Question

Question #d4c10

Answer 1

The order is `"NaCl" > "NH"_3 > "NF"_3 > "He"`.

Explanation:

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"`.