Trigonal Planar
Definition and meaning of Trigonal Planar in chemistry.
Trigonal planar is a molecular shape in which a central atom is bonded to three other atoms that lie flat in one plane, spaced 120 degrees apart. This geometry appears when a central atom is surrounded by three regions of electrons and no lone pairs.
In more detail
The shape comes from VSEPR theory, which says that groups of electrons around a central atom spread out to get as far apart as possible. With three bonding groups and no lone pairs, the lowest-repulsion arrangement is a flat triangle. The three bonds point toward the corners of that triangle, giving ideal bond angles of 120 degrees, and all four atoms sit in the same plane.
Boron trifluoride, BF3, is a textbook example. The boron atom forms three single bonds to fluorine and has no lone pair, so the fluorines settle into a flat triangle around it. Other examples include the carbonate ion, CO3<sup>2-</sup>, the nitrate ion, NO3<sup>-</sup>, and the carbon atoms in molecules such as formaldehyde and the alkene ethene.
The central atom in a trigonal planar molecule is usually described as sp2 hybridized. In this picture, one s orbital and two p orbitals mix to form three equal orbitals aimed at the triangle's corners, while any leftover p orbital stands perpendicular to the plane and can form a pi bond.
That leftover orbital is why trigonal planar carbons often take part in double bonds. It is important to separate electron geometry from molecular geometry. Trigonal planar describes both when all three electron regions are bonds.
If one of the three regions were a lone pair instead of a bond, the electron geometry would still be trigonal planar, but the visible shape of the atoms would become bent. Counting lone pairs correctly is the key to predicting the final shape.
Key facts
| Field | General Chemistry |
|---|---|
| Electron regions | 3 bonding, 0 lone pairs |
| Bond angle | 120 degrees |
| Shape | flat triangle, all atoms in one plane |
| Hybridization | sp2 |
| Example molecule | BF3 |
| Example ions | CO3<sup>2-</sup>, NO3<sup>-</sup> |
| Theory used | VSEPR |
In boron trifluoride, BF3, boron sits at the center with three fluorine atoms arranged in a flat triangle 120 degrees apart. Because boron has no lone pair, the molecule is a clean example of trigonal planar geometry.
Frequently asked questions
Why is the bond angle 120 degrees?
With three bonding groups and no lone pairs, the electron regions spread out as far as possible in a plane. Dividing 360 degrees among three directions gives 120 degrees between each bond.
What hybridization goes with trigonal planar?
The central atom is sp2 hybridized. Three sp2 orbitals form the flat triangle, and a leftover unhybridized p orbital can form a pi bond above and below the plane.
How is trigonal planar different from trigonal pyramidal?
Trigonal planar has three bonds and no lone pair, so it is flat with 120-degree angles. Trigonal pyramidal has three bonds plus one lone pair, which pushes the bonds into a pyramid shape.
Can a molecule with a double bond be trigonal planar?
Yes. A double bond counts as one electron region. A carbon with one double bond and two single bonds, as in formaldehyde, is trigonal planar.