Is a molecule with a central atom that has three electron groups and three bonded atoms assume a trigonal pyramidal shape
The Correct Answer and Explanation is:
No, a molecule with a central atom that has three electron groups and three bonded atoms does not assume a trigonal pyramidal shape. Instead, it adopts a trigonal planar shape.
Here’s why:
- Electron Groups and Bonding: When a molecule has three electron groups (bonding pairs or lone pairs) around the central atom, and all three are bonded atoms, these groups arrange themselves in a way that minimizes electron-electron repulsion. According to VSEPR theory (Valence Shell Electron Pair Repulsion), three electron groups will spread out in a flat, trigonal planar geometry, where the bond angles between the atoms are 120°.
- Trigonal Pyramidal vs Trigonal Planar: The trigonal pyramidal shape is observed when there are three bonded atoms but one lone pair of electrons on the central atom. The lone pair repels the bonding pairs, causing the molecule to adopt a pyramid-like shape with the central atom at the apex. In contrast, trigonal planar geometry is flat, with no lone pairs affecting the arrangement.
- Example: A classic example of a trigonal planar molecule is BF₃ (boron trifluoride), where the boron atom has three bonded fluorine atoms and no lone pairs. If there were a lone pair, as in NH₃ (ammonia), the shape would be trigonal pyramidal because of the lone pair pushing down on the bonding pairs.
In summary, for a molecule with three electron groups and three bonded atoms (no lone pairs), the geometry is trigonal planar, not trigonal pyramidal. The presence of lone pairs is essential for a trigonal pyramidal structure.
