if k2O, SrO, and Al2O3 are all correct formulas, predict the formulas for the oxides of Rb, Mg, & Ga.

Question

i think the answers are Rb2O, MgO,& Ga2O3 but im not sure why

Answer 1

All are correct. Why?
K2O means K is +1, Rb is in the same column so we predict +1 for it too; therefore, Rb is Rb2O. Same reasoning for Mg and Ga.