Lanthanum (La): electron configuration, atomic mass and facts
Lanthanum (La) is element 57 of the periodic table, a lanthanide in period 6 (f-block). Its electron configuration is [Xe] 5d1 6s2, with electrons arranged in shells 2, 8, 18, 18, 9, 2, and its standard atomic weight is 138.91 u.
▶ Watch Lanthanum’s electrons orbit in the interactive periodic tableLanthanum at a glance
| Symbol | La |
|---|---|
| Atomic number | 57 |
| Atomic mass | 138.91 u |
| Category | Lanthanide |
| Group | f-block (no group number) |
| Period | 6 |
| Block | f-block |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d1 6s2 |
| Noble-gas shorthand | [Xe] 5d1 6s2 |
| Electrons per shell | 2, 8, 18, 18, 9, 2 |
| Electronegativity (Pauling) | 1.1 |
| State at 20 °C | solid |
Where is Lanthanum on the periodic table?
Lanthanum is in period 6 of the f-block, the lanthanide row printed below the main table. It has 6 electron shells, and this row is where the 4f subshell fills (its own configuration is an exception, explained below).
How to work out the electron configuration of Lanthanum
Put 57 electrons into subshells in order of rising energy, filling each before starting the next: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), 3d¹⁰ (30), 4p⁶ (36), 5s² (38), 4d¹⁰ (48), 5p⁶ (54), 6s² (56), 4f¹ (57). Lanthanum is one of the exceptions to this filling order: experiments show its ground state is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹ rather than 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹, because the neighbouring subshells are so close in energy. In shorthand, [Xe] stands for the 54 electrons of xenon, giving [Xe] 5d¹ 6s².
How electronegative is Lanthanum?
Lanthanum has a Pauling electronegativity of 1.1, ranking 87 of the 100 elements with a known value (fluorine is highest at 3.98). That is low: it tends to give up electrons and form positive ions.
Frequently asked questions
What is the electron configuration of Lanthanum?
The electron configuration of Lanthanum (La) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d1 6s2. In noble-gas shorthand it is written [Xe] 5d1 6s2.
How many electrons does Lanthanum have in each shell?
Lanthanum has 57 electrons arranged in shells as 2, 8, 18, 18, 9, 2.
What is the atomic mass of Lanthanum?
The standard atomic weight of Lanthanum is 138.91 u, so one mole of Lanthanum atoms has a mass of about 138.91 g.
Is Lanthanum a metal?
Yes. Lanthanum is a lanthanide.
Explore nearby elements
More lanthanides: Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium. See all elements on the periodic table.