Question

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

Short answer

The presence of nonbonding electron pairs affects the molecular shape for molecules (b) \(\operatorname{PF}_{3}\) (trigonal pyramidal) and (e) \(\operatorname{SO}_{2}\) (bent or V-shaped).

Step-by-step solution

For each molecule or ion, we need to identify the central atom. For a simple molecule, the central atom is usually the one with the most significant valence, or it is least electronegative. The central atom will be surrounded by the other atoms in the molecule. (a) \(\operatorname{SiH}_{4}\): The central atom is Si (Silicon). (b) \(\operatorname{PF}_{3}\): The central atom is P (Phosphorus). (c) \(\operatorname{HBr}\): The central atom is Br (Bromine). (d) \(\operatorname{HCN}\): The central atom is C (Carbon). (e) \(\operatorname{SO}_{2}\): The central atom is S (Sulfur). #Step 2: Determine the number of valence electrons for each central atom#

Next, we need to determine the number of valence electrons for the central atom in each of the molecules or ions. This will help us later to figure out how many bonding and nonbonding electron pairs there are for each central atom. (a) Si: Group 14 - 4 valence electrons. (b) P: Group 15 - 5 valence electrons. (c) Br: Group 17 - 7 valence electrons. (d) C: Group 14 - 4 valence electrons. (e) S: Group 16 - 6 valence electrons. #Step 3: Identify the bonding and nonbonding electron pairs for each central atom#

Now, let's identify the number of bonding and nonbonding electron pairs for each central atom: (a) SiH4: 4 bonding electron pairs (4 H atoms around the Si atom). (b) PF3: 3 bonding electron pairs (3 F atoms around the P atom), one nonbonding electron pair (5 valence electrons of P - 6 electrons used for bonding = 1 nonbonding pair). (c) HBr: 1 bonding electron pair (1 H atom around the Br atom), 3 nonbonding electron pairs (7 valence electrons of Br - 2 electrons used for bonding = 6 nonbonding electrons / 2 = 3 nonbonding electron pairs). (d) HCN: 4 bonding electron pairs (1 H atom and 3 N atoms around the C atom), no nonbonding electron pairs. (e) SO2: 2 bonding electron pairs (2 O atoms around the S atom), 1 nonbonding electron pair (6 valence electrons of S - 4 electrons used for bonding = 2 nonbonding electrons / 2 = 1 nonbonding electron pair). #Step 4: Analyze the effect of nonbonding electron pairs on molecular shape#

The presence of nonbonding electron pairs will affect the molecular shape when they force bonding electron pairs to adjust their angles, creating a deviation from the predicted molecular geometry. We can now identify which of the molecules/ions have nonbonding electron pairs that significantly affect their molecular shape. (a) SiH4: No nonbonding electron pairs - the molecular shape is not affected. (b) PF3: One nonbonding electron pair - the molecular shape is affected (Trigonal pyramidal instead of trigonal planar). (c) HBr: Does not apply as it is a diatomic molecule, and the shape is unaffected by nonbonding electron pairs. (d) HCN: No nonbonding electron pairs - the molecular shape is not affected. (e) SO2: One nonbonding electron pair - the molecular shape is affected (Bent or V-shaped instead of linear). Conclusion: The presence of nonbonding electron pairs affects the molecular shape for molecules (b) \(\operatorname{PF}_{3}\) and (e) \(\operatorname{SO}_{2}\).

Key concepts

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom, which can participate in forming chemical bonds.
Understanding valence electrons is crucial since the number of these electrons determines the bonding behavior of an atom when it interacts with others to form compounds. For example, atoms in the same group of the periodic table have the same number of valence electrons; carbon in group 14 has four valence electrons, while sulfur in group 16 has six.

In a covalent bond, valence electrons can be shared between atoms, leading to the formation of molecular structures. The knowledge of how many valence electrons are present helps us predict how many and what kind of bonds an atom can form, distinguishing between single, double, or triple bonds. For instance, in our exercise, carbon forms a triple bond with nitrogen in HCN, utilizing all its four valence electrons.

Nonbonding Electron Pairs

Nonbonding electron pairs, often referred to as lone pairs, are valence electrons not used in bonding. They belong exclusively to one atom in the molecule and can majorly influence the molecular shape.
The presence of nonbonding electron pairs leads to electron-electron repulsion, which in turn affects the spatial arrangement of atoms bonded to the central atom. The exercise solution highlights this concept by identifying nonbonding electron pairs for each central atom. For example, PF3 has one nonbonding pair on the phosphorus atom, altering its geometry from a trigonal planar to a trigonal pyramidal shape.

Understanding nonbonding electron pairs is essential when using the VSEPR (Valence Shell Electron Pair Repulsion) theory to predict molecular geometry. This concept emphasizes that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion.

Molecular Geometry

Molecular geometry refers to the three-dimensional shape or arrangement of atoms within a molecule. It is a critical concept in chemistry because the shape of a molecule can determine its properties and reactivity.
The VSEPR theory is commonly used to predict molecular geometry by considering the number of bonding and nonbonding electron pairs surrounding a central atom. For instance, SiH4 is predicted to be tetrahedral based on its four bonding pairs, and the absence of nonbonding pairs allows it to maintain that shape. Conversely, SO2, with one nonbonding electron pair and two bonding pairs, adopts a bent or V-shaped geometry instead of a planar structure.

As the exercise demonstrates, the presence of nonbonding electron pairs can cause deviations from idealized geometries, revealing the necessity to consider both bonding and nonbonding electron pairs when predicting the shape of a molecule.