Manganese

Manganese is a silvery-gray metal, hard and brittle, that resembles iron and is often found in minerals alongside it. This transition metal, with the symbol Mn and atomic number 25, was first isolated in the 1770s. It is widely used in industrial alloys, especially stainless steels, where it boosts strength, workability, and resistance to wear. Manganese oxides serve as oxidizing agents, rubber additives, and ingredients in glass, fertilizers, and ceramics, while manganese sulfate works as a fungicide.

The element is essential for human health, playing a role in macronutrient metabolism, bone formation, and the body's defense against free radicals. It is a key part of dozens of proteins and enzymes, and is stored mainly in bones, but also in the liver, kidneys, and brain. In the brain, manganese binds to metalloproteins like glutamine synthetase in astrocytes. A deep violet salt, potassium permanganate, is a common laboratory oxidizer and is used as a biocide in water treatment. Manganese also sits at the active sites of certain enzymes; notably, a manganese-oxygen cluster called the oxygen-evolving complex is critical for oxygen production in plants.

Physically, manganese is hard, brittle, difficult to melt, and oxidizes easily. It and its common ions are paramagnetic. It tarnishes slowly in air and rusts in water with dissolved oxygen, much like iron. Naturally occurring manganese has just one stable isotope, ⁵⁵Mn. Numerous radioisotopes, from ⁴⁶Mn to ⁷²Mn, have been identified; the most stable are ⁵³Mn (half-life 3.7 million years), ⁵⁴Mn (312.08 days), and ⁵²Mn (5.591 days). All other radioactive isotopes have half-lives under three hours, most under a minute. Lighter isotopes decay mainly by electron capture, heavier ones by beta decay. Manganese also has three meta states. It belongs to the iron group of elements, thought to be synthesized in massive stars just before supernova explosions. ⁵³Mn decays to ⁵³Cr with a 3.7-million-year half-life, and is rare, produced by cosmic rays hitting iron. Manganese and chromium are found together often enough that their isotopic ratios are used in geology to date the early Solar System; Mn-Cr ratios support evidence from ²⁶Al and ¹⁰⁷Pd. Variations in ⁵³Cr/⁵²Cr and Mn/Cr ratios in meteorites suggest an initial non-zero ⁵³Mn/⁵⁵Mn ratio, meaning chromium isotopic variations come from in situ decay of ⁵³Mn in differentiated planetary bodies, providing evidence for nucleosynthesis just before the Solar System formed.

Solid manganese has four allotropes—α, β, γ, and δ—each stable at successively higher temperatures. All are metallic at standard pressure with cubic crystal lattices, but their atomic structures vary widely. α-Mn is the room-temperature phase, with a body-centered cubic lattice and a complex unit cell of 58 atoms (29 per primitive cell) in four different site types. It is paramagnetic at room temperature and antiferromagnetic below 95 K. β-Mn forms above 973 K and has a primitive cubic structure with 20 atoms per unit cell at two site types, as complex as any elemental metal. It can be metastable at room temperature if rapidly quenched from 850 °C in ice water, and remains paramagnetic down to 1.1 K. γ-Mn forms above 1,370 K with a simple face-centered cubic structure (four atoms per cell). Quenching it to room temperature converts it to β-Mn, but alloying with at least 5% of elements like carbon, iron, nickel, copper, palladium, or gold stabilizes it, distorting it into a face-centered tetragonal structure. δ-Mn forms above 1,406 K and is stable up to the melting point of 1,519 K, with a body-centered cubic structure (two atoms per cell).

Common oxidation states for manganese are +2, +4, and +7, though all from −3 to +7 have been observed. The +7 state is seen in purple permanganate salts (MnO₄⁻). Potassium permanganate is a common lab oxidizer and topical medicine, used for fish diseases, and was among the first stains and fixatives for electron microscopy. Other Mn(VII) compounds include unstable Mn₂O₇ and powerful oxidizing oxyhalides (MnO₃F and MnO₃Cl). The +6 state appears as the green manganate anion (MnO₄²⁻), which is an intermediate in extracting manganese from ores.