Atomic number 57

Lanthanum

La
+ phase
− phase

Shells Orbitals 5d 6s

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57 electrons in 6 shells: 2, 8, 18, 18, 9, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Lanthanide
Period: 6
Block: f
Standard atomic weight: 138.91(1) Da
Electron configuration: [Xe] 5d¹ 6s²
Phase at 25 °C: solid
Melting point: 920 °C
Boiling point: 3464 °C
Density: 6.15 g/cm³
Electronegativity (Pauling): 1.1
Discovered: 1839, Carl-Gustav Mosander

Neighbours in the table

Ba Ce

The Greek verb lanthanein, “to escape notice”, gave lanthanum its name. It hid in cerium ore and was hard to part from it, in the history by CIAAW, the atomic-weights commission of IUPAC, the chemists’ international union. Carl-Gustav Mosander, a Swedish chemist and surgeon, was seeking impurities in cerium. In 1839 he treated cerium nitrate with dilute nitric acid and found a new oxide, lanthana, La₂O₃, in Jefferson Lab’s account. By 1842, in CIAAW’s history, he had split his lanthana into two oxides, keeping the old name for one and naming the other didymium, or twin.

Lanthanum’s usual oxidation state, a bookkeeping charge counted as if every bond were ionic, is +3 in Jefferson Lab’s listing. Los Alamos lists +3, +2 and +1 and calls the oxide strongly basic, one that reacts with acids as a base does. The metal combines directly with sulfur, phosphorus, carbon, nitrogen, boron, silicon, selenium and the halogens, Los Alamos reports.

The silvery-white metal is soft enough to cut with a knife. It ranks among the most reactive rare-earth metals on Los Alamos’s page, oxidising quickly in air, and hot water attacks it far faster than cold. It also changes crystal form twice on heating, from hexagonal to face-centred cubic at 310 °C and to body-centred cubic at 865 °C.

Where it is found

Lanthanum makes up an estimated 39 milligrams per kilogram of the Earth’s crust, in Jefferson Lab’s figures. Seawater holds 3.4 × 10⁻⁶ milligrams per litre. It occurs in cerite, monazite, allanite and bastnasite, according to Los Alamos. Monazite and bastnasite are its principal ores, carrying up to 25 per cent and 38 per cent lanthanum respectively.

Monazite sand is a phosphate of lanthanum together with cerium, thorium, neodymium and yttrium. Today most lanthanum comes from that sand through an ion-exchange process, Jefferson Lab reports. The metal itself can be made by reducing the anhydrous, or water-free, fluoride with calcium, and it was first isolated in relatively pure form in 1923, according to Los Alamos.

In the United States, bastnaesite, a rare-earth fluorocarbonate and one of lanthanum’s two principal ores, was mined as a primary product at Mountain Pass, California, the US Geological Survey reports. The US government stockpile listed potential acquisitions of 1,300 tons of lanthanum for fiscal year 2024 and 1,100 tons for 2025.

Why it matters

Rare-earth compounds containing lanthanum have fed the carbon arc lamps of film studios and cinema projectors. That use takes about 25 per cent of rare-earth compounds produced, by Los Alamos’s count. Lanthanum oxide, La₂O₃, makes glass more resistant to alkalis and goes into special optical glasses and camera lenses. Misch metal, the alloy in lighter flints, is about 25 per cent lanthanum, Los Alamos reports. The US Geological Survey lists a grade of 65 per cent cerium and 35 per cent lanthanum.

Hydrogen-sponge alloys containing lanthanum can take in as much as 400 times their volume of hydrogen, and give it back. Each uptake gives off heat, so the alloys have possibilities in energy conservation, in Los Alamos’s view. Lanthanum metal also reduces the oxides of europium, samarium and thulium to those metals, taking their oxygen, according to Los Alamos.

The rarer natural isotope, lanthanum-138, makes up 0.08881 per cent of the element. Its half-life is 1.03 × 10¹¹ years in the evaluation of the International Atomic Energy Agency (IAEA). Of its decays, 65.5 per cent turn it into barium-138 and 34.5 per cent into cerium-138, in IAEA’s evaluation. Barium-138 building up in rare-earth-rich rocks such as allanite dates them, in IUPAC’s isotope notes. The stable lanthanum-139, bombarded with protons, yields cerium-139, a medical radioisotope.

Properties

Properties of lanthanum Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	57
Standard atomic weight	138.91(1) Da	Da	IUPAC/CIAAW 2024
Melting point	920 °C	°C	Los Alamos: 920 °C
Boiling point	3464 °C	°C	Los Alamos: 3464 °C
Density	6.15 g/cm³	g/cm³
Electronegativity (Pauling)	1.1
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹
Phase at 25 °C	solid

Safety

Lanthanum metal can catch fire and reacts with water. Among company notifications to the European Chemicals Agency, 23.4% class it as a flammable solid and 19% as releasing flammable gas with water, while 58.9% report no hazard. The acute toxicity of the metal and its compounds is rated low to moderate in Haz-Map’s summary and on Los Alamos’s page. The PubChem Laboratory Chemical Safety Summary for lanthanum gives the classification and handling guidance.

Questions

What is lanthanum used for?

Lanthanum oxide, La₂O₃, goes into the glass of camera lenses and other special glasses, in Jefferson Lab's account on PubChem's record. Small additions of the metal turn ordinary cast iron into nodular cast iron, Los Alamos reports. Oxides of lanthanum, cerium, yttrium and zirconium can stand in for thorium in welding electrodes, according to the US Geological Survey. Lighter flints and hydrogen-storage alloys use it too.

Is lanthanum radioactive?

Only a sliver of it. Stable lanthanum-139 makes up 99.91119 per cent of natural lanthanum, in the table of CIAAW, IUPAC's atomic-weights commission. The rest is lanthanum-138, with a half-life of 1.03 × 10¹¹ years in the International Atomic Energy Agency's evaluation. Los Alamos recognises 23 other radioactive isotopes. With cerium-138 and cerium-136, lanthanum-138 can measure the time since a supernova that produced large numbers of neutrinos, in IUPAC's isotope notes.

Is lanthanum dangerous to touch?

The acute toxicity of lanthanum and its compounds is rated low to moderate by Haz-Map and by Los Alamos, which advises care in handling them for that reason. The metal can also burn: some notifications to the European Chemicals Agency class it as a flammable solid that releases flammable gas with water. The PubChem Laboratory Chemical Safety Summary for lanthanum sets out its handling guidance.

How much does lanthanum oxide cost?

Lanthanum oxide of at least 99.5 per cent purity averaged about 1 dollar per kilogram in 2022, 2023 and 2024, down from 2 dollars in 2020 and 2021, on US Geological Survey figures. A misch metal of 65 per cent cerium and 35 per cent lanthanum sold for about 5 dollars per kilogram in 2024. China supplied 70 per cent of US rare-earth imports from 2020 to 2023.