In this problem we're going to find the formula for
the compound formed from aluminium and phosphorus.
The first thing I'm going to do is write my chemical symbols for
each of these elements.
So we have aluminum and phosphorus.
The next thing I'm going to do is actually figure out what the charges are on those
elements when they form an ion for an ionic compound.
Notice that aluminum is in this column, the third column of the main group.
And we don't count those transition groups in there when we're looking at
these ionic charges.
And so we have aluminum, we have a 3 plus charge.
I look at phosphorus, which is in the same column as nitrogen here, and
that actually has a 3 minus charge.
And so what we want to do is actually crisscross down,
and here we end up with Al3P3.
But for our ionic compounds,
we always show them as our empirical formula with the lowest ratio.
So we can actually show this as AlP.
When I look at the charges on these, I see I have one aluminum with a plus 3 charge,
and plus the phosphorus with a minus 3.
And I get a total of 0.
So that lets me know that I have the correct formula for this compound.
Because the sum of the charges on the ions in the compound sum to zero.