Zirconium has 40 electrons arranged as 2, 8, 18, 10, 2 across 5 shells, which is why it sits in period 5.
Start from the previous noble gas core, written in square brackets, then add the remaining electrons in filling order. For zirconium that gives [Kr] 4d2 5s2.
| Atomic number | 40 |
| Symbol | Zr |
| Atomic mass | 91.224 u |
| Group | 4 |
| Period | 5 |
| Block | d-block |
| Category | Transition metal |
| Element | Configuration |
|---|---|
| Titanium | [Ar] 3d2 4s2 |
| Hafnium | [Xe] 4f14 5d2 6s2 |
| Rutherfordium | [Rn] 5f14 6d2 7s2 |