00:02
Hi there, in this question we are trying to write the correct formulas for two ionic compounds here.
00:08
The first one is chromium and we have a roman numeral there.
00:15
Roman numeral tells us the charge of the chromium ion in this compound and it is with oxide which means oxygen.
00:25
That's the oxygen atom.
00:27
Okay so to write the correct formula the first thing we need to do is to write the correct symbols for each of the elements.
00:36
So chromium is cr and oxygen is o.
00:42
Then i'm going to put the charges in.
00:44
Chromium has a 3 plus charge.
00:46
Oxygen, since it's in group 6 of the periodic table, it has 6 valence electrons so when it forms an ion it gains 2 giving it a 2 negative charge.
00:57
Well ionic compounds are neutral.
00:59
Even though they're composed of charged particles they're neutral which means that the total positive charges and total negative charges have to add up to zero.
01:08
So we have 3 positive and 2 negative.
01:12
The only way to make those add up to zero is by increasing both of them and we need to increase them to the least common multiple of 2 and 3.
01:23
So we need 2 of the cr because 2 times 3 is going to be 6 positive and we need 3 of the 2 negative because 3 times 2 negative is 6 negative.
01:39
So these would then add up to zero...