Question

Consider the \(\mathrm{CH}_{3}\) anion. (a) Draw and name its three-dimensional shape and indicate the predicted values of its bond angles. (b) Explain how the structure of this anion would compare with that of \(\mathrm{NH}_{3}\) and why both would have bond angles that are less than ideal.

Short answer

The CH3- anion has a trigonal pyramidal molecular geometry (AX3E1 notation) with H-C-H bond angles deviating from the ideal 109.5 degrees, due to the presence of a lone pair on the central carbon atom. NH3 also has a trigonal pyramidal shape (AX3E1) but with a smaller H-N-H bond angle (~107 degrees) as nitrogen's higher electronegativity causes its lone pair to repel the hydrogen atoms further. Both structures have bond angles less than 109.5 degrees due to the lone pair's repulsion effect on the bonded atoms.

Step-by-step solution

Draw CH3- ion structure

Let's start by drawing the 3-dimensional structure of CH3-. Carbon forms four bonds, but since CH3- has a negative charge, it holds an extra lone pair as well. The molecule has a total of 4 groups surrounding the central atom: 3 hydrogens and 1 lone pair. Now let's use the VSEPR theory to predict the molecule's shape. The formula for the VSEPR notation is AXE, where A stands for the central atom, X is the number of bonded atoms, and E is the number of lone pairs. So for CH3-, the VSEPR notation is AX3E1. According to the VSEPR theory, AX3E1 corresponds to a trigonal pyramidal molecular geometry.

Predict bond angles and their values

Given the trigonal pyramidal shape, the bond angles in the CH3- anion are the H-C-H bond angles. The ideal bond angle for a trigonal pyramidal structure is 109.5 degrees; however, the actual bond angle for CH3- may not be exactly 109.5 degrees due to the presence of a lone pair, which takes up more space and repels the H atoms slightly, causing them to be closer to each other.

Compare CH3- and NH3 structures

The structure of NH3 also follows the AX3E1 notation (A is nitrogen, X is 3 hydrogen atoms, and E1 represents one lone pair). NH3 also has a trigonal pyramidal shape. However, the electronegativity of nitrogen is higher than that of carbon, which results in its lone pair occupying more space, repelling the hydrogen atoms further, and causing the H-N-H bond angle to be smaller (~107 degrees) than the H-C-H bond angle in CH3-. Both CH3- and NH_counts3 display bond angles that are less than the ideal 109.5 degrees due to the repulsion effect of the lone pair on the bonded hydrogen atoms. In summary, both anions have trigonal pyramidal structures, with bond angles less than 109.5 degrees due to the presence of the lone pair.

Key concepts

Trigonal Pyramidal Geometry

In chemical structures where trigonal pyramidal geometry is observed, a central atom is surrounded by three bonds and one lone pair of electrons.
The lone pair significantly influences the spatial arrangement of the molecule. For example, in the case of the \( \text{CH}_3^- \) anion, the central carbon atom forms three bonds with hydrogen atoms, while retaining one lone pair.
This arrangement leads to a shape akin to a three-sided pyramid with the carbon atom near the apex. Although this geometry is similar to the tetrahedral shape, it lacks the symmetry due to the presence of the lone pair.

• The trigonal pyramidal is not flat; it forms a pyramid.
• The bond angles are affected by the lone pair, making it smaller than in a regular tetrahedron.

Lone Pair Repulsion

Lone pair repulsion is a key concept in understanding molecular shapes. Lone electron pairs are somewhat like invisible structures that occupy space around an atom, and they repel bonded pairs more forcefully than the bonded pairs repel each other.
In \( \text{CH}_3^- \), the lone pair on the carbon atom pushes the hydrogen atoms a bit closer than they would be in a perfect tetrahedral arrangement.
This is why instead of a perfect 109.5-degree angle, you often see slightly smaller bond angles.

• Lone pairs repel more than bonded pairs because they are closer to the nucleus.
• This repulsion decreases the bond angles in the molecule.
• Understanding lone pair repulsion helps predict molecular shapes accurately.

Bond Angles

The bond angles in molecular structures are crucial as they flesh out the 3D shape of the molecule. Typically, in a trigonal pyramidal geometry, without any variations, you would expect bond angles around 109.5 degrees, similar to a tetrahedral shape.
However, the presence of lone pairs changes this. In \( \text{CH}_3^- \), due to the influence of the lone pair, the H-C-H bond angles are reduced. The lone pair occupies space and exerts a stronger repulsion compared to bonded pairs, leading to these smaller deviated angles.
For instance, when comparing molecules like \( \text{NH}_3 \) and \( \text{CH}_3^- \), even though both share a trigonal pyramidal geometry, the bond angles of \( \text{NH}_3 \) (around 107 degrees) are smaller due to nitrogen's higher electronegativity than carbon's.

• Bond angles define how far apart atoms sit from each other due to electron repulsion.
• Lone pairs cause a reduction from the idealized bond angles.
• Electronegativity can affect how much lone pairs repel bonded atoms, thus altering bond angles.

Three-Dimensional Molecular Shape

The three-dimensional molecular shape helps predict and illustrate how atoms in a molecule interact with each other. The shape in 3D space is determined by the VSEPR Theory, which considers both bonded atoms and lone pairs.
For the \( \text{CH}_3^- \) anion, its 3D shape is described as trigonal pyramidal. This means the molecule is not flat, and the atoms are arranged in such a way that they provide a 3D coverage.
\( \text{NH}_3 \) shares this 3D symmetry due to its lone pair arrangement. This characteristic results in similar spatial arrangements in both molecules, even though different atoms are involved.

• Three-dimensional shape takes into account all interactions between electrons and atoms within a molecule.
• It helps to understand the molecule's physical properties and chemical reactivity.
• This visualization aids in predicting the molecular interactions in different environments.