When a collision between atoms occur like in heat conduction between two gases, does the energy transferred cause an electron to move to a higher energy excitation state, or does it just increase the kinetic energy of the atom (i.e. its temperature) or both?

For example let’s say that atom A has higher kinetic energy than atom B, and that atom B needs 10 or 20 units of energy to get to more excited levels. If the collision between A and B gives 15 units of energy to atom B, how much of that energy goes to get an electron of atom B to a higher energy level and how much goes to increasing the kinetic energy of atom B (i.e. increasing its temperature)? Also what about atom A, it lost 15 units of energy does that only go to reduce its kinetic energy (i.e. its temperature) or does it also allow electrons in atom A to go to a lower excitation level.


1 Answer 1


The answer depends on the details.

Room temperature is about 1/40th of an eV kinetic energy. For atomic gases (I.e. helium), the electronic excitations require a lot more energy than is available from thermal motion. So collisions just bounce the atoms around without electron excitation.

At much higher temperatures, for example in a flame, there’s enough energy, and you do get excited atoms which can then emit light.

Back at lower temperatures, more complex molecules can have lower energy excitations. The more complex molecules tend to have lots of vibration and rotation modes that can be excited at room temperature. That leads to observable effects, such as higher heat capacities. Those modes can also radiate (and absorb) in the infrared, which leads to black-body radiation emission and absorption.

If you want to include them as "excited energy states", thermal energy can also break the inter-molecular bonds that hold solids and liquids together. Collisions with enough energy, i.e. at a high enough temperature, break apart the weak bonds that hold molecules together in a liquid, so that it is instead a gas. Cool it a bit and it condenses; add more heat energy and the bonds will break apart, transforming it again to a gas.


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