Lanthanum

Lanthanum (La) is a lanthanide with atomic number 57 and an atomic mass of 138.91. Its electron configuration is [Xe] 5d1 6s2.

Lanthanum key data

Symbol	La
Atomic number	57
Atomic mass (standard atomic weight)	138.91
Molar mass	138.91 g/mol
Protons	57
Neutrons (most abundant isotope, lanthanum-139)	82
Electrons (neutral atom)	57
Category	Lanthanide
Period	6
Group	3 or none, by convention
Block	d-block or f-block, by convention
Noble gas notation	[Xe] 5d1 6s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d1 6s2
Electrons per shell	2, 8, 18, 18, 9, 2

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of lanthanum

From PubChem, the US National Institutes of Health. Public domain.

Melting point	1191 K (917.85 °C), 52nd of 103 by melting point
Boiling point	3737 K (3463.9 °C), 20th of 93 by boiling point
Density	6.15 g/cm³, 57th of 96 by density
State at room temperature	Solid
Electronegativity	1.1 (Pauling), joint 84th of 95 by electronegativity
Van der Waals radius	240 pm, joint 17th of 99 by atomic radius
First ionisation energy	5.577 eV (538.1 kJ/mol), 89th of 102 by first ionisation energy
Electron affinity	0.5 eV (48.2 kJ/mol), joint 40th of 57 by electron affinity
Common oxidation states	+3
Discovered	1839

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Lanthanum electron configuration

The full electron configuration of lanthanum is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d1 6s2. With the noble-gas core collapsed it is [Xe] 5d1 6s2, and the outermost subshell is 6s2.

Lanthanum does not follow the simple filling order. Its measured configuration differs from the one the aufbau rule predicts, because at this point in the table the 4f and 5d subshells lie so close in energy that one electron goes into 5d rather than 4f. This is a measured fact about the atom rather than an exception to be derived.

Bohr model of lanthanum (La): a nucleus and six shells holding 2, 8, 18, 18, 9 and 2 electrons from the inside out. — Bohr model of lanthanum (La). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of lanthanum (La), [Xe] 5d1 6s2, with the xenon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 5d. — Orbital diagram of lanthanum (La). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one lanthanum atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 138.91 / (6.02214076 × 10²³)
= 2.3067 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 138.91
= 4.3353 × 10²¹ atoms

Lanthanum on the periodic table

Lanthanum is in period 6. Tables differ on its group: some place it in group 3, in the d-block, and others in the f-block, outside the numbered groups.