Arsenic has 33 electrons arranged as 2, 8, 18, 5 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 Arsenic's 33 electrons in that order gives 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p3.
| Atomic number | 33 |
| Symbol | As |
| Atomic mass | 74.922 u |
| Group | 15 |
| Period | 4 |
| Block | p-block |
| Category | Metalloid |
| Valence electrons | 5 |
| Element | Configuration |
|---|---|
| Nitrogen | [He] 2s2 2p3 |
| Phosphorus | [Ne] 3s2 3p3 |
| Antimony | [Kr] 4d10 5s2 5p3 |
| Bismuth | [Xe] 4f14 5d10 6s2 6p3 |
| Moscovium | [Rn] 5f14 6d10 7s2 7p3 |
Elements in the same group have the same number of outer electrons, which is why they behave alike.