Understanding Trigonal Planar Geometry
* Central Atom: The central atom in a trigonal planar molecule has three bonding pairs of electrons and no lone pairs.
* Bond Angles: The bond angles between the three atoms bonded to the central atom are approximately 120°.
* Shape: The three atoms bonded to the central atom form a flat triangle, with the central atom at the center.
Examples of Trigonal Planar Molecules and Ions:
* Boron trifluoride (BF₃): Boron has three valence electrons, and it forms three single bonds with fluorine atoms. This results in a trigonal planar structure.
* Carbon dioxide (CO₂): Carbon has four valence electrons, and it forms two double bonds with oxygen atoms. The double bonds are treated as single bonds for the purpose of determining shape, so CO₂ has a trigonal planar structure.
* Nitrate ion (NO₃⁻): Nitrogen has five valence electrons. It forms three single bonds with oxygen atoms and has one lone pair of electrons. However, the lone pair is delocalized, meaning it's spread out over the three oxygen atoms, and it doesn't significantly affect the overall shape. The nitrate ion has a trigonal planar structure.
* Phosphine (PH₃): Phosphorus has five valence electrons. It forms three single bonds with hydrogen atoms and has one lone pair of electrons. This lone pair pushes the three hydrogen atoms down, resulting in a trigonal pyramidal structure, not trigonal planar.
Key Takeaway:
To determine if a molecule or ion has a trigonal planar shape, look for a central atom with three bonding pairs and no lone pairs of electrons.
