Question

Give reason :$AlC{l_3}$ is an electron deficient compound but $Al{F_3}$ is not. This is because
A. Atomic size of F is smaller than Cl which makes $Al{F_3}$ more covalent
B. $AlC{l_3}$ is a covalent compound while $Al{F_3}$ is an ionic compound.
C. $AlC{l_3}$ exists as a dimer but $Al{F_3}$ does not.
D. Al in $AlC{l_3}$ is in $s{p^3}$ hybrid state but Al in $Al{F_3}$ is in $s{p^2}$ state.

Answer 1

This question is based on Fajan’s rule. Fajan’s rule states that with increasing the size of anion and decreasing the size of cation, the ionic character of the compound decreases while the covalent character increases. Or simply smaller the size of cation or grater the size of anion more will be the covalent character.

Complete step by step answer:
Electronic configuration of Al: 2, 8, 3
According to Fajan’s rule increasing the size of anion and decreasing the size of cation ionic character of the compound decreases while the covalent character increases. Or simply smaller the size of cation or grater the size of anion more will be the covalent character.
It means the nature of compounds depends upon the size of cation and anion.
Now in this example cation is similar that is aluminium (Al) while anion is different that is in $AlC{l_3}$ anion is chlorine ion while in $Al{F_3}$ anion is fluorine ion. So the nature of $AlC{l_3}$ and $Al{F_3}$ depends upon the size of anion only.
Now, the size of chlorine is more than fluorine so according to Fajan’s rule greater size of anion more is covalent character that is $AlC{l_3}$ is covalent in nature while $Al{F_3}$ is ionic.
So, in $Al{F_3}$ central element Al is in +3 state. So its configuration is
Al (+3): 2, 8
Since there are 8 electrons in the valence shell of the central atom in $Al{F_3}$ so $Al{F_3}$ is not an electron deficient compound.
Hence, the correct answer is option A.

Note: The molecule in which the central atom has less than 8 electrons that is this electron deficient compound called hypervalent compound. Example: $AlC{l_3},BC{l_3}$
The molecule in which the central atom has more than 8 electrons is this electron rich compound called hypervalent compound. Example: $I{F_7},{H_2}S{O_4}$