Question

Which of the following options is/are true?

• A. Energy gap between HOMO and LUMO is lower in $F_2$ than $B{r}_2$
• B. $N{H}_3>N{F}_3$ is the correct order of dipole moments
• C. $Al{F}_3$ is ionic while $AlC{l}_3$ is covalent
• D. $AlC{l}_3$ dimerizes while $BC{l}_3$ does not

Answer 1

Answer: (B), (C), (D)

The correct options are
B $N{H}_3>N{F}_3$ is the correct order of dipole moments
C $Al{F}_3$ is ionic while $AlC{l}_3$ is covalent
D $AlC{l}_3$ dimerizes while $BC{l}_3$ does not

(a) Down the group, the energy gap between HOMO and LUMO decreases for $X_2$, so the energy gap decreases in the order $F_2>C{l}_2>B{r}_2>I_2$.
(b) In case of $N{H}_3$, nitrogen is more electronegative than hydrogen,so it tries to pull the electrons from hydrogen atoms.
whereas in case of $N{F}_3$, fluorine being more electronegative than nitrogen tries to pull the electrons from nitrogen opposite from that of nitrogen.


(c) Fajan's rule states that with an increase in the size of the anion and with a decrease in the size of the cation, ionic character of the compound decreases while the covalent character increases.
Here, since the cation is the same in both the compounds, the size of the anion decides the nature of the compound. The size of chlorine is larger than fluorine, $AlC{l}_3$ is covalent while $Al{F}_3$ is ionic.
(d) The electron deficiency (octet) of Boron in $BC{l}_3$ is compensated by the formation of a co-ordinate bond between the lone pair of the peripheral chlorine atom and the empty unhybridised p-orbital of the boron atom, forming pπ–pπ bonding, so dimerisation does not occur.