Question

Use the various molecular modeling techniques (balland-stick, space-filling, two-dimensional pictures using wedges and dashed lines) to illustrate these simple molecules: (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{CO}_{2}\)

Short answer

Create ball-and-stick, space-filling, and 2D models of each molecule's structure.

Step-by-step solution

Understanding the Molecular Structure

To begin illustrating each molecule, we need to understand their molecular geometry. \(\mathrm{NH}_{3}\) is a trigonal pyramidal shape, \(\mathrm{H}_{2} \mathrm{O}\) is a bent or V-shaped molecule, and \(\mathrm{CO}_{2}\) is a linear molecule. This basic geometry will guide the creation of models.

Creating Ball-and-Stick Models

Ball-and-stick models represent atoms as balls and bonds as sticks. For \(\mathrm{NH}_{3}\), place a nitrogen ball in the center with three hydrogen balls connected with sticks extending at about \(107°\). For \(\mathrm{H}_{2} \mathrm{O}\), place an oxygen ball in the center, connecting to two hydrogen balls with sticks at an angle of approximately \(104.5°\). For \(\mathrm{CO}_{2}\), place a carbon ball in the center with two oxygen balls directly in line on either side, creating a 180° linear model.

Creating Space-Filling Models

Space-filling models depict the relative sizes of atoms and how they occupy space. In the model, each atom is represented by a sphere sized proportionally to its electron cloud. For \(\mathrm{NH}_{3}\), position the nitrogen sphere with partial overlap from three small hydrogen spheres. For \(\mathrm{H}_{2} \mathrm{O}\), the oxygen sphere overlaps slightly with two smaller hydrogen spheres. For \(\mathrm{CO}_{2}\), align the carbon sphere with two oxygen spheres on either side, slightly overlapping due to bonding.

Drawing Two-Dimensional Diagrams

Two-dimensional structures use wedges and dashes to indicate 3D orientation. For \(\mathrm{NH}_{3}\), draw a central nitrogen atom with three hydrogen atoms: one connected by a solid wedge (coming out of the page), one by a dash (going into the page), and one by a solid line (lying in the plane). For \(\mathrm{H}_{2} \mathrm{O}\), draw an oxygen atom with two hydrogen atoms using solid lines, each angled away from a central angle. For \(\mathrm{CO}_{2}\), draw a straight line connecting the carbon to each oxygen without wedges or dashes.

Key concepts

Ball-and-Stick Model

The ball-and-stick model is a classic way to represent molecules. This model uses spheres to represent atoms and sticks to show chemical bonds between them. It is an effective tool to illustrate the angles between bonds, making it easier to understand the shape of a molecule.
In the case of the molecule \(\mathrm{NH}_{3}\), the nitrogen atom is placed at the center, surrounded by hydrogen atoms at an approximate angle of 107°. This illustrates the trigonal pyramidal shape of ammonia. For water, \(\mathrm{H}_{2} \, \mathrm{O}\), the oxygen atom is centered with hydrogen atoms at about 104.5°, showing a bent shape. Lastly, in the \(\mathrm{CO}_{2}\) molecule, the carbon atom is linear between two oxygen atoms, at an angle of 180°.

• Atoms are represented as spheres.
• Bonds are shown as sticks connecting these spheres.
• This model focuses on angles between bonds.

This model is excellent for studying molecular geometry, as it emphasizes individual bond angles.

Space-Filling Model

The space-filling model is a different approach that highlights the occupied space of a molecule. In this model, each atom is shown as a full sphere, representing the space its electron cloud takes up. This model provides a more realistic picture of molecular interactions.
For \(\mathrm{NH}_{3}\), the nitrogen sphere is the center, partially overlapping with smaller hydrogen spheres. In \(\mathrm{H}_{2} \, \mathrm{O}\), the larger oxygen sphere slightly overlaps with hydrogens. The molecule \(\mathrm{CO}_{2}\) is shown by a central carbon sphere flanked by two oxygen spheres, all slightly overlapping.

• Highlights the actual size of atoms.
• Shows how atoms touch or overlap in a molecule.
• Provides a more "real-world" perspective compared to other models.

It's a valuable model for understanding molecular surfaces and how molecules may interact within a cell or other biological environments.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is essential not only for visualizing molecules but also for predicting their behavior and interactions.
The geometry of \(\mathrm{NH}_{3}\) is trigonal pyramidal due to the lone pair that pushes hydrogen atoms slightly closer. \(\mathrm{H}_{2} \, \mathrm{O}\), with its two lone pairs on oxygen, results in a bent shape. \(\mathrm{CO}_{2}\) has a linear geometry with two double bonds providing a symmetric shape.

• Geometry determines physical and chemical properties.
• It often predicts reactivity and interaction with other molecules.
• VSEPR theory helps predict shapes based on repulsions.

Understanding these shapes is crucial in fields like chemistry and biology where molecular interactions are key.

Two-Dimensional Diagrams

Two-dimensional diagrams simplify the visualization of molecules on paper. They use symbols like wedges and dashed lines to indicate how atoms are oriented in space.
In these diagrams, ammonia \(\mathrm{NH}_{3}\) is depicted with wedges and dashes showcasing its 3D orientation: one hydrogen coming out and another going into the plane. The \(\mathrm{H}_{2} \, \mathrm{O}\) molecule uses plain solid lines as it lies mostly within a plane. Carbon dioxide \(\mathrm{CO}_{2}\) is represented as a simple straight line without wedges, as it's linear.

• Uses wedges to show bonds coming out or going in.
• Simplifies 3D models for paper representation.
• Helpful in organic chemistry notation.

This model is great for drawing molecular structures in textbooks and notes.