Molecular Geometry: Trigonal Planar

Molecular Shape and Hybridization

• In BF3, the electron pairs are placed in a plane, forming a triangle, to minimize electron pair repulsion.
• BF3 has three shared electron pairs around the central atom, resulting in a trigonal planar structure with a 120° bond angle.
• Compounds with central atoms in the same group of the periodic table have similar geometry, such as AlH3, which is also trigonal planar.

sp2 Hybridization

• Mixing two p orbitals and one s orbital in the same plane produces three sp2 orbitals pointing towards the corners of an equilateral triangle.
• This leaves one p orbital, containing one electron, perpendicular to the plane.

Molecular Geometry

• Trigonal planar molecular geometry has three atoms bonded to the central atom, with a bond angle of 120°.
• Tetrahedral molecular geometry has four atoms bonded to the central atom, with a bond angle of 109.5°.
• Trigonal pyramidal molecular geometry has three atoms bonded to the central atom, with a bond angle of <120°.
• Linear molecular geometry has two atoms bonded to the central atom, with a bond angle of 180°.

Hybridization and Bonding

• sp3 hybridization results in four degenerate hybrid orbitals, with a bond angle of 109.5°.
• sp2 hybridization results in three degenerate hybrid orbitals, with a bond angle of 120°.
• sp hybridization results in two degenerate hybrid orbitals, with a bond angle of 180°.
• The Lewis structure of ethene is trigonal planar, with a planar overall shape.

Molecular Examples

• Ammonia (NH3) is an example of trigonal pyramidal molecular geometry, due to the sp3 hybridization of the nitrogen atom.
• Water (H2O) is an example of bent molecular geometry, due to the sp3 hybridization of the oxygen atom and the bonding of only two atoms to the oxygen.
• Methane (CH4) is an example of tetrahedral molecular geometry, due to the sp3 hybridization of the carbon atom.