Which molecules have an overall dipole moment? 1 carbon monoxide, CO 2 phosphine, PH3 3 carbon dioxide, CO2

Question 31 [Chemical Bonding > Covalent bonding and co-ordinate (dative covalent) bonding including shapes of simple molecules]

Which molecules have an overall dipole moment?

1 carbon monoxide, CO

2 phosphine, PH3

3 carbon dioxide, CO2

Reference: Past Exam Paper – Chemistry (9701) March 2016 Paper 12 Q31

Solution:

  Answer: B

   Firstly, to begin elimination, remove CO₂. The molecule is linear AND symmetrical. Therefore the dipoles of the otherwise polar C=O cancel out.

Coming to Carbon Monoxide, CO, it contains only one polar carbon-oxygen bond and therefore is a polar molecule.

PH₃ was the one molecule that confused the majority of the candidates. DO NOT make the mistake of assuming that all the three bonds are equally spaced, at 120°. If that were the case then the molecule would have been symmetrical and ultimately non polar. However the first protocol in dealing with an unknown compound is, draw it. You will notice that there is still a lone pair of electrons on the central P atom; this would distort the symmetrical shape and thus, PH₃ has an overall dipole.