why CO exists?

CO exists in the real world if the oxygen content is not enough when burning carbon.

It is formed by 2 covalent bonds (i.e. double bond) and one dative covalent bond, which the oxygen atom donates 2 electrons.

As you can see, the structure is highly unstable in the sense that it will react with O2 to form CO2 if there is adequate oxygen content.

Practical examples:
1. Someone committed a suicide by burning carbon in a closed room. It is because burning carbon reacts with oxygen to form CO2 as long as oxygen is enough. In a closed room, oxygen will be used up and hence CO will be formed. The death is due to CO poison. CO will bind with haemoglobin to form carbmino-haemoglobin that blocks the binding of oxygen. Without oxygen, people will die.

The electronic structure of CO showing the electrons in outermost shells is shown below :

圖片參考：http://home.lovehk.com/ernest_hml/CO.jpg


There are 4 outermost electrons in a C atom (in red) , and 6 outermost electrons in an O atom (in blue).
In CO molecules, both C and O atoms have 8 electrons in the outermost shell. C and O atoms are held together by a C≡C triple bond, in which 2 electrons are given from the C atom and 4 electrons are from the O atom.