Which of the following correctly describes the bond angle in a molecule of the general type ABX, where A is the central atom, B is a surrounding atom, and X represents a lone pair?

• A. The bond angle is always 180° in ABX type molecules.
• B. The bond angle is less than 109.5° due to the presence of a lone pair on the central atom.
• C. The bond angle is exactly 120° because lone pairs do not affect bond angles.
• D. The bond angle is greater than 109.5° due to increased repulsion from the lone pair.

Answer: (B)

Verified step by step guidance

1. Identify the electron domain geometry around the central atom A by considering both bonding pairs (B atoms) and lone pairs (X). For a molecule of type ABX, there are typically three regions of electron density (two bonds and one lone pair), which corresponds to a trigonal pyramidal or bent shape derived from a tetrahedral electron geometry.
2. Recall that the ideal bond angle for a tetrahedral electron geometry (four electron domains) is 109.5°, which applies when there are no lone pairs affecting the shape.
3. Understand that lone pairs occupy more space than bonding pairs because lone pairs are localized closer to the central atom and repel bonding pairs more strongly, which tends to compress the bond angles between bonded atoms.
4. Apply the concept that the presence of a lone pair on the central atom reduces the bond angle between the bonded atoms to less than the ideal tetrahedral angle of 109.5°, due to increased lone pair-bond pair repulsion.
5. Conclude that for an ABX molecule with one lone pair on the central atom, the bond angle will be less than 109.5°, making the correct description: 'The bond angle is less than 109.5° due to the presence of a lone pair on the central atom.'