Question

Which molecules have an overall dipole moment?
1. Carbon Monoxide, $ CO $
2. Phosphine, $ P{H_3} $
3. carbon dioxide, $ C{O_2} $
(A) 1,2,3 are correct
(B) 1 and 2 only are correct
(C) 2 and 3 only are correct
(D) 1 only is correct

Answer 1

Hint: As we know that there is some relation between polarity and an overall dipole moment. If the polarity of any compound is zero then the compound hasn’t any dipole moment. Overall dipole moment means the compound's structure is not symmetrical.

Complete answer:
When there is a charge separation, dipole moments occur. Dipole moments are caused by changes in electronegativity and can occur between two ions in an ionic bond or between atoms in a covalent binding. The dipole moment is a measurement of a molecule's polarity.
Firstly, to begin elimination, remove $ C{O_2} $ . The molecule is linear and symmetrical as well. Therefore the dipoles of the otherwise $ C = O $ cancel out. So, carbon dioxide is nonpolar and it has no dipole moment.
In the case of Carbon monoxide, $ CO $ : Despite the stronger electronegativity of oxygen, theoretical and experimental research demonstrate that the dipole moment points from the more negative carbon end to the more positive oxygen end. The three bonds are polar covalent bonds with a high degree of polarisation. Therefore, carbon monoxide has an overall dipole moment.
In the case of phosphine, $ P{H_3} $ : It forms dipole-dipole because it is a polar molecule. Here is why: $ P{H_3} $ is called phosphine and it is quite toxic and flammable. $ P{H_3} $ must be polar since it is not symmetrical. Therefore, phosphine also has an overall dipole moment.
Thus, carbon monoxide and phosphine both have an overall dipole moment, whereas carbon dioxide doesn’t have an overall dipole moment.
Hence, the correct option is (B) 1 and 2 only are correct.

Note:
Do not make the mistake of assuming that all the three bonds are equally spaced in phosphine, at $ {120^\circ } $ . If this was the case, the molecule would have been symmetrical and, as a result, non-polar. When dealing with an unknown compound, the first rule is to draw it.