Absorption of infrared radiation by the main components of the atmosphere, namely nitrogen and oxygen molecules, is forbidden by quantum laws. These indicate that molecules that are allowed to absorb and emit infrared radiation must either possess a permanent dipole moment—for example the bent water molecule in which the oxygen atom has a partially negative charge balanced by partially positive charges on the hydrogen atoms—or undergo vibrational motions that alter the dipole moment. An example of the latter is the linear CO₂ molecule that has zero dipole moment because the partially negatively charged oxygen atoms are diametrically opposed, but when it undergoes a bending vibration the dipole moment is transiently non-zero. Thus water vapour and CO₂ are greenhouse gases, as are to a lesser extent methane (CH₄), nitrous oxide (dinitrogen oxide, N₂O) and ozone (O₃).