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

Feb 22, 2016

$C O$ is a polar molecule and ${O}_{2}$ is a non polar molecule.

Explanation:

We can answer this question by looking at the polarity of both molecules.

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 ($\sum \vec{\mu} = 0$).

In a non polar molecule, the electron density is equally distributed over the atoms making the molecule.

However, $C \equiv O$ is a molecule that is made from two atoms of two different element, and therefore, the two atoms have different electronegativity. Thus, $C O$ is a polar molecule since the net dipole moment is not nil ($\sum \vec{\mu} \ne 0$).

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 $C O$.

Feb 22, 2016

Refer to the Explanation.

Explanation:

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 $\left(\Delta \text{EN}\right)$.

A $\Delta \text{EN}$ of $< 0.4$ indicates a nonpolar covalent bond. A $\Delta \text{EN}$ of $0.4 - 1.7$ indicates a polar covalent bond, and a $\Delta \text{EN}$ of $> 1.7$ indicates an ionic bond. Some authors have different guidelines, so you should always check with your instructor or your book.
http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx71/tx71.html

The electronegativity of an oxygen atom is 3.44. Since two oxygen atoms make up a molecule of oxygen $\left(\text{O"_2}\right)$, the difference in electronegativity is $\left\mid 3.44 - 3.44 \right\mid = 0$. A $\Delta \text{EN}$ of 0 means the bond is nonpolar covalent, and the electrons are shared equally.

The electronegativity of a carbon atom is 2.55. The $\Delta \text{EN}$ between carbon and oxygen is $\left\mid 2.55 - 3.44 \right\mid = 0.89$. 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 ${\delta}^{+}$.
https://en.wikipedia.org/wiki/Electronegativity#Pauling_electronegativity