Chem1 help please (exam tomorrow!)

Basically, if I'm trying to find the ratio of let's say, aluminium chloride, and I know that Al = +3 and Cl = -1, what on earth do I do next? I know the answer is AlCl3, I just don't know how to get there..

Well you know that AlCl3 has an overall charge of 0, so the charges of the different ions have to add up to make 0.
As Al has a charge of +3 and Cl- has a charge of -1, you need +3-3 = 0, so you need 3 -1 charges (therefore 3 Cl- ions)

Can you explain this without me knowing that it's AlCl3? I only know it's AlCl3 because I looked at the mark scheme haha.

Can you explain this without me knowing that it's AlCl3? I only know it's AlCl3 because I looked at the mark scheme haha.

Ok, so it's to do with the number of electrons in the atom's outer shell. Al is in Group 3, so you know it must have 3 outer electrons. Therefore, the stable ion of Al would be Al3+ (because it wants a full outer shell so it 'loses' 3 electrons, giving it a charge of +3).

Cl is in Group 7, so it has 7 electrons in its outer shell. Now, this means one chlorine atom is very reactive as it wants to gain one more electron to get a full outer shell. When it gains another electron (e-), the charge of that ion now becomes more negative (because electrons are negatively charged). Seeing as it's only gained one electron, the charge decrease by one, therefore giving it a charge of -1.

Now that you know an aluminium ion has a charge of +3 and a chloride ion has a charge of -1, any molecule that it makes must have a charge of 0.
To make 0, you need to cancel any charges of the ions making up that molecule: Al3+ and Cl-. +3-3 = 0
So three -1 charges are needed to cancel out the +3 charge on the aluminium ion --> three Cl- ions needed --> AlCl3

Really hoped this helped

Yes, this helps massively, thank you so so so much!