Group 7: the manganese group

Seven electrons, from +2 to +7

The three lighter members all end in (n−1)d⁵ ns²: manganese is [Ar] 3d⁵ 4s², technetium [Kr] 4d⁵ 5s² and rhenium [Xe] 4f¹⁴ 5d⁵ 6s². With seven electrons to use, they show many oxidation states. Manganese is commonly +2, +3, +4 and +7: Mn²⁺ is its most stable ion, manganese dioxide (MnO₂) is +4, and permanganate is +7.

Technetium and rhenium favor +4, +6 and +7. Their pertechnetate (TcO₄⁻) and perrhenate (ReO₄⁻) ions have the same shape as permanganate but are much weaker oxidizers.

Trends down the group

Melting point and density rise sharply. Manganese melts at 1519 K, technetium at 2430 K and rhenium at 3459 K, one of the highest melting points of any element. Density goes from 7.21 g/cm³ to 11 g/cm³ and then 21.02 g/cm³, nearly three times that of manganese. Electronegativity rises from 1.55 for manganese to 1.9 for technetium and rhenium. Manganese itself is hard but brittle and almost never used on its own.

Where you meet group 7

Most manganese ends up in steel, where it removes sulfur and oxygen and makes the metal tougher. Manganese dioxide is inside ordinary alkaline batteries, and potassium permanganate is used to treat water. Plants need manganese to split water and release oxygen during photosynthesis. Rhenium goes into the nickel superalloys of jet engine turbine blades and into catalysts that help make high-octane gasoline.

The artificial element in the middle

Technetium is the lightest element with no stable isotopes. In 1937 it became the first element made artificially, and its name comes from the Greek for artificial. Its short-lived isotope technetium-99m is the most common tracer in nuclear medicine, used in tens of millions of scans every year. Rhenium, found in 1925, was the last stable element to be discovered. Bohrium (107), named after Niels Bohr, is known only from a few atoms; it has formed a compound like rhenium’s, but its physical properties are unknown.