---
title: "Lanthanum (La) — element 57"
description: "Lanthanum is element 57, symbol La, atomic weight 138.91. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/lanthanum/
site: "At One Place"
---

# Lanthanum (La)

**Lanthanum — element 57: La**

138.91 u · Lanthanide · period 6

Lanthanum means "to lie hidden", because Mosander found it concealed inside what everyone had taken for pure cerium — a fair description of the whole rare-earth story, in which one "element" after another turned out to be several. A Toyota Prius battery pack of the nickel-metal hydride generation contained on the order of 10 kg of lanthanum. Its electron configuration is anomalous: [Xe] 6s² 5d¹ with no 4f electron at all, which is exactly why it is arguable that lanthanum, not lutetium, belongs in group 3.

## Properties

- **Atomic number:** 57
- **Symbol:** La
- **Standard atomic weight:** 138.91 u
- **Category:** Lanthanide
- **Period:** 6
- **Block:** d-block
- **Electron configuration:** [Xe] 6s² 5d¹
- **Electrons per shell:** 2, 8, 18, 18, 9, 2
- **Melting point:** 1,191 K — 917.9 °C — 1684.1 °F
- **Boiling point:** 3,737 K — 3463.8 °C — 6266.9 °F
- **Density:** 6.15 g/cm³
- **Electronegativity:** 1.1 (Pauling)
- **Oxidation states:** +3
- **Discovered:** 1839 by Carl Gustaf Mosander

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹
- **Noble gas shorthand:** [Xe] 6s² 5d¹
- **Shells:** K: 2 · L: 8 · M: 18 · N: 18 · O: 9 · P: 2

## What it is used for

- Nickel-metal hydride battery electrodes, including in hybrid cars
- Fluid catalytic cracking catalysts in oil refineries
- High-refractive-index camera and telescope lenses
- Carbon arc lamps for studio and projector lighting
- Mischmetal in lighter flints, alloyed with cerium

> Anomalous electron configuration: [Xe] 6s² 5d¹ rather than the predicted 6s² 4f¹ — the first lanthanide has no f electrons. Filed in the d-block for that reason, though tables that place lutetium in group 3 put lanthanum in the f-block instead.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 55 | Cesium | Cs | 132.91 |
| 56 | Barium | Ba | 137.33 |
| 57 | Lanthanum ← | La | 138.91 |
| 58 | Cerium | Ce | 140.12 |
| 59 | Praseodymium | Pr | 140.91 |

## Common questions

### What is the atomic number of lanthanum?

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

### What is the symbol for lanthanum?

La.

### What is the electron configuration of lanthanum?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹, or [Xe] 6s² 5d¹ in noble-gas shorthand.

### What is the melting point of lanthanum?

1,191 K — 917.9 °C — 1684.1 °F

## Related pages

- [Cesium (Cs)](https://atoneplace.net/element/cesium/)
- [Barium (Ba)](https://atoneplace.net/element/barium/)
- [Cerium (Ce)](https://atoneplace.net/element/cerium/)
- [Praseodymium (Pr)](https://atoneplace.net/element/praseodymium/)

## Sources

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