Assorted AP Chemistry -- radii, formal charges, & intermolecular forces

Question

a.) Using principles of atomic structure, explain why the atomic radius of Kr is less than the ionic radius of Se²⁻.

The boiling point of SeOF2 (diagram D) is less than the boiling point of SeOCl2 (diagram E).

f) Identify the type(s) of intermolecular force(s) that the two substances have in common. What about them?

g.) Explain the difference in boiling points based on the types and relative strengths of intermolecular forces.

Answer 1

[Kr] as the same electron configuration of Se[2-], however Krypton has a greater effective nuclear charge, so --let's say--in a manner of speaking--as you 'add' an electron onto a Selenium atom (into the n=4 level), the new 4p5 electron feels a weaker effective pull from the protons in the nucleus of Selenium than the 4p5 does from Krypton's more proton-heavy nucleus

(recall in neutral atoms, the greater effective charge going right along a period shows additional valence electrons are held closer to the nucleus, smaller atomic radii left to right, and that this was attributed to the greater effective nuclear charge felt as a result of one more proton being in the nucleus and being felt by an electron placed at roughly the same distance for a given n shell). . . .

The same idea goes for a hypothetical addition of one electron into the 4p6 orbital position to give Se[2-]:

that is, it feels a weaker net effective charge compared to the Krypton atom's 4p6 electron (primarily due to the relatively comparable distance of the 4p orbitals with Krypton having greater total proton/positive charge in the nucleus...

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(NEXT + (f) + (g) COVERED BELOW)

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Best guess for the following I could offer that would be based on reasoning YOU are responsible to know rather than knowledge previously gained knowledge I actually have would go as follows:

The dipole moments of the sp2 hybridized SeOX2 molecules you show may be very roughly equal---other than dipole-dipole (Permanent dipole) intermolecular forces, only the van der walls (INDUCED dipole-dipole) interactions are present in these molecules...... generally speaking.

A very small difference between the measure of the strength of the former (permanent dipole-dipole interactions) might not account for an appreciably greater or lesser boiling point of one compared to the other.... the other salient intermolecular force governing the bp (the induced dipole-induced dipole--the van der waals SA/NP force) would therefore have to carry the load in this explanation . . .

Here, the greater polarizability of the chlorine analogue of this molecule may be the key to answering why there is what might be considered a partly unexpected order of boiling points here.

In other words, a permanent dipole dictates electrostatic attraction between nearby O=Se(F)2 molecules in a pure sample at a strength that is on the same order as the attraction due to the same force between nearby O=Se(Cl)2 molecules in its pure sample,

(( I do know the dipole moments, for instance, of CF2=CH2 and CCl2=CH2---are 1.38 and 1.3 respectively, and approximately))

If THIS dipole magnitude is not enough to warrant a greater boiling point in the former compared to the later, then we have to invoke an alternative explanation. If we are told, in fact, that the OSe(Cl)2 has a higher boilling point, then we know for sure that either (A) xor (B) below MUST be true (one

(A) the chlorine analogue has a stronger dipole moment, and that explains the higher boiling point

(greater permanent dipole-dipole interaction)

XOR

(B) the fluorine analogue has slightly strong dipole moment which is not appreciably larger than the chlorine analogue's; However, the PERMANENT DIPOLE-INDUCED DIPOLE, and PERMANENT INDUCED-DIPOLE--INDUCED DIPOLE interactions would be expectted to be greater (greater attraction) in the case of the chlorine analogue due to the greater polarizability (lesser charge distribution) ....

GOOD LUCK WITH THAT ONE.