Group 9

Group 9 is one of the columns (groups) of the periodic table of chemical elements. It contains cobalt (Co), rhodium (Rh), iridium (Ir), and meitnerium (Mt). All are transition metals, meaning their electrons occupy an incomplete d-shell.

The designation 'Group 9' follows the IUPAC numbering system in which columns are numbered 1 to 18. In older systems, cobalt, rhodium, and iridium were classified under a broader group called 'Group VIII' or the 'iron group', which included columns 8, 9, and 10.

Members

Cobalt is a hard, magnetic metal found in ores containing nickel and copper. Rhodium and iridium belong to the platinum group metals; both are rare, highly resistant to corrosion, and expensive. Meitnerium is a synthetic element produced in tiny quantities of atoms; its chemical properties are not fully understood.

General properties

Elements in this group typically display multiple oxidation states, including +2 and +3 for cobalt, while rhodium and iridium more commonly show +1 and +3. They form coordination complexes important in coordination chemistry. Melting and boiling points generally increase on descending the group, with iridium being one of the most refractory elements known.

Applications

Cobalt is used in alloys for high-temperature resistance, in blue dyes for ceramics and glazes, and in lithium-ion battery electrodes. Rhodium is widely employed in automobile catalytic converters. Iridium is used in high-temperature applications, such as spark plugs and crucibles for crystal growth.