Question

Predict the geometry of the following molecules and ion using the VSEPR model: (a) ${\mathrm{CBr}}_4,(\mathrm{b}){\mathrm{BCl}}_3,(\mathrm{c}){\mathrm{NF}}_3,$ (d) ${\mathrm{H}}_2\mathrm{Se},$ (e) ${\mathrm{NO}}_2^{-}$.

Short answer

(a) Tetrahedral, (b) Trigonal planar, (c) Trigonal pyramidal, (d) Bent, (e) Bent.

Step-by-step solution

Understanding VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) theory helps predict the geometry of molecules by considering the repulsions between electron pairs around a central atom. The electron pairs, including bonding and lone pairs, will arrange themselves as far apart as possible.

Determine Electron Regions for Each Molecule

Count the number of electron regions (bonding and lone pairs) around the central atom for each molecule. For example, in ${\mathrm{CBr}}_4$, there are 4 bonding pairs. Analyze similarly for ${\mathrm{BCl}}_3$, ${\mathrm{NF}}_3$, ${\mathrm{H}}_2\mathrm{Se}$, and ${\mathrm{NO}}_2^{-}$.

Assign Basic Molecular Shapes

Using VSEPR, assign a basic shape based on electron regions. For ${\mathrm{CBr}}_4$, with 4 bonding pairs, the shape is tetrahedral. ${\mathrm{BCl}}_3$, with 3 bonding pairs, is trigonal planar. ${\mathrm{NF}}_3$ has 3 bonding pairs plus 1 lone pair, making it trigonal pyramidal. ${\mathrm{H}}_2\mathrm{Se}$ has 2 bonding pairs and 2 lone pairs, making it bent. ${\mathrm{NO}}_2^{-}$ has 2 bonding pairs and 1 lone pair, resulting in a bent shape.

Verify for Lone Pairs Influence

Check the presence of lone pairs that might alter the shape from the theoretical predictions. Lone pairs exert more repulsion than bonding pairs, so they can modify idealized shapes, such as in ${\mathrm{NF}}_3$ and ${\mathrm{H}}_2\mathrm{Se}$.

Conclude the Molecular Geometries

Summarize the VSEPR-based shapes: (a) ${\mathrm{CBr}}_4$ is tetrahedral, (b) ${\mathrm{BCl}}_3$ is trigonal planar, (c) ${\mathrm{NF}}_3$ is trigonal pyramidal, (d) ${\mathrm{H}}_2\mathrm{Se}$ is bent, and (e) ${\mathrm{NO}}_2^{-}$ is bent.

Key concepts

Understanding Molecular Geometry

Molecular geometry is all about the 3D shape that forms when atoms bond together. By knowing this shape, we can understand a molecule's properties and behavior. The shape is influenced by how electron pairs surround a central atom. They'll spread out to minimize repulsion. This, in turn, influences how atoms bond and dictate the molecule’s structure.

The VSEPR model (Valence Shell Electron Pair Repulsion) is the key approach used in predicting these shapes. For instance, in a molecule like ${\mathrm{CBr}}_4$, it has four bonding pairs that arrange themselves in a way that they are as far apart as possible, leading to a tetrahedral shape.

Understanding molecular geometry offers insights into a molecule's characteristics, such as polarity, reactivity, and physical properties. It also forms the basis of more advanced chemical theories and applications.

Role of Electron Pairs in Molecular Geometry

Electron pairs play a central role in determining how molecules form their shapes. They come in two main types: bonding pairs and lone pairs. In the VSEPR model, these pairs are considered around a central atom to predict the molecular geometry.

Bonding pairs are shared between atoms and contribute directly to the structure of a molecule. Lone pairs, on the other hand, are not involved in bonding but still influence the shape due to their repulsive force.

For example, ${\mathrm{NF}}_3$ has three bonding pairs and one lone pair. The lone pair pushes the bonded atoms downwards, creating a trigonal pyramidal shape. This interaction between electron pairs is foundational for understanding how molecules assume their specific forms.

Significance of Bonding Pairs

Bonding pairs are the electrons that form chemical bonds between atoms. In molecular geometry, these are key in determining the shape since they connect the different atoms together.

For example:

• ${\mathrm{CBr}}_4$: The four carbon-bromine bonds form a tetrahedral shape as they are evenly spaced.
• ${\mathrm{BCl}}_3$: Three chlorine atoms bonded to boron form a trigonal planar shape.

The arrangement of these pairs decides the angles between bonds and the overall geometry. Bonding pairs help predict the molecule’s reactivity and how it interacts with other molecules.

Influence of Lone Pairs

Lone pairs of electrons have a big impact on molecular geometry even though they aren’t part of bonds. Their presence can alter the shape from what would be expected if only bonding pairs were considered.

Lone pairs tend to occupy more space than bonding pairs, leading to increased repulsion. This change can drastically modify the geometry. For instance:

• ${\mathrm{NF}}_3$: The lone pair causes the molecule to be trigonal pyramidal instead of trigonal planar.
• ${\mathrm{H}}_2\mathrm{Se}$: Two lone pairs bend the shape, creating a more angular form.

This influence highlights the need to consider both lone and bonded electron pairs for accurate geometrical predictions.