For solving this first of all you figure out the formula of iron(3)oxide so you look at the periodic table as you said valency of iron is 3 positive because metals are always positive and oxygen’s valency is 2 (negative because non-metal). You criss-cross both you take 2 from oxygen and put it under the iron and then 3 from iron under oxygen.
Valency is the number of electrons needed to be a noble gas.
Oxygen has 6 electrons in the outer shell and it needs 2 more to become noble gas.
The total number of electrons needed in every shell is 8 so to figure out valency you subtract the number of electrons in the outer shell from 8. Valency also shows how many electrons you need to share for bonding.
