Question

In which of these molecules or ions does the presence of nonbonding electron pairs produce an effect on molecular shape? (a) \(\mathrm{SiH}_{4}\), (b) \(\mathrm{PF}_{3}\), (c) \(\mathrm{HBr}\), (d) \(\mathrm{HCN}\), (e) \(\mathrm{SO}_{2}\).

Short answer

In molecules (b) PF3 and (e) SO2, the presence of nonbonding electron pairs affects the molecular shape due to repulsion between the lone pairs and bonding pairs of electrons. In the other molecules, there are no nonbonding electron pairs on the central atom, so they do not have an effect on the molecular shape.

Step-by-step solution

(a) SiH4

: Silicon is the central atom with 4 hydrogen atoms bonded to it, creating a tetrahedral arrangement. The silicon atom has a total of 4 valence electrons which are entirely involved in bonding with the hydrogens. There are no lone pairs on silicon, so the molecular geometry is also a tetrahedron. The effect of nonbonding electron pairs is not seen.

(b) PF3

: In PF3, phosphorus is the central atom with 3 fluorine atoms bonded to it. Phosphorus has 5 valence electrons, 3 of which form bonds with the fluorine atoms, leaving behind 1 lone pair (2 nonbonding valence electrons). These lone pairs can distort the molecular shape due to their repulsion with the bonding pairs. The presence of nonbonding electron pairs has an effect on the molecular shape.

(c) HBr

: HBr is a linear diatomic molecule with no central atom. Hence nonbonding electron pairs do not have an effect on molecular shape.

(d) HCN

: In this linear molecule, carbon is the central atom with 2 bonding electron pairs: one with hydrogen and one with nitrogen. Carbon has 4 valence electrons, all of which are involved in bonding, leaving no nonbonding electron pairs. Hence, there is no effect due to nonbonding electron pairs on the molecular shape.

(e) SO2

: In SO2, sulfur is the central atom, and it is bonded to 2 oxygen atoms. Sulfur has 6 valence electrons, 4 of which are involved in the bonding with 2 oxygen atoms, leaving behind 1 lone pair (2 nonbonding valence electrons). These lone pairs lead to repulsion with the bonding pairs, distorting the molecular shape. The presence of nonbonding electron pairs has an effect on the molecular shape. In conclusion, the presence of nonbonding electron pairs has an effect on the molecular shape of (b) PF3 and (e) SO2.

Key concepts

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used in chemistry to predict the shape of individual molecules based on the number of electrons surrounding their central atoms. It operates under the principle that electron pairs around a central atom will organize themselves to minimize repulsion between each other. This repulsive force mainly arises because electron pairs are negatively charged and like charges repel.

The arrangement of electron pairs is determined first by totaling the number of bonding (shared with other atoms) and nonbonding (lone pairs) electron pairs. Then, VSEPR theory predicts the geometric distribution of these pairs to minimize repulsion and determine the molecular shape. The shapes can range from linear, trigonal planar, tetrahedral, to more complex geometries such as trigonal bipyramidal and octahedral, depending on the number of electron pairs.

Lone Pair Repulsion

Lone pair repulsion is a concept within the VSEPR theory that explains how nonbonding electron pairs can influence the shape of a molecule. When electrons are in lone pairs, they occupy more space around the central atom than bonding pairs because they are solely under the influence of one nucleus, as opposed to bonding pairs which are 'shared' between two nuclei.

As a result, lone pairs exert greater repulsive forces on each other and on bonding pairs of electrons. This increased repulsion can lead to distortions in the idealized molecular shape. For instance, in a molecule with one lone pair, you might expect the bonds to deviate from their positions when compared to a similar molecule with all bonding pairs. Understanding how the presence and number of lone pairs affect molecular structure is crucial in predicting how molecules will look and behave.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The geometry of a molecule significantly influences its physical and chemical properties, including reactivity, polarity, and interaction with other molecules.

The VSEPR theory helps in understanding these geometries by considering the number of bonding and nonbonding electron pairs. Some common geometries are linear, bent, trigonal planar, pyramidal, and tetrahedral. Lone pair repulsions can cause deviations from these basic geometries, leading to 'bent' or 'distorted' shapes like those seen in PF3 and SO2 molecules from the original exercise. It's essential to distinguish between the electron-pair geometry, which describes the arrangement of all electron pairs (bonding and nonbonding), and the molecular shape, which describes the spatial arrangement of just the atoms.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are significant in determining how the atom can bond with others. These electrons have the highest energy level and are usually the only electrons involved in chemical bonds. The number of valence electrons an atom has influences its chemical properties and the type of bonds it can form.

Atoms tend to follow an 'octet rule,' where they are most stable with eight valence electrons. In bonding, atoms will share, lose, or gain electrons to achieve a full valence electron shell. During this process, lone pairs can be formed when atoms have extra valence electrons that aren't shared in bonds. By analyzing these electrons through the VSEPR theory, we can predict the molecular shape and understand the structural reasons behind the molecule's behavior and reactivity.