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

1 Answer
Dec 5, 2016

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.