* Calcium (Ca) is in group 2 of the periodic table, meaning it has 2 valence electrons (electrons in its outermost shell).

* Oxygen (O) is in group 16, meaning it has 6 valence electrons. It wants to gain 2 electrons to achieve a stable octet (8 electrons) in its outer shell.

During the formation of CaO (calcium oxide):

* Calcium donates its 2 valence electrons to oxygen.

* Oxygen accepts these 2 electrons, completing its octet.

Therefore, one atom of calcium donates 2 electrons to one atom of oxygen during the formation of CaO.