What is the molecular geometry of a molecule with 4 outer atoms and 1 lone pair on the central atom?

• A. Trigonal bipyramidal
• B. See-saw
• C. Square planar
• D. Tetrahedral

Answer: (B)

Verified step by step guidance

1. Identify the electron domain geometry based on the total number of electron groups (bonding + lone pairs) around the central atom. Here, there are 4 outer atoms (bonding pairs) and 1 lone pair, making a total of 5 electron groups.
2. Recall that 5 electron groups around a central atom correspond to a trigonal bipyramidal electron domain geometry, which arranges electron pairs to minimize repulsion.
3. Consider the effect of the lone pair on the molecular shape. Lone pairs occupy more space and tend to occupy equatorial positions in a trigonal bipyramidal arrangement to minimize repulsion.
4. With one lone pair in an equatorial position and four bonded atoms, the molecular geometry is called 'see-saw' because the shape resembles a seesaw with one position occupied by the lone pair.
5. Therefore, the molecular geometry is 'see-saw', which is distinct from the electron domain geometry (trigonal bipyramidal) and different from other options like square planar or tetrahedral.