Having said that, there is a significant difference between CO2
and XeF2, which is : in CO2, the central partially positively
charged C atom has no lone pairs, while the central partially
positively charged Xe atom has 3 lone pairs. Therefore the
(relative) magnitude of permanent dipole - permanent dipole
attractions will be expected to be more significant for CO2 than
for XeF2. And yet XeF2 has a much higher melting point than CO2,
because XeF2 has a lot more electrons, has much larger molecular
size, and therefore has significantly more polarizable electron
charge clouds.)
Why CO2's Pd-Pd attraction is relatively stronger than XeF2's
Pd-Pd attraction??
I thought XeF2 would have a larger net dipole moment than CO2,
hence more significant Pd-Pd than CO2??
Both XeF2 and CO2 are linear, and hence have no overall net dipole.
However, because C has no lone pairs, it is more electron-deficient
compared to Xe with 3 lone pairs. Hence, based on this factor
alone, you might expect C to be more partial positive compared to
Xe. However, as I pointed out years ago, the most important factor
remains polarizability of electron charge clouds, and Xe being in
Period 5, has a much larger atomic radius than C, as well as
greater shielding effect from greater number of inner shell
electrons, and as such, coupled with the greater electronegativity
of F over O, the magnitude of partial positive charge on Xe will
still be greater than on C. Consequently, the intermolecular van
der Waals interactions (all 3 types of van der Waals forces :
permanent dipole - permanent dipole Keesom forces, permanent dipole
- induced dipole Debye forces, and instantaneous dipole - induced
dipole London dispersion forces) will still be far stronger between
XeF2 molecules compared to between CO2 molecules.
Based on the simplified (and often erroneous) teachings of
Singapore JCs, almost all H2 Chem students (and even some JC
teachers) will be instantly dismissive of the idea of any permanent
dipole - permanent dipole intermolecular interactions between
(overall) non-polar molecules like CO2. And yet permanent dipole -
permanent dipole Keesom forces *does* exist between some non-polar
molecules like CO2, On a related note, during hydration which
occurs before hydrolysis, (overall) non-polar CO2 can actually even
participate in hydrogen bonding. I won't elaborate or explain why
here on the forum (where's the fun in that?), interested students
can go figure out for themselves (or ask their school teachers or
private tutors).
