Add electron dots and charges as necessary to show the reaction of calcium and oxygen to form an ionic compound.
neutral atoms ions
ca + o > ca + o

The two atoms shown in the equation are CALCIUM and oxygen. 

You start off with a neutral calcium atom with a shell of two electrons, a shell of 8 around that, a shell of 8 around that, and a shell containing 2...with no charge. 
20 protons + 20 electrons. 

You also have an oxygen atom with a shell of 2, and a shell of 6...with no charge. 
8 protons + 8 electrons. 

Each ionizes to form a calcium ion with 2 electrons removed (from the outer shell), leaving a +2 charge (20 protons, 18 electrons)... 
and an oxygen ion with 2 electrons added (to the outer shell), leaving a -2 charge (8 protons, 10 electrons). 

Their electrostatic attraction causes them to come together to form an ionic compound of CaO in a crystal lattice.