Question

If $AlC{{l}_{3}}$ is an electron deficient compound but $Al{{F}_{3}}$ is not. This is because:
A. The 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 dimer but $Al{{F}_{3}}$ does not.
D. Al in $AlC{{l}_{3}}\,is\,s{{p}^{3}}$hybrid state but Al in $Al{{F}_{3}}$is in $s{{p}^{2}}$state.

Answer 1

Electron deficient compounds are those in which the central metal atom is with incomplete octet. Most covalent compounds are electron deficient because in ionic compounds an atom can donate its electron to others, so as to make an electron precise or electron rich octet. This is not possible in case of covalent bond.

Complete step-by-step answer:
In this question we have to assign a suitable reason why $AlC{{l}_{3}}$ is electron deficient and $Al{{F}_{3}}$ not. Firstly, we need to identify which compound from both is less covalent or more ionic and which one is more covalent.

To identify this, we will use the Fajan’s rule. This rule states that “The smaller the size of the cation and the larger the size of the anion, the greater the covalent character in the bond. In this question, since we can see that the size and charge of the cations of both the given compounds are the same, the size of the anion determines the character. And we have already seen that as per fajan’s rule, the greater the size of anion the greater the covalent character.
In the compounds $AlC{{l}_{3}}\And Al{{F}_{3}}$,Chlorine $\left( Cl \right)$and fluorine $\left( F \right)$are the respective anions.

We know that both of them belong to group 17, Fluorine being at the top and then chlorine below it. And in a group the size increases from top to bottom. Therefore, Chlorine will have larger size as compared with Fluorine. Hence, by fajan’s rule $AlC{{l}_{3}}$ has more covalent character, as compared to $Al{{F}_{3}}$ which has less covalent and more ionic character.

Since, $Al{{F}_{3}}$ has ionic character or it forms an ionic bond, it cannot be electron deficient. As in ionic bonds we know that one atom donates electrons to another. Similarly, in these compounds fluorine after completing its own octet, donates a few electrons to Aluminum to complete its octet and hence the compound becomes stable.

Such donation of electrons is not possible in covalent bonds, as this type of bond involves only sharing of electrons and hence the central metal remains electron deficient and in turn the whole compound.

Hence, the correct answer is option ‘B’

Note: If we understand the fajan’s rule then it states that a compound can have both the ionic or covalent character. But in this question, we have done a relative comparison, so as to know which compound has more covalent character and less ionic character or vice versa.