---
title: "Zirconium (Zr) — element 40"
description: "Zirconium is element 40, symbol Zr, atomic weight 91.222. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/zirconium/
site: "At One Place"
---

# Zirconium (Zr)

**Zirconium — element 40: Zr**

91.222 u · Transition metal · period 5, group 4

Zirconium is nearly transparent to neutrons, which is why every water-cooled reactor in the world clads its fuel in a zirconium alloy — steel would soak up too many neutrons to sustain the chain reaction. That choice has a sting: above about 1,200 °C zirconium strips oxygen out of steam and releases hydrogen, which is what produced the explosions at Fukushima Daiichi. Zircon crystals from Jack Hills in Western Australia are the oldest known terrestrial material at about 4.4 billion years, because zircon is tough enough to survive being eroded, buried and re-eroded for most of Earth's history.

## Properties

- **Atomic number:** 40
- **Symbol:** Zr
- **Standard atomic weight:** 91.222 u
- **Category:** Transition metal
- **Group:** 4
- **Period:** 5
- **Block:** d-block
- **Electron configuration:** [Kr] 5s² 4d²
- **Electrons per shell:** 2, 8, 18, 10, 2
- **Melting point:** 2,128 K — 1854.8 °C — 3370.7 °F
- **Boiling point:** 4,682 K — 4408.9 °C — 7967.9 °F
- **Density:** 6.52 g/cm³
- **Electronegativity:** 1.33 (Pauling)
- **Oxidation states:** +4
- **Discovered:** 1789 by Martin Heinrich Klaproth

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d²
- **Noble gas shorthand:** [Kr] 5s² 4d²
- **Shells:** K: 2 · L: 8 · M: 18 · N: 10 · O: 2

## What it is used for

- Fuel rod cladding in nuclear reactors, as zircaloy
- Cubic zirconia as a diamond simulant
- Zirconia dental crowns and hip joint ceramics
- Foundry sand and refractory linings
- Chemical plant pipework handling hot acids

> CIAAW revised the standard atomic weight to 91.222 in 2023; tables printed before then show 91.224.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 38 | Strontium | Sr | 87.62 |
| 39 | Yttrium | Y | 88.906 |
| 40 | Zirconium ← | Zr | 91.222 |
| 41 | Niobium | Nb | 92.906 |
| 42 | Molybdenum | Mo | 95.95 |

## Group 4

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Zirconium sits in group 4 with titanium, hafnium, rutherfordium.

## Common questions

### What is the atomic number of zirconium?

40. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for zirconium?

Zr.

### What is the electron configuration of zirconium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d², or [Kr] 5s² 4d² in noble-gas shorthand.

### What is the melting point of zirconium?

2,128 K — 1854.8 °C — 3370.7 °F

## Related pages

- [Strontium (Sr)](https://atoneplace.net/element/strontium/)
- [Yttrium (Y)](https://atoneplace.net/element/yttrium/)
- [Niobium (Nb)](https://atoneplace.net/element/niobium/)
- [Molybdenum (Mo)](https://atoneplace.net/element/molybdenum/)
- [Titanium (Ti)](https://atoneplace.net/element/titanium/)
- [Hafnium (Hf)](https://atoneplace.net/element/hafnium/)
- [Rutherfordium (Rf)](https://atoneplace.net/element/rutherfordium/)

## Sources

- Official vendor product documentation — At One Place (/about/)