Answer is: the two electrons in the fourth shell.
Atomic number of calcium is 20, it means that it has 20 protons and 20 electrons, so atom of calcium is neutral.
Electron configuration of calcium atom: ₂₀Ca 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²; calcium has two valence electrons (4s²).
In reactions it lose those two electrons ang form cation with positive charge 2+.
Atomic number is the number of protons, which is characteristic of a chemical element.
Beryllium, magnesium,calcium and strontium are alkaline earth metals. The elements have very similar properties.
Alkaline earth metals have in common an outer s- electron shell (two electrons).
