IMAT 2018 Q47 [Polarity]

Which row correctly shows the shape of each molecule and whether the molecule is polar (i.e. has a permanent dipole moment) or non-polar?

[Atomic numbers: H = 1; B = 5; C = 6; N = 7; O = 8; F = 9; P = 15; Cl = 17]

image

A. row 3
B. row 1
C. row 4
D. row 2
E. row 5

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Polarity: Polarity is determined by the shape of the molecule (is it asymmetrical or symmetrical) and electronegativity within the molecule. A polar molecule is one with an unequal distribution of charge, so it means the electrons within the molecule will spend more time towards one pole than another, creating regions of partial positive and partial negative (net-dipole). Symmetry is important because, even if there is a big enough difference in electronegative, if the molecule is symmetrical then the attraction towards the poles is cancelled out by each other.

Electronegativity Scale
0.0-0.4 (non-polar - covalent bond)
0.4-1.7 (polar - covalent bond)
greater than (>) 1.7 (Ionic bond)

Molecular shape:

The molecular shape of a molecule is based on the number of molecules bonded to a central atom. Since these molecules want to be in a favourable position since there is repulsion within these molecules, they take certain configurations depending on how many atoms are present.

There is also electron geometry, which is almost the same as molecular geometry but it includes the electron groups like lone pairs on the central atom (they give further repulsion).

For more info on geometry, search for a table online, you will need to memorize the different shapes.

Now onto our question:


Here we have the electronegativities and any dipole moments labelled in purple. If there is an equal pull, then the dipoles are cancelling each other out.

As we can see, PCl_5 has a trigonal bipyramidal shape and is non-polar.

\fcolorbox{red}{grey!30}{Therefore the answer is E, row $5$.}