Calcium contains approximately 20 protons. It also contains 20 electrons and 20 neutrons. The element has an atomic number of 20, which coincides with its proton and electron values.
Calcium has an atomic mass of 40 atomic mass units, which is how it is determined that the element also has 20 neutrons. The number of neutrons can be found by subtracting the atomic number from the atomic mass. Calcium also has approximately four energy levels in which all of the electrons reside. Each energy level contains a different number of electrons, based on the amount of energy that the electron shell contains.