$\ce{Ca^2+(g) + O^2-(g) -> CaO(s)}$

$\ce{Ca(g) + 1/2 O2(g) -> CaO(s)}$

$\ce{Ca(s) + O(g) -> CaO(s)}$

$\ce{2Ca(s) + O2(g) -> 2CaO(s)}$

$\ce{Ca(s) + 1/2 O2(g) -> CaO(s)}$