Atomic number 42

Molybdenum

Mo
+ phase
− phase

Shells Orbitals 4d 5s

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42 electrons in 5 shells: 2, 8, 18, 13, 1.

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

Category: Transition metal
Period: 5
Group: 6
Block: d
Standard atomic weight: 95.95(1) Da
Electron configuration: [Kr] 4d⁵ 5s¹
Phase at 25 °C: solid
Melting point: 2623 °C
Boiling point: 4639 °C
Density: 10.2 g/cm³
Electronegativity (Pauling): 2.16
Discovered: 1778, Carl Wilhelm Scheele

Neighbours in the table

Nb Tc

Until 1778 molybdenite, the sulfide MoS₂, was mixed up with graphite and with lead ore. That year Carl Wilhelm Scheele recognised it as the ore of a new element, Los Alamos records. The name keeps the confusion. CIAAW, IUPAC’s atomic-weights commission, derives it from molybdos, Greek for lead, the ancients’ word for any black mineral that left a mark on paper. Peter-Jacob Hjelm isolated the metal in 1781 by CIAAW’s and Jefferson Lab’s dates.

Molybdenum’s six valence electrons, 5s and 4d, belong to the configuration [Kr] 4d⁵ 5s¹, which spreads as 2, 8, 18, 13 and 1 across the shells. Its neighbours show the same lone s electron. Chromium, directly above, moves an electron from 4s to 3d for the extra stability of a half-filled subshell. Niobium, to the left, ends [Kr] 5s¹ 4d⁴, which the book blames on electron repulsion across a small 5s–4d energy gap, warning that no simple method predicts such exceptions.

In water molybdenum mostly holds the +6 oxidation state, a bookkeeping charge assuming ionic bonds, OpenStax says. Second-row transition metals keep high oxidation states better than first-row ones, because their larger ions lose electrons from orbitals farther out. Water-stable species are oxyanions, metal-oxygen negative ions such as MoO₄²⁻, while Mo³⁺ reacts readily with air’s oxygen.

Where it is found

Jefferson Lab’s figures in PubChem’s record give 1.2 milligrams of molybdenum per kilogram of crust and 1 × 10⁻² milligrams per litre of seawater. The metal never occurs free. Molybdenite supplies most of it, with wulfenite, PbMoO₄, and powellite, CaMoO₄, as minor ores that often lie alongside tin and tungsten ores.

In large, low-grade porphyry deposits, rock bodies with scattered sulfides, molybdenum sulfide is the main metal, the US Geological Survey says. In porphyry copper deposits it comes along with the copper. Los Alamos adds tungsten mines as a source; the survey notes falling copper-mine ore grades weighing on output. Of the five largest producers, only China and the United States mine it both ways.

Three US roasting plants turn the concentrate into molybdic oxide, the starting point for ferromolybdenum, metal powder and chemicals. Los Alamos adds that the metal itself comes from powder made by reducing purified molybdic trioxide or ammonium molybdate with hydrogen. US scrap may furnish up to 30 per cent of apparent supply, the survey estimates, yet no process recovers the molybdenum separately; it returns inside steel, superalloy and catalyst scrap.

Why it matters

Alloying in cast iron, steel and superalloys is the principal use, the US Geological Survey says, finding little that can replace molybdenum in steels. Los Alamos gives its value: the metal adds hardenability and toughness to quenched and tempered steels and keeps them strong when hot. Almost every ultra-high-strength steel carries 0.25 to 8 per cent of it. It also goes into Hastelloys, nickel alloys that resist heat and chemical corrosion, Los Alamos notes.

Jefferson Lab pairs a high melting point, about 2623 °C, with electrodes for electrically heated glass furnaces. Its disulfide, MoS₂, is a lubricant where oils would decompose at high temperature, Los Alamos notes. The survey’s graphite report adds the other side: as a dry lubricant it copes worse with oxidising conditions than graphite does.

In medicine molybdenum-99 is the parent of technetium-99m, the radiopharmaceutical used more than any other worldwide, according to IUPAC’s text in PubChem’s record. Its half-life of about 66 hours, against 6 hours for technetium-99m, makes it easier to ship. Its technetium-99m generator was first developed at Brookhaven National Laboratory in the early 1960s. IUPAC names fission of uranium-235 as the commercial route, while Jefferson Lab’s technetium note describes neutron capture by molybdenum-98. That note gives 65.94 hours and says the isotope beta-decays to technetium-99; the Atomic Mass Data Center gives 65.932 hours.

Properties

Properties of molybdenum 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	42
Standard atomic weight	95.95(1) Da	Da	IUPAC/CIAAW 2024
Melting point	2623 °C	°C	Los Alamos: 2623 °C
Boiling point	4639 °C	°C	Los Alamos: 4639 °C
Density	10.2 g/cm³	g/cm³
Electronegativity (Pauling)	2.16
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d⁵
Phase at 25 °C	solid

Safety

Bulk molybdenum is essentially harmless in its everyday forms: Haz-Map, in PubChem’s safety summary, reports low toxicity in animals and people. The cautions that summary lists include fine dust or powder, which can burn or explode, and contact that irritates the eyes, skin and airways. The PubChem Laboratory Chemical Safety Summary and NIOSH’s card for molybdenum metal are the sheets for workplaces handling the powder.

Questions

Do plants and people need molybdenum?

Yes, in trace amounts. Los Alamos calls it a trace element in plant nutrition and notes that some land is barren because its soil lacks molybdenum. Bacteria that fix nitrogen, such as the rhizobia in legume root nodules, rely on nitrogenase enzymes containing iron and molybdenum, OpenStax says. Its anatomy text lists molybdenum as a cofactor for human enzymes, found in legumes, whole grains and nuts, with deficiency rare.

Which country produces the most molybdenum?

China. The US Geological Survey estimates 2024 mine output at 110,000 metric tons of molybdenum out of a world total near 260,000. Peru, Chile, the United States and Mexico follow, and those five countries together supplied 90 per cent. China also holds about 5.9 million of the world's 15 million metric tons of reserves, ahead of the United States with 3.5 million and Peru with 1.9 million.

Is molybdenum toxic?

The metal has low toxicity in animals and people, according to Haz-Map in PubChem's safety summary. The Agency for Toxic Substances and Disease Registry adds that people meet molybdenum almost only in compounds, since the pure metal is not found in nature. Of 1,523 company reports to European regulators, 57.4% assign the metal no hazard. Among reports that name a hazard, 30% class it as a flammable solid and 29.5% as a suspected threat to fertility or an unborn child.

How is molybdenum different from tungsten?

The two sit in group 6 and behave alike in water, where OpenStax finds both mostly in the +6 oxidation state. Both also form sulfides with the metal at +4 or +6. Los Alamos describes molybdenum as very hard, but softer and more ductile than tungsten, which melts higher. Haz-Map notes the resemblance goes into the body: excess tungsten absorbed may work against molybdenum's action as a trace nutrient.