The hybridisation of any atom can be determined instantly using: Hybridisation state = number of sigma bonds + number of lone pairs on that atom. This equals the steric number. sp = 2, sp2 = 3, sp3 = 4, sp3d = 5, sp3d2 = 6, sp3d3 = 7. For carbon in organic molecules: single bond = sp3, double bond = sp2, triple bond = sp. For atoms in resonance structures, use any single valid Lewis structure. Example: CO2 — carbon has 2 sigma bonds, 0 lone pairs, SN = 2, hybridisation = sp. Each oxygen has 1 sigma bond + 2 lone pairs = sp2 (not sp, because we count the lone pairs).