Question

write the correct formula for the ionic compounds containing the following pairs of elements:
A. Li & O
B. Al & F
C. Mg & S

Answer 1

You'll need a periodic table for this - find the metallic elements (Li, Al and Mg) on the left and the group they are in will tell you how many electrons they will need to gain the most stable form: note that Al forms 3+ ions since it has three unpaired electrons as an element.

Likewise, if you figure out which group O, F and S are in then that will help you find out how many electrons they need to gain: O needs 2 electrons. If we do Li and O, then each Li needs to lose 1 electron whilst each O needs to gain two electrons, so the formula will be Li2O; ie 2 lithium ions per oxygen and the ions are stable because they have paired their outer electrons.

I'll give you a hint for the last one: the compound is called magnesium silicide and the silicon gains four electrons rather than loses four electrons. Hope this helps