What are the electron-domain and molecular geometries of a molecule that has the following electron domains on its central atom? (a) three bonding domains and no nonbonding domains, (b) three bonding domains and one nonbonding domain, (c) two bonding domains and two nonbonding domains.
The correct answer and explanation is:
Here are the electron-domain and molecular geometries for each case based on the number of bonding and nonbonding electron domains around the central atom:
(a) Three bonding domains and no nonbonding domains
- Electron-domain geometry: Trigonal planar
- Molecular geometry: Trigonal planar
When there are three bonding pairs and no lone pairs on the central atom, the electron domains arrange themselves evenly in a plane to minimize repulsion. This results in a trigonal planar shape, where all atoms are 120 degrees apart.
(b) Three bonding domains and one nonbonding domain
- Electron-domain geometry: Tetrahedral
- Molecular geometry: Trigonal pyramidal
With four electron domains total (three bonding pairs and one lone pair), the electron-domain geometry is tetrahedral. However, the lone pair occupies one vertex, which is not visible in the molecular shape. This pushes the bonded atoms downward, creating a trigonal pyramidal shape with bond angles slightly less than 109.5 degrees due to lone pair repulsion.
(c) Two bonding domains and two nonbonding domains
- Electron-domain geometry: Tetrahedral
- Molecular geometry: Bent (or V-shaped)
Again, there are four electron domains total (two bonding pairs and two lone pairs), so the electron-domain geometry is tetrahedral. The molecular geometry considers only the atoms, so with two lone pairs pushing down on the bonding pairs, the shape is bent or V-shaped. Bond angles are less than 109.5 degrees, usually around 104-105 degrees, because lone pairs repel more strongly than bonding pairs.
Explanation
Molecular shape depends on the total number of electron domains (bonding plus nonbonding) around the central atom. The electron-domain geometry describes the arrangement of all electron pairs, while the molecular geometry considers only the positions of the atoms. Lone pairs occupy more space than bonding pairs, causing deviations from ideal bond angles.
In the first case, with only bonding domains, the shape is straightforward. In the second and third cases, lone pairs distort the geometry because their electron density repels bonding pairs more strongly. This leads to shapes like trigonal pyramidal and bent, which differ from the symmetrical arrangements of the electron domains.
Understanding these geometries helps predict molecular properties such as polarity, reactivity, and intermolecular interactions. This is foundational in chemistry for explaining how molecules behave in different environments.