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QUESTION

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.