---
title: "Lanthanum (La): Atomic Mass, Electron Configuration & Properties"
url: https://www.moldraw.com/elements/lanthanum.html
description: "Lanthanum (La), element 57: atomic mass 138.91, electron configuration [Xe] 5d¹⁶s², group 3, period 6, f-block lanthanide. Melting and boiling point, density, electronegativity and shells."
lang: en
publisher: Scidart Academy
generated: 2026-11-03
---
57La138.91

# Lanthanum (La) – element 57

Lanthanum is a chemical element with the symbol **La** and atomic number **57**. It is a lanthanide in group 3 (lanthanides), period 6 of the periodic table (f-block), and is a solid at room temperature.

Lanthanum (La) has atomic number 57, atomic mass 138.91 u and electron configuration [Xe] 5d16s2.

[Download periodic table PDF](https://www.moldraw.com/printables/periodic-table.pdf)[Open interactive periodic table](https://www.moldraw.com/tools/free-chem-tools/interactive-periodic-table.html)

## Lanthanum facts

| Symbol | La |
| --- | --- |
| Atomic number | 57 |
| Atomic mass | 138.91 u (g/mol) |
| Category | lanthanide |
| Group | group 3 (lanthanides) |
| Period | 6 |
| Block | f-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 5d1 |
| Noble-gas notation | [Xe] 5d16s2 |
| Electrons per shell | 2, 8, 18, 18, 9, 2 |

## Bohr model and shells

Lanthanum has 57 electrons in 6 shells: 2, 8, 18, 18, 9, 2. Full configuration and noble-gas notation are in the table; work out any element or ion with the [electron configuration calculator](https://www.moldraw.com/tools/free-chem-tools/electron-configuration-calculator.html).

## Physical and atomic properties

| Melting point | 1193 K (919.9 °C) |
| --- | --- |
| Boiling point | 3737 K (3463.8 °C) |
| Density | 6.162 g/cm³ |
| Electronegativity (Pauling) | 1.1 |
| Electron affinity | 53 kJ/mol |
| Ionization energies | 538.1, 1067, 1850.3 kJ/mol (1st, 2nd, 3rd) |
| Molar heat capacity | 27.11 J/(mol·K) |
| Discovered by | Carl Gustaf Mosander |

Use the atomic mass in the [molar mass calculator](https://www.moldraw.com/tools/free-chem-tools/molar-mass-calculator.html) for any formula containing La.

## Position in the periodic table

Tap any element to open its page.

## Frequently asked questions

### What is the atomic mass of lanthanum?

The atomic mass of lanthanum is 138.91 u, so one mole of lanthanum atoms weighs 138.91 g.

### What is the electron configuration of lanthanum?

Lanthanum has 57 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹, or [Xe] 5d¹⁶s² in noble-gas notation. Electrons per shell: 2, 8, 18, 18, 9, 2.

### Is lanthanum a metal?

Yes. Lanthanum is a lanthanide.

### Where is lanthanum on the periodic table?

Lanthanum is in group 3 (lanthanides), period 6, in the f-block.

Data: [Periodic-Table-JSON](https://github.com/Bowserinator/Periodic-Table-JSON) (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.