Why are electrons shared equally in oxygen, #O_2# but not in carbon monoxide #CO#?

Oxygen, #O=O# is a diatomic molecule that is made from two atoms of the same element, and therefore, both atoms have the same electronegativity. Thus, #O_2# is a non polar molecule since the net dipole moment is equal to zero (#sumvecmu=0#).

However, #C-=O# is a molecule that is made from two atoms of two different element, and therefore, the two atoms have different electronegativity. Thus, #CO# is a polar molecule since the net dipole moment is not nil (#sumvecmu!=0#).

In a polar molecule, the electron density is pulled by the more electronegative atom (which is oxygen), and therefore, the electron density is not distributed evenly on both atoms of the molecule #CO#.

Every element has a property called electronegativity (EN), which is the tendency of a bonded atom to attract electrons to itself. Bond character, such nonpolar and polar covalent, and ionic, is determined by the difference in electronegativities #(Delta"EN")#.

A #Delta"EN"# of #<0.4# indicates a nonpolar covalent bond. A #Delta"EN"# of #0.4-1.7# indicates a polar covalent bond, and a #Delta"EN"# of #>1.7# indicates an ionic bond. Some authors have different guidelines, so you should always check with your instructor or your book.
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The electronegativity of an oxygen atom is 3.44. Since two oxygen atoms make up a molecule of oxygen #("O"_2")#, the difference in electronegativity is #abs(3.44-3.44)=0#. A #Delta"EN"# of 0 means the bond is nonpolar covalent, and the electrons are shared equally.

The electronegativity of a carbon atom is 2.55. The #Delta"EN"# between carbon and oxygen is #abs(2.55-3.44)=0.89#. Therefore the bond is polar covalent and the electrons are not shared equally. The oxygen end will have a partial negative charge, designated by # delta^-#, and the carbon end will have a partial positive end, designated by #delta^+#.
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