Final answer:
In chemistry, 'trigonal planar' describes a molecular geometry where three electron groups form an equilateral triangle around the central atom lying in the same plane, each 120° apart, like in the molecule Boron trifluoride (BF3). This shape minimizes electron group repulsion and is distinct from the trigonal bipyramidal shape, which involves five electron groups.
Step-by-step explanation:
Trigonal planar is a term used in chemistry to describe a molecular geometry where a molecule with three electron groups orients these groups as far apart as possible. This arrangement causes the molecule to adopt a shape where the three groups form the points of an equilateral triangle, which are 120° apart, all lying in the same plane. This particular distribution minimizes the repulsion between the electron groups and is often observed in molecules without any lone pairs affecting the geometry.
An example of a trigonal planar molecule is Boron trifluoride (BF3). The boron atom is at the center with three fluorine atoms spaced equally around it. The geometry of sp² hybrid orbitals contributes to this flat, triangular shape, as the orbitals align themselves 120° apart, allowing for the formation of the molecule.
A similar but different geometry is the trigonal bipyramidal shape, where there are five electron groups around the central atom. Three electron groups form a plane with 120° angles (equatorial positions), whereas the remaining two are positioned above and below this plane (axial positions).