Question

Which of the following molecular species/compounds is not hypovalent?
A. $CH_3^ + $
B. ${B_2}{H_6}$
C. $NH_2^ + $
D. $Al{F_3}$

Answer 1

Hint: The valency is the number of electrons that an atom required or to be donated for completing its octet. If an atom has less than 8 electrons in its outermost shell after bond formation, we can say it is a hypovalent. If the number of electrons is greater than 8 after the bond formation, then it is a hypervalent.

Complete step by step answer:
We can check each of the molecules one by one. In $CH_3^ + $, the number of electrons in the outermost electrons of carbon is 6. One atom is released from the carbon atom and the electrons from the last shell of carbon becomes 3 instead of 4. Then it gains 3 electrons from three hydrogen atoms and it becomes a total 6 electrons which is hypovalent.
In ${B_2}{H_6}$ molecules, it is formed by the dimerization of $B{H_3}$ molecules. In $B{H_3}$ also, there are only three electrons in boron and three electrons from hydrogen. Total electron count becomes 6 in the last shell, still it is hypovalent.
In $NH_2^ + $, from the total 5 electrons in the last shell of nitrogen, one electron is released and it becomes a positive charge. The remaining four electrons and the two electrons from hydrogen makes the electron count 7. Still it becomes hypovalent.
In the case of $Al{F_3}$, the oxidation number of aluminium is +3. The three electrons in the outermost electrons are donated to three monovalent chlorine atoms and complete its octet in the penultimate shell. So, $Al{F_3}$ is an electron precise molecule and not a hypervalent molecule.
Therefore the correct answer is option D.

Note:
The ${B_2}{H_6}$ molecule will form a bridge bond or banana bond by dimerization in order to overcome the electron deficiency. Still it will be counted as an electron deficient system.