# Reason for vibration of molecules

Why do molecules of a body exhibit vibrational motion? Is it due to interaction between various molecules of the body, or interatomic interactions between atoms in the same molecule? If it is because of any of them, how can these interaction cause the molecules to vibrate?

• Atoms and molecules have energy even at 0 K. This energy causes them to vibrate (among other things). Interactions between various components of the molecules (nuclear/nuclear repulsion, electron/nuclear attraction, electron/electron repulsion, etc.) determine their motion (rotational, vibrational, translational). Each component has a finite amount of energy associated with it. – LordStryker Jan 31 '14 at 20:14
• Geeze all the answers are too complicated. In the simplest sense a molecule in free space can "vibrate" because the atoms which make up the molecule can move in different directions relative to the position of the whole molecule. See images within en.wikipedia.org/wiki/Infrared_spectroscopy#Theory – MaxW Feb 9 '17 at 18:37

Here is a picture and an equation that may add to the comment LordStryker made. Molecules vibrate because they have energy. First the equation, as $T \to 0$ (absolute zero), the first term will vanish, but the molecule will still have an energy given by $h\nu/2$. So even at absolute zero a molecule will have some vibrational energy, this is called the "zero point energy". The parabola (representing the hydrogen well, for example, that two hydrogen atoms will "fall into" when they bond and become a stable hydrogen molecule) gives a pictorial representation of why this must be - energy levels are quantized. Even the lowest level is above the bottom of the parabola, so even in the lowest energy level at absolute zero, the atom or molecule will still have a non-zero energy which will cause it to vibrate.

$\displaystyle\varepsilon = \frac{h\nu}{\mathrm e^{h\nu/kT} - 1} + \frac{h\nu}{2}$

• Can we tap that for free, infinite energy? – LordStryker Feb 4 '14 at 16:20
• @LordStryker Lots of people have tried / are trying, but no success to date, that I am aware of (and I'm sure it would be big news!). Google "zero-point energy" and you'll see some links. – ron Feb 4 '14 at 22:35

Heat

Molecules vibrate because of heat. Or we could say this the other way round: heat is because molecules vibrate. If we ignore quantum stuff (molecules vibrate even at zero kelvin) then the major reason for vibration is because things are warm. And the reason why any particular molecule vibrates is because is is constantly bumping into all the other molecules around it. The more vibration, the hotter.

In a molecular gas, the molecules hit each other a lot and this ensures that any energy they have is well distributed among the molecules (in a gas much of the thermal energy is tied up in the kinetic motion of the molecules or atoms, but for molecules, some of it will be distributed in vibrations and the constant bumping into each other will ensure that the energy is well distributed).

Solid or liquids are similar but much of the energy is distributed to those vibrational models. Solids, where the atoms or molecules in them don't have kinetic energy by definition (that is, they are locked in relative place, which is more or less the definition of a solid), most of the energy is tied up in vibrations either of atoms/molecules vibrating around their location in the solid or by stretching vibrations of the bonds inside the molecules. Molecules in the solid interaction with each other (if a neighbour molecule is vibrating strongly, you will pick up some of that energy because you are close to and interacting with that neighbour).

For example, heat one end of a bar of metal and the atoms will vibrate more strongly. But those vibrations excite the atoms near the hot end and they start to vibrate more strongly as well. They in turn do the same until the whole bar is warmer and all the atoms are vibrating more vigorously.

Molecules have zero-point energy as a consequence of the Heisenberg Uncertainty Principle, which directs that it is not possible to simultaneously know the position and momentum of a particle, in this case a vibrational quantum. The zero point energy, $h\nu/2$, is that minimum energy needed to satisfy uncertainty.

In contrast rotational motion can have zero energy, i.e. the molecule does not rotate, but it is then not possible to know in which direction the molecule is pointing. As soon as it starts rotating its spatial orientation is limited to only certain values; again uncertainty limits this.