I'm trying to understand the meaning of kT (energy) in molecular systems, how to define the temperature of an individual molecule, and how a molecule recieves thermal energy or dissipates thermal energy. Here is my 'gendanken': One molecule is floating in a box, in vacuum. The temperature of all the walls of the box are room temperature, 298 K. I think the walls are emitting black-body radiation. If we use Wien's Law, the peak wavelength of this radiation is about 9.7 μm. So let us say the molecule is being bombarded with photons with wavelength of 9.7 μm, this corresponds to the photons having an energy of about 0.128 eV. What does it mean if the molecule is in thermal equilibrium with the box? Is the molecule also emitting photons at 0.128 eV? Where is this thermal energy stored - in vibrations of the bonds of the molecule, or in the electrons of the molecule? 0.128 eV is about 4.9* kT, where kT is ~ 0.026 eV. I have read that the energy associated with a degree-of-freedom is about kT/2. Does this mean that the number of degrees of freedom available to the molecule depend on the temperature of the molecule? What if it is a monatomic gas molecule - He or Ar - it can't have 10 degrees of freedom! Is it reasonable that a blackbody radiator at room temperature emits photons with an energy of 5*kT?