Chapter 2 - Atoms, Molecules, and Ions - Active Learning Questions - Page 73: 11

Iron has two variable valencies - $Fe^{2+}$ and $Fe^{3+}$ But, on the other hand, Barium has no variable valency. So, when we write compounds of Iron, we specify which valency we are using, to avoid any confusion. e.g. Iron Oxide can mean - FeO (Ferrous Oxide) and $Fe_{2}O_{3}$ (Ferric Oxide)

For distinguishing between Fe(II) and Fe(III), we write Iron (Fe) along with its valency when writing names of compounds. Barium does not have any variable valency.