Question

What word or two-word phrase describes the shape of the formaldehyde (`CH_2O`) molecule?

Answer 1

From counting valence electrons:

• Carbon has four valence electrons, so it tends to make four bonds in covalent compounds.
• Oxygen has six valence electrons, but as it tends to accept two, it tends to make two bonds in covalent compounds.
• Hydrogen is almost always going to make only one bond, given that it only has one valence electron.

Eventually, you should get that formaldehyde looks like this:

You can add the two remaining valence electrons onto `"O"` and check that you have a Lewis structure containing `4 + 2 + 6 = 12` valence electrons.

Since the `"C"="O"` bond does not rotate when the oxygen is held fixed, its presence forces a planar structure (you should compare with `"C"_2"H"_4`).

You can draw a triangular plane onto this molecule, and there are three electron groups (which are all bonding groups), so we call this molecular geometry around carbon trigonal planar.