Question

In which of the following species the bond angle around the central atom is equal to ${120^{\text{o}}}$
A.$B{F_3}$
B.$BC{l_3}$
C.$B{F_4}$
D.$S{O_3}$

Answer 1

To answer this question you must recall the VSEPR theory. The Valence shell electron pair repulsion theory proposes that the hybridized orbitals in an atom arrange themselves in such a way so as to minimize the repulsion between them, hence determining the geometry of a molecule on the basis of its hybridization. The bonds will be at ${120^{\text{o}}}$ if the hybridization is $s{p^2}$

Complete answer:
First we consider $B{F_3}$ molecules.
Boron has atomic number 5 and has 3 valence electrons. One electron from its $2s$ orbital is excited to its $2p$orbital. The boron atom undergoes hybridization of $s{p^2}$ and forms three hybrid orbitals. The geometry of the molecule is trigonal planar and the bond angle around the central atom is equal to ${120^{\text{o}}}$
First we consider $BC{l_3}$ molecules.
Boron has atomic number 5 and has 3 valence electrons. One electron from its $2s$ orbital is excited to its $2p$ orbital. The boron atom undergoes hybridization of $s{p^2}$ and forms three hybrid orbitals. The geometry of the molecule is trigonal planar and the bond angle around the central atom is equal to ${120^{\text{o}}}$
Now we consider $B{F_4}$ molecules.
Boron is an electron deficient atom and has only three electrons present. It cannot form more than 4 covalent bonds. The fourth bond is formed by fluorine and it is a coordinate bond. There are 4 bonds in the molecule and the shape is tetrahedral.
Now we consider $S{O_3}$ molecule.
Sulphur has six valence electrons. It forms two bonds with each oxygen atom, one $\sigma$ and one $\pi$bond. Thus only three electrons are hybridized. The hybridization of the molecule is $s{p^2}$ and shape of the molecule is trigonal planar and the bond angle around the central atom is equal to ${120^{\text{o}}}$

Thus, the correct answer is A, B and D.

Note:
The concept of mixing of atomic orbitals in order to form new hybrid orbitals that possess different shapes and energies as compared to the original parent atomic orbitals is known as hybridisation. Hybrid orbitals are suitable to form chemical bonds of equal energies. Also hybridization of orbitals leads to the formation of more stable compounds because hybrid orbitals have lower energy than the unhybrid orbitals.