Lanthanum (La)
Lanthanum (La) is a lanthanide with atomic number 57 and a standard atomic weight of 138.91. At room temperature it is a solid that melts at 920 °C and boils at 3,464 °C.
Lanthanum key facts
Lanthanum at a glance
Identity
| Symbol | La |
|---|---|
| Atomic number (Z) | 57 |
| Atomic weight | 138.91 ± 0.01 (CIAAW 2024) |
| Category | Lanthanide |
| Group | — (f-block: lanthanide series) |
| Period | 6 |
| Block | d-block |
| CAS number | 7439-91-0 |
Atomic properties
| Electron configuration | [Xe] 5d1 6s2 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d1 6s2 |
|---|---|
| Electrons per shell | 2, 8, 18, 18, 9, 2 |
| Oxidation states | +3 Less common: 0, +1, +2 |
| Electronegativity (Pauling) | 1.1 |
| First ionisation energy | 5.577 eV (538.1 kJ/mol) 2nd: 11.185 eV · 3rd: 19.177 eV |
| Electron affinity | 0.47 eV |
| Atomic radius (empirical) | 195 pm |
| Covalent radius | 180 pm |
| Van der Waals radius | 243 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 920 °C · 1,688 °F · 1,193.15 K |
| Boiling point | 3,464 °C · 6,267.2 °F · 3,737.15 K |
| Density | 6.15 g/cm³ |
| Specific heat capacity | 0.195 J/(g·K) |
| Thermal conductivity | 13.4 W/(m·K) |
| Crystal structure | Hexagonal |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 139La 99.9112%, 138La 0.0888% (radioactive) |
| Stable isotopes | 1 |
| Known isotopes | 42 |
| Abundance in Earth's crust | 39 mg/kg |
| Abundance in seawater | 0.0000034 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Lanthanum electron configuration
Lanthanum has 57 electrons. Written in short form, its ground-state configuration is [Xe] 5d1 6s2 — the electrons of the noble gas xenon, plus 5d1 6s2.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d1 6s2.
An exception to the filling rule. The simple aufbau (Madelung) order predicts 4f1, 5d0, but lanthanum actually has 4f0, 5d1. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.
Discovery and name
1839 — Carl Gustaf Mosander · Sweden.
Greek lanthanein, to lie hidden.
Known for
The first lanthanide. Used in nickel–metal hydride batteries, camera-lens glass and petroleum-cracking catalysts.
Isotopes of lanthanum
Naturally occurring lanthanum is a mix of 2 isotopes. Each has 57 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 139La | 57 | 82 | 99.9112% | stable |
| 138La | 57 | 81 | 0.0888% | radioactive · half-life 103 billion years |
42 isotopes of lanthanum are known in our data set (half-lives as published in nuclear data tables).
Lanthanum and its neighbours in the lanthanide series
Neighbouring elements in the same f-block series:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Lanthanum | 57 | 138.91 | 920 °C | 3,464 °C | 6.15 g/cm³ | 1.1 |
| Cerium | 58 | 140.12 | 799 °C | 3,443 °C | 6.77 g/cm³ | 1.12 |
| Praseodymium | 59 | 140.91 | 931 °C | 3,520 °C | 6.773 g/cm³ | 1.13 |
| Neodymium | 60 | 144.24 | 1,016 °C | 3,074 °C | 7.01 g/cm³ | 1.14 |