Final answer:
The geometry around the carbon atom in CH₃NH₂ is tetrahedral, while the geometry around the nitrogen atom is trigonal pyramidal. The electron-pair geometry is tetrahedral for both atoms, yet the lone pair on the nitrogen atom results in a trigonal pyramidal shape for the molecule at the nitrogen site. So the correct answer is option B.
Step-by-step explanation:
To determine the geometry around each interior atom in the molecule CH₃NH₂, we apply the VSEPR (Valence Shell Electron Pair Repulsion) theory. This molecule has a carbon atom (C) bonded to three hydrogens (H) and a nitrogen (N) atom, which in turn is bonded to two hydrogens (H) and the carbon (C).
The carbon atom has four electron density regions (the three H atoms and one bond to N), so the electron-pair geometry is tetrahedral. Considering all four positions are occupied by bonds to hydrogen or nitrogen, the local structure of the carbon is also tetrahedral.
For the nitrogen atom, the electron-pair geometry is also tetrahedral with one of the electron pairs being a lone pair and the other three areas being bonds to hydrogen and carbon. The molecular structure for nitrogen in this case is trigonal pyramidal due to the lone pair arrangement.