How do you write chemical formulas for ionic compounds?

[ncmm16]

So its the beginning of chemistry and I don't know how to make formulas. BUT its very important I learn now otherwise I will have a very hard year...

For ex:
It says, "Fill in the correct formulas for the given anion and cation."


SO^2-₄ and Al³⁺ (how do i combine those two to make a formula)


Another ex:
Al³⁺ and Cr₂O^2-₇
PLEASE EXPLAIN HOW TO DO THESE.

Thanks so much for any help:)!

 

[mgb_phys]

The resulting molecule must be uncharged.
So if you have Al(3+) and SO4(2-) you need to find how many 3+ you need to combine with how many 2- to get zero.
This is just a least common multiple elementary school maths problem.
In this case the LCM is 6, so 2*(3+) and 3*(2-) =0 giving Al₂(SO₄)₃

 

[pzona]

Write out the cation and the anion with their charges. For your second example, this would be written as Al^{3+}Cr_{2}O_{7}^{2-}. Then write the charge of the cation as the subscript of the anion, and vice versa, ignoring the plus and minus of course. It then looks like Al_{2}(Cr_{2}O_{7})_{3}. This is basically what mgb phys said, only it's a quicker (and many people starting chemistry find it easier) way to do it. It's also important to note that if you have charges like 4+ and 2-, you must simplify it to 2 and 1 when you write the subscripts.

EDIT: When I wrote the subscripts they came up as superscripts for some reason. Hopefully you can still tell what I meant.