Cobalt (Co)

Cobalt sits at the centre of vitamin B12, the only vitamin containing a metal, and the only one animals cannot get from plants — it is made by bacteria. Its blue compounds have coloured glass and pottery since at least ancient Egypt, and the name comes from Kobold, the German mine goblin blamed by miners whose "silver" ore smelted into worthless, arsenic-fuming slag. Roughly 70% of world supply now comes from the Democratic Republic of the Congo, a concentration that makes battery supply chains nervous.

Properties

Atomic number

27

Symbol

Co

Standard atomic weight

58.933 u

Category

Transition metal

Group

9

Period

4

Block

d-block

Electron configuration

[Ar] 4s² 3d⁷

Electrons per shell

2, 8, 15, 2

Melting point

1,768 K — 1494.8 °C — 2722.7 °F

Boiling point

3,200 K — 2926.8 °C — 5300.3 °F

Density

8.86 g/cm³

Electronegativity

1.88 (Pauling)

Oxidation states

+2, +3

Discovered

1735 by Georg Brandt

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷

Noble gas shorthand

[Ar] 4s² 3d⁷

Shells

K: 2 · L: 8 · M: 15 · N: 2

What it is used for

• Lithium-ion cathodes in phones, laptops and many electric cars
• Samarium-cobalt and alnico permanent magnets
• The binder that holds tungsten carbide cutting tools together
• Cobalt blue pigment in glass, ceramics and paint
• Cobalt-60 gamma sources for sterilising medical equipment and radiotherapy

Hazards

• Cobalt dust and hard-metal fume cause lung disease
• Cobalt-60 is a strong gamma emitter; orphaned sources have caused fatal accidents

Neighbours in the table

Group 9

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Cobalt sits in group 9 with rhodium, iridium, meitnerium.