Question

Which one of the following molecules has the smallest bond angle? (a) \(\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{H}_{2} \mathrm{Se}\) (d) \(\mathrm{H}_{2} \mathrm{~S}\)

Short answer

The molecule H2Se has the smallest bond angle.

Step-by-step solution

Identify the Molecular Geometry

To determine the bond angle, first identify the molecular geometry of each molecule. - 2 1 2 3 2. N : Triangular pyramidal geometry. 2 4 3. H2Se : Bent geometry similar to water due to the two lone pairs on Se. H2S : Bent shape ; similar to water

Review Typical Bond Angles

Recognize typical bond angles associated with each geometry: - The bond angle in linear molecules is 180°. - Water ( 9 H2O) and its analogs such as H2S and H2Se have bent geometries with typical angles of about 104.5° to 95. 3°. 3. XeF2: The bond angle is 180° in this linear shape, NH3 has a bond angle of approximately 107° due to its trigonal pyramidal shape.

Compare Electron Repulsion Effects

Consider the effect of lone pairs in reducing bond angles compared to the ideal tetrahedral angle of 109.5°: - NH3 has one lone pair causing slight reduction to approximately 107°. - H2O, H2S, and H2Se have two lone pairs each, causing greater reduction of the bond angle below that of NH3. - Between H2S and H2Se, the larger size of the central atom in H2Se causes the bond angle to be slightly less due to lesser repulsion between lone pairs.

Determine the Smallest Bond Angle

Based on size of central atom and lone pairs, predict bond angle size: - H2O: Predominantly known as 104.5°. - NH3: 107°. - H2Se: Smaller angle than H2S due to larger Se atom, potentially below 92°. - H2S: Around 92°. - H2Se: Estimated to have the smallest angle based on these factors.

Key concepts

Molecular Geometry

Molecular geometry refers to the spatial arrangement of atoms within a molecule. It influences not only the shape of the molecule but also its physical and chemical properties. In the context of bond angles, molecular geometry provides insight into how atoms and electron pairs are distributed around a central atom. For instance, water ( H_2O ) and its analogs, such as hydrogen sulfide ( H_2S ) and hydrogen selenide ( H_2Se ), possess a bent geometry due to the presence of two lone pair electrons repelling the bonded pairs. This geometry results in a bond angle that is less than 109.5°, typical for a tetrahedral arrangement. This aspect is crucial for determining which molecule out of the given options has the smallest bond angle. It's important to note that differing molecular geometries are often described using specific terms like linear, trigonal planar, tetrahedral, and bent, which reference the number of surrounding atoms and lone pairs impacting the central atom's configuration.

Lone Pair Electrons

Lone pair electrons are pairs of valence electrons that are not involved in forming chemical bonds. They occupy more space around the central atom than bond pairs, leading to greater electron repulsion. This repulsion can decrease the bond angles in a molecule. For example, in molecules like water ( H_2O ), sulfur dihydride ( H_2S ), and selenium dihydride ( H_2Se ), the presence of two lone pairs on the central atom causes the bond angle to reduce significantly from the ideal tetrahedral angle of 109.5°. Lone pairs push the bonded hydrogen atoms closer together, resulting in the bent geometry with reduced bond angles compared to molecules like ammonia ( NH_3 ), which has only one lone pair and thus a slightly larger bond angle.

Intermolecular Forces

Intermolecular forces are the forces that influence the interaction between molecules. While these forces primarily affect physical properties like boiling point and melting point, they can indirectly impact molecular shape and bond angles. Polar molecules, such as H_2O and NH_3 , experience hydrogen bonding due to the presence of highly electronegative atoms like oxygen and nitrogen. This effect influences molecular interactions but plays less of a direct role in determining bond angles when lone pairs are the dominant factor in shape determination. For the exercise's purpose, it is important to focus on how internal electron repulsion, rather than intermolecular forces, primarily dictates the bond angle.

VSEPR Theory

The VSEPR (Valence Shell Electron Pair Repulsion) theory is a cornerstone concept in understanding molecular geometry and bond angles. It posits that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. This theory helps predict the geometry of a molecule by considering both bonding and lone pairs. According to VSEPR theory, the presence and quantity of lone pair electrons significantly alter bond angles. For example, H_2O , H_2S , and H_2Se all exhibit bent geometries with bond angles reduced from the ideal 109.5° due to repulsion from the lone pairs. In contrast, NH_3 , with its trigonal pyramidal shape, has a slightly larger bond angle of around 107°, due to having only one lone pair. VSEPR theory is crucial for determining which molecular structures result in the smallest bond angles among various compounds.