Calcium has electrons in more energy levels than magnesium. The number of electron in calcium atom is 20 which means 4 energy levels. (2 electron in first shell+ 8 electron in second shell+8 electron in third shell+ 2 electron in fourth shell). While number of electron in magnesium atom is 12 which means 3 energy levels. (2 electron in first shell+ 8 electron in second shell+ 2 electron in third shell). Also the Calcium third energy level contains more electron than magnesium.