is incorrect as you can divide both by 2.

[Borek]

you take the smallest numbers without common denominator possible.

That's the case for ionic compounds, that's not a general rule.

[DevaDevil]

of course, but we are dealing with an ionic compound here

[jackfiore]

Two questions: why do ionic compounds have to be neutral, (I know the reason they are is because the anion's charge=cation's charge, but why do they have to be the same?) and how would it apply to covalent compounds (Can you swap valencies with covalent compounds, and, if so, why can you do that? Do the two valencies always -or at least as a general rule of thumb- have to be equal so that valency1-valency2=0?)

[DevaDevil]

Two questions: why do ionic compounds have to be neutral

every compound has to be neutral (not just ionic), local positive or negative charges are always counterbalanced by its opposite to get neutrality in macroscopic compounds. A stable, charged substance just will not exist.

I know the reason they are is because the anion's charge=cation's charge, but why do they have to be the same?

if for example the cation is 2+ the anion can be 1-, but then there would be 2 anions per 1 cation in the eventual compound to have neutrality again.

how would it apply to covalent compounds (Can you swap valencies with covalent compounds, and, if so, why can you do that? Do the two valencies always -or at least as a general rule of thumb- have to be equal so that valency1-valency2=0?)

covalent bonds work differently, they do not have the valency ionic bonds do. here you apply the octet rule. Every atom likes to be surrounded by 8 electrons. For example, oxygen has 6 of its own in its outer shell, so likes to get 2 more through bonds, thus, oxygen likes to form 2 bonds. Carbon has 4 electrons in its outer shell, thus, carbon likes to obtain 4 more through bonds.

[jackfiore]

Thanks for a lot for the reply. Is that not the same thing as "valency"? I thought valency of an element was the number of electrons involved in bonding. So the valency will reflect the octet rule?

So when determining the covalent compounds, can you list the valencies e.g. methane: C : 4 H:1 and then use the swapped valencies as the ratio subscripts? (and why can you do that - I know now the reason you can in ionic is to have the charges add up to zero, but here it is the valencies, because in covalent bonding, there are no ions?). Thanks so much, you've both been very helpful indeed.

[Lord Hades]

Valency is the ability to bond. For example, the valency of Na would be 1 because it loses 1 electron to become Na¹⁺.

For covalent compounds, CH4 is true but so is C2H8 which has the same formula (CnH2n, where n is the number). In covalent bonds there are no ions. This is because electrons are shared between atoms of elements, not transferred.

And lastly, valencies and charges are the same thing. A atom having a charge of 1+ would have a valency of 1+ too.

[rabolisk]

For covalent compounds, CH4 is true but so is C2H8 which has the same formula (CnH2n, where n is the number).

I would love to see this compound, C₂H₈...

[Lord Hades]

My mistake that should have been (CnH(2n-2)) so C2H6

My apologies