 One of the first quantum effects to be explicitly noticed ( but not understood at the time ) was a Maxwell observation involving hydrogen , half a century before full quantum mechanical theory arrived . Maxwell observed that the specific heat capacity of H2 unaccountably departs from that of a diatomic gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures . According to quantum theory , this behavior arises from the spacing of the ( quantized ) rotational energy levels , which are particularly wide @-@ spaced in H2 because of its low mass . These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures . Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect . 
