Question

The shapes and hybridisation of \(\mathrm{BF}_{3}\) and \(\mathrm{BH}_{4}^{-}\) respectively are (a) \(\mathrm{BF}_{3}\) - Trigonal, sp \(^{2}\) hybridisation ; \(\mathrm{BH}_{4}^{-}\)- Square planar, sp \(^{3}\) hybridisation (b) \(\mathrm{BF}_{3}\) - Triangular, \(s p^{3}\) hybridisation ; \(\mathrm{BH}_{4}^{-}\)- Hexagonal, \(s p^{3} d\) hybridisation (c) \(\mathrm{BF}_{3}\) - Trigonal, sp \(^{2}\) hybridisation ; \(\mathrm{BH}_{4}^{-}\)- Tetrahedral, \(s p^{3}\) hybridisation (d) Tetrahedral, \(s p^{3}\) hybridisation ; \(\mathrm{BH}_{4}^{-}\)- Tetrahedral, \(s p^{3}\) hybridisation.

Short answer

The correct shapes and hybridisations are: BF3 - Trigonal, sp^2 hybridization and BH4- - Tetrahedral, sp^3 hybridization, which is option (c).

Step-by-step solution

Determine the shape and hybridization of BF3

Count the number of electron groups (bond pairs and lone pairs) around the central boron atom. In BF3, boron has three bond pairs and no lone pairs, making a total of three electron groups. According to the VSEPR theory, three electron groups arrang in a trigonal planar shape to minimize repulsion, which requires sp^2 hybridization.

Determine the shape and hybridization of BH4-

Similarly, count the electron groups around the central boron atom in BH4-. Boron has four bond pairs and no lone pairs, since it carries a negative charge indicating an additional electron, leading to four electron groups. Following VSEPR theory, these four electron groups form a tetrahedral shape to minimize repulsion, which correlates with sp^3 hybridization.

Match the determined shapes and hybridizations with the given options

BF3 has a trigonal planar shape with sp^2 hybridization, and BH4- has a tetrahedral shape with sp^3 hybridization. Option (c) is the only choice that corresponds with these determinations and is therefore the correct answer.

Key concepts

VSEPR theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used in chemistry to predict the geometry of individual molecules based on the number of electron pairs surrounding their central atoms. The theory posits that electron pairs will arrange themselves in space to be as far apart as possible to minimize the repulsion between negative charge clouds. Understanding VSEPR theory is critical for predicting molecular shapes.

For instance, if a molecule like boron trifluoride (BF3) has three bonding pairs around the central boron atom, the electron pairs repel each other equally and adopt a trigonal planar structure to stay as distant as possible. This concept of electron repulsion dictates that a molecule's shape is directly influenced by its electronic structure.

sp2 hybridisation

Hybridization is the process of mixing atomic orbitals to create new hybrid orbitals suitable for the pairing of electrons to form chemical bonds in molecules. sp2 hybridization occurs when one s orbital mixes with two p orbitals to create three equivalent hybrid orbitals. This type of hybridization is seen in molecules where the central atom is bonded to three other atoms and has no lone electron pairs.

For example, in BF3, the boron atom undergoes sp2 hybridization to form three new hybrid orbitals that lie in the same plane, leading to a trigonal planar molecular shape. Each of the three fluorine atoms forms a sigma bond with the boron atom, utilizing one of the sp2 hybrid orbitals.

sp3 hybridisation

sp3 hybridization involves the combination of one s and three p orbitals to form four equivalent sp3 hybrid orbitals. This hybridization is typical in molecules where the central atom has four electron pairs, whether they are bonding pairs, such as those in methane (CH4), or a combination of bonding pairs and lone pairs, like in ammonia (NH3).

In the BH4- ion, the boron atom undergoes sp3 hybridization to accommodate four bonding pairs of electrons, resulting in a tetrahedral molecular geometry. Here, the negative charge indicates the addition of an extra electron, which requires the boron atom to expand its valence shell to form four sp3 hybridized orbitals.

Molecular shapes

Molecular shapes, also known as molecular geometries, are determined by the spatial arrangement of atoms bonded to a central atom. The shape of a molecule significantly influences its physical and chemical properties, including reactivity, polarity, and color. The VSEPR theory is instrumental in predicting the three-dimensional structures of molecules.

Some common shapes are linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. Each shape is associated with a specific hybridization state of the central atom. For example, trigonal planar molecules like BF3 exhibit sp2 hybridization, while tetrahedral molecules such as BH4- demonstrate sp3 hybridization.

Chemical bonding

Chemical bonding refers to the attractive forces that hold atoms together in molecules and compounds. There are three primary types of chemical bonds: ionic, covalent, and metallic. Covalent bonding occurs when atoms share one or more pairs of electrons, as seen in molecules like BF3 and BH4-.

In BF3, each boron and fluorine atom forms a sigma bond by the overlapping of sp2 hybridized and p orbitals, respectively. In BH4-, four sigma bonds form due to the overlap of boron's sp3 hybridized orbitals with hydrogen's s orbitals. These bonds create the stable compounds we observe with distinctive shape and hybridization in line with VSEPR theory.