Question #d4c10

The most common intermolecular forces in pure substances are, in decreasing order of strength:

#"Ion-ion > dipole-dipole > London dispersion"#

We get an idea of the strengths the forces by looking at the bond electronegativity differences #ΔEN# and the molecular geometries.

#bb"NaCl"#

For #"NaCl"#, #ΔEN = |3.16 - 0.93| = 2.23#.

#ΔEN >1.6#, so the bond is ionic.

The strongest intermolecular forces are ion-ion attractions, so #"NaCl"# is first in the list.

#"He"#

Helium is a noble gas. It does not form ionic or covalent bonds.

The only intermolecular forces in helium are London dispersion forces.

Helium is last in the list.

#bb"NH"_3# and #bb"NF"_3#

Both #"NH_3" and #"NF"_3# have a trigonal pyramidal geometry (the yellow ball represents a lone pair".

For #"NH"_3#, #ΔEN = |3.04 - 2.20| = 0.84#.

For #"NF"_3#, #ΔEN = |3.04 - 3.98| = 0.94#.

Thus, we would predict the compounds to have about the same polarities, with #"NF"_3# perhaps being slightly more polar.

However, #"NH"_3# is much more polar than #"NF"_3#.

The reason is that the lone pair itself contributes a dipole to the molecule.

In #"NH"_3# the bond dipoles reinforce that of the lone pair, while in #"NF"_3# the bond dipoles oppose it.

Thus, #"NF"_3# is less polar than #"NH"_3#.

The order of polarity for all the compounds is

#"NaCl" > "NH"_3 > "NF"_3 > "He"#.