Group 6: the chromium group

Six electrons and the +6 state

Chromium ([Ar] 3d⁵ 4s¹) and molybdenum ([Kr] 4d⁵ 5s¹) put one electron in the s orbital and five in the d subshell, because a half-filled d subshell is especially stable. Tungsten is [Xe] 4f¹⁴ 5d⁴ 6s². Either way there are six electrons beyond the core, and all three metals reach +6, as in CrO₃, MoO₃ and WO₃.

Chromium also forms Cr²⁺ and Cr³⁺, and +3 is its most stable state. Its +6 compounds, such as yellow chromate (CrO₄²⁻) and orange dichromate (Cr₂O₇²⁻), are strong oxidizers and toxic. Down the group +6 becomes more stable, so molybdenum and tungsten sit comfortably at +6, the reverse of main groups such as group 14, where lead prefers +2.

Trends down the group

Melting point and density climb steeply. Chromium melts at 2180 K, molybdenum at 2896 K and tungsten at 3695 K. Density rises from 7.19 g/cm³ to 10.28 g/cm³ and then to 19.25 g/cm³, almost exactly the density of gold (19.3 g/cm³). Unusually, electronegativity also increases down the group, from 1.66 to 2.16 and 2.36, while in most main groups it falls.

Where you meet group 6

Chromium gives stainless steel its rust resistance by forming an invisible oxide film, and it puts the mirror shine on chrome-plated faucets. Traces of chromium make rubies red and many emeralds green, fitting for an element whose name comes from the Greek word for color. Molybdenum strengthens steel for pipelines and tools, molybdenum disulfide is a slippery lubricant, and the bacteria that turn nitrogen from the air into ammonia usually rely on a molybdenum enzyme. Tungsten carbide tips drill bits and saw blades, and tungsten wire glowed in old incandescent bulbs.

Seaborgium

Seaborgium (106) is radioactive and has been made only a few atoms at a time. It was named after the chemist Glenn Seaborg while he was still alive. Single-atom experiments show it forms volatile compounds like those of molybdenum and tungsten, but its melting point and density have never been measured.