Scandium has 21 electrons arranged as 2, 8, 9, 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 Scandium's 21 electrons in that order gives 1s2 2s2 2p6 3s2 3p6 4s2 3d1.
| Atomic number | 21 |
| Symbol | Sc |
| Atomic mass | 44.956 u |
| Group | 3 |
| Period | 4 |
| Block | d-block |
| Category | Transition metal |