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.

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.

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.

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.