Chemical compounds have different 3-dimensional arrangement of their atoms, depending on the number of their bonds and electrons. In this lesson, we’ll learn about the structure of a compound with trigonal planar molecular geometry.

Molecular Geometry

In chemistry class, we often see structures of chemical compounds written on paper, and sometimes, it’s hard to visualize their 3-dimensional structures. So, let’s imagine a compound where atoms are bonded together. The specific 3-dimensional arrangement of bonded atoms is known as molecular geometry.

Different compounds have different molecular geometries or spatial arrangements. The molecular geometries of atoms in compounds depend on the number of atoms or bonds attached to the central atom and lone pairs of electrons, if any, around the central atom. Each atom and lone pair of electrons around the central atom count as one group.

Now, let’s discuss the molecular geometry of a trigonal planar compound. A trigonal planar compound has a central atom attached to three atoms arranged in a triangular shape around the central atom. All four atoms lie flat on a plane. Note that there are no lone pairs of electrons around the central atom.

What do the wedges and the dashed lines mean? To answer that question, let’s imagine a central atom, A, lying flat on a piece of paper. The dashed or broken line means that the attached atom X1 is behind the paper, while the thick, wedged line means that the X2 atom juts out of the paper. X3, on the other hand, lies flat on the paper.

Another important thing to remember about trigonal planar compounds is the angle between the atoms. Let’s draw AX3 with all the atoms laying flat on the paper. We can see that the bond angles between the atoms attached to the central atom are all 120 degrees.

Examples of Trigonal Planar Compounds

Let’s look at a few chemical compounds that have a trigonal planar geometry. The compounds boron hydride (BH3) and boron trifluoride (BF3) both have trigonal planar molecular geometries.

Earlier, we looked at examples of trigonal planar, where the compounds’ central atom shares a single bonded with the other atoms. Trigonal planar geometries are also possible if the central atom of a compound shares double bonds with the other atoms. Even if the atom has a double bond, it still counts as one group. Examples include formaldehyde (CH2O), sulfur trioxide (SO3), and phosgene (COCl2).

Earlier, we explored some examples of trigonal planar geometries of neutral compounds, which are compounds that don’t have a charge. Chemical species with charge, also known as polyatomic ions, can also exhibit trigonal planar geometry. Carbonate, which has a charge of -2, and nitrate, which has a charge of -1, are some polyatomic ions that are trigonal planar.

Lesson Summary

A compound with trigonal planar molecular geometry has a central atom bonded to three other atoms or groups. As it has no lone pairs of electron pairs, the three groups to which it is bonded are arranged like a triangle around the central atom, with the bond angles measuring 120 degrees. In different compounds with trigonal planar geometry, the atoms share single bonds with the central atom, like boron hydride. Other compounds and polyatomic ions with trigonal planar geometry not only have single bonds, but double bonds as well, like formaldehyde and carbonate ion.