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In chemistry, 'trigonal planar' is a molecular geometry with one atom at the center and three atoms at the corners of a triangle all in one plane. In a trigonal planar molecule where all three attached atoms are identical, all bond angles will be 120° and the molecule will belong to point group D_3h. Molecules where these atoms differ − such as H₂CO − as well as more complex molecules − such as alkenes − will deviate from this ideal geometry due to lowered molecular symmetry. In general the atomic orbitals of a trigonal planar molecule are sp² hybridized.
Examples of molecules with a trigonal planar geometry include boron trifluoride BF₃, formaldehyde H₂CO, and sulfur trioxide SO₃. Some ions with trigonal planar geometry include the nitrate ion, NO₃⁻, the carbonate ion, CO₃²⁻, and the borate ion, BO₃³⁻.
'Pyramidalization' is a distortion of this molecular shape towards a tetrahedral molecular geometry. One way to observe this distortion is in pyramidal alkenes.