What is the formula for iron(II) oxide?

##FeO##

To write this formula, know that the roman numeral in iron (II) simply denotes that this ion of iron has a ##2+## charge. So, the iron here will be ##Fe^(2+)##

Oxygen ion always has a ##2-## charge since all Group A are monatomic ions - ions of atoms which have only one charge. So oxygen will always be ##O^(2-)##

You can use this guide for predicting the charges of ions:

Now to write the formula is to simply let the charges of each be the subscript of the other and neglecting their signs. Like this:

Now by doing this, you'll end up with a formula unit ##Fe_2O_2##. However, in writing formula units ##-## chemical formula for , we always write the lowest ratio of atoms that we have. Since you have 2 iron ions and 2 oxygen ions, their ratio is 1:1 and therefore we write the formula as ##FeO##.