Question

Valency of $Boron$ in $B{F_3}$ molecules is ________
A) Zero
B) One
C) two
D) three

Answer 1

Group- $13$ have the following elements- $Boron,Alu\min ium,Gallium,Indium,Thallium$ and the most common oxidation state among them is $+ 3$ . They are also known as p-block elements. These metals mostly have non-metallic character.

Complete step by step solution:
$Boron$ , is the 1st member of Group- $13$ . It belongs to the p- block family.
The elements of the p-block family are non-metals as they have high electronegativities, their size increases as we move down the group so the value of ionization energy decreases.
Non-metals are responsible for the formation of acidic hydroxides.
In the halogen family, $Fluorine$ is the 1st member and also it is the most electronegative atom in the entire periodic table.
The compound is covalent in nature. It is a colourless gas with boiling point $- {101^\circ }C$ . It can be prepared by
${B_2}{O_3} + 3Ca{F_2} + conc.3{H_2}S{O_4} \to 2B{F_3} + 3CaS{O_4} + 3{H_2}O$
Also there is another method-
${B_2}{O_3} + 6N{H_4}B{F_4} \to 8B{F_3} + 6N{H_3} + 3{H_2}O$
This compound has a trigonal planar structure, with bond angle ${120^\circ }$ which is predicted by $VSEPR$ theory. The bond lengths are $1.30A^\circ$ which is shorter than the sum of covalent radii. The bond energy is very high $646KJmo{l^{ - 1}}$ which is due to the strong $p\pi - p\pi$ bond which induces a double bond like character.
Also halogens do not show a positive oxidation state due to high electronegativities.
Hence the oxidation state of $Fluorine$ here will be $- 1$ .
Now calculating the oxidation state $Boron$ in $B{F_3}$ , let’s say it is “a”
$\begin{gathered} a + 3 \times ( - 1) = 0 \\\ \Rightarrow a - 3 = 0 \\\ \Rightarrow a = + 3 \\\ \end{gathered}$
The electronic configuration of $Boron$ in $+ 3$ oxidation state $1{s^2}2{s^0}2{p^0}$
$\therefore$ $B{F_3}$ Has three bond pairs and thus the Valency is three.
Correct option is (D).

Note:
$Boron$ also forms trihalides with chlorine, bromine and iodine. $BC{l_3}$ , $BB{r_3}$ are liquid unlike $B{F_3}$ whereas $B{I_3}$ is a solid compound. $Boron$ Has electron deficiency in the trihalides thus it can be used as Lewis acids in many chemical reactions.