Manganese, technetium and rhenium, situated in group 7 of the periodic table, are silvery-grey metals with high melting and boiling points. The melting point figures rise from 1246 degrees Celsius for manganese to 3186 degrees Celsius for rhenium, which has the highest melting point of any transition metal save for tungsten.
Manganese is a more reactive element than its fellow group members. It forms an oxide layer on exposure to air, and burns when in powdered form. It reacts with water, and dissolves in acids, and combines with the non-metallic elements - nitrogen, phosphorus, sulphur, silicon, carbon and the halogens - when heated.
Technetium and rhenium are more resistant to corrosion. They can be induced to oxidize in moist air, but only slowly, and are resistant to attack from hydrchloric and hydrofluoric acids. In powdered form, they become more reactive.
Both manganese and rhenium are very hard. The properties of technetium, a very rare, radioactive element, are often difficult to determine.
