Calcium has 20 electrons arranged as 2, 8, 8, 2 across 4 shells, which is why it sits in period 4.
Electrons fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… Each s orbital holds 2 electrons, p holds 6 and d holds 10. Placing Calcium's 20 electrons in that order gives 1s2 2s2 2p6 3s2 3p6 4s2.
| Atomic number | 20 |
| Symbol | Ca |
| Atomic mass | 40.078 u |
| Group | 2 |
| Period | 4 |
| Block | s-block |
| Category | Alkaline earth metal |
| Valence electrons | 2 |
Elements in the same group have the same number of outer electrons, which is why they behave alike.