The atomic number of calcium is 20. The atomic number represents the number of protons in the nucleus of an atom and, therefore, is the atom's positive charge. It is also typically the number of electrons contained in the atom.
Calcium does not appear in isolation in nature because of its propensity to form compounds in the presence of oxygen and water. It is an alkaline earth metal found in period 4 and group 2 of the periodic table. Calcium ranks as the fifth most common element in the Earth's crust and is necessary for the nutritional heath of plants and animals.