electron dot structure of calcium oxide
calcium has 2 valence electrons while oxygen has 6 valence electrons. when adding calcium and oxygen together they for calcium oxide or CaO and calciums 2 valence electrons join with oxygens 6 to fill the Lewis structure.
Ca 2+ [O with 8e-] 2-
xx Ca xx O xx
Ca 's two electrons and O 's 6 electrons combine and forms a covalent bond.