chemical element with symbol Co and atomic number 27

Cobalt (chemical symbol Co) is a chemical element. It has an atomic number of 27 and an atomic mass of about 59. It is a metal.[1]

Sheet of cobalt metal


Cobalt(II) chloride with water
Cobalt(II) chloride without water

Cobalt is a transition metal. It is shiny and conducts electricity. It is magnetic. It is a hard metal. It is moderately reactive. Iron is more reactive and copper is less reactive. It dissolves slowly in acids. This reaction makes hydrogen and a salt of cobalt. Cobalt is normally in its +2 oxidation state as an ion. Some chemical compounds contain cobalt ions in its +4 oxidation state. Cobalt(II) chloride is one of the most common cobalt compounds. Many cobalt compounds are blue or pink. One of them is black.

Chemical compounds


Cobalt comes in two oxidation states, +2 and +3. Most soluble cobalt compounds are red. They can also be green, blue, brown, and black.

Cobalt(II) compounds
Mixed oxidation state
Cobalt(III) compounds

Occurrence and preparation


Cobalt is too reactive to occur as a pure element in the earth. It is found in certain minerals. It is found with copper and nickel deposits. Normally the three metals are bonded to arsenic and sulfur. The majority of cobalt mining is in central Africa.

It is found as a byproduct (left over substance) when copper and nickel are produced. It is made by reaction with the sludge from copper and nickel processing.

Glass colored with cobalt. Cobalt is most commonly associated with the color blue and gray. Blue is shown here.

Cobalt is used in some types of steel. It hardens the steel. It is also used to make very strong tough alloys. These alloys are known as superalloys. Some cobalt compounds are used in the lithium-ion battery. Cobalt compounds were used as an artificial food coloring until 1971. It was discovered that it has harmful effects. It is used to make glass blue. It is also used as a catalyst, and in some medicines, and for ink, pigments, dyes, and varnishes.

The human body needs small amounts of cobalt for certain vitamins. Cobalt compounds are used to stop cyanide from poisoning the body.



People need cobalt compounds in small amounts, but cobalt is toxic in large quantities. Sometimes cobalt compounds were added to beer, and people that drank it were poisoned. It can cause skin irritation when touched.


  1. "How many valence electrons does Cobalt (Co) have?".