All the atoms involved in double bonds are planar. In addition to that, if one or both of the atoms involved in double bond formation be carbon, then next atoms attached to it (or them) are planar as well. In short all the atoms sharing in sp2 hybrid orbitals area planar. Atoms attached to triple bonds (sp hybrid) are linear and hence planar. In other words atoms attached to multiple bonds are planar. For example, both the carbons and all the hydrogens in ethene (C2H4) are planar. Both of the carbons, and both the hydriogens in ethyne are linear and planar. In acetaldehyde (CH3CHO) molecule, however, the carbonyl carbon is sp2 hybridised, hence planar. Its attachments viz.O, C and one aldehydic H, all lie in one plane. All the carbons of benzene (C6H6) and all of its hydrogens are planar for the same reason. Ethane (C2H6) is a saturated hydrocarbon. All of its carbon atoms are sp3 hybridised, hence tetrahedral (does not lie in the same plane).