As molecules absorb energy from their surroundings, their incipient kinetic energy increases. One of the ways that molecules dissipate energy is by molecular vibrations that include the stretching and the bending of covalent bonds. The C—C single bonds found in an organic molecule can rotate. Propane is symmetrical with a central CH2 unit with a methyl group on either end. The “ethane model” used for propane and butane can be extended to other alkanes. Halogens have the same valence as hydrogen. The increase in steric hindrance is apparent and it influences the rotamer populations in organic compounds bearing halogen atoms. In planar cyclic alkanes there is another form of strain because all the C—H bond eclipse, which leads to a steric interaction and an increase in the strain energy.