Question

Why is `CO_2` a nonpolar molecule, whereas `CO` is a polar molecule?

Answer 1

Consider the vector sum of the individual bond dipoles........

Explanation:

The `C-O` bond in both gases is polar to a certain, limited extent. `""^(+delta)C-=O^(delta-)` has some polarity; on the other hand, for `""^(delta-)O=^(delta+)C=O^(delta-)`, the vectors of the bond dipoles clearly sum to ZERO.

This polarity is not reflected in the boiling points of the 2 gases, `-191.5^@` for `CO` at `1*atm`, versus a normal sublimation point of `-78^@C` for carbon dioxide, however, carbon dioxide is a bigger molecule, with extra dispersion forces.