Formaldehyde

Formaldehyde is the simplest molecule with a carbonyl group, a carbon double-bonded to an oxygen (C=O). Carbonyls show up all over organic chemistry: in aldehydes, ketones, acids, esters and amides.

The carbon is bonded to three atoms (two hydrogens and the oxygen), so just like the carbons in ethene it is sp² hybridized, with its three σ bonds flat in one plane and angles close to 120°. The leftover p orbital on carbon overlaps sideways with a p orbital on oxygen to make the C=O π bond. Turn on p orbitals and look edge-on to see the two parallel p orbitals. Oxygen also keeps two lone pairs, which lie in the plane of the molecule.

The big difference from ethene’s C=C bond is that oxygen is much more electronegative than carbon, so it pulls the bonding electrons toward itself. Turn on π electrons: the π cloud is lopsided, with more of it around the oxygen than around the carbon. That leaves the carbon slightly positive and the oxygen slightly negative. Most carbonyl chemistry comes down to that partial positive charge on carbon attracting electron-rich reagents.

Turn on Angles & lengths to compare the C=O bond with the C–H bonds and to check the angles around carbon.