Tempas

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Molecular Vibrations

Molecular Vibrations are one of three different kinds of motion for molecules that include translational motion (when the whole molecule goes in the same direction) and rotational motion (when the molecule spins like a top)

A vibrational motion for a molecule is when the bonds between two atoms within a molecule move. Think of the atoms as two round balls that are attached by a spring that can stretch back and forth. This back and forth motion is the simplest example of a vibration for a molecule and is called stretching. Some examples of molecules that are like this include hydrogen H₂, nitrogen N₂ and oxygen O₂

But if the molecule has more than two atoms than things get more complicated. Suppose just one more atom is added so now there are three atoms in the molecule. Take water H₂O for instance where the two hydrogen atoms are both attached to the central oxygen atom. Remember with hydrogen there was one kind of stretching but in water there are two kinds of stretching and four other kinds of vibration called bending vibrations as shown below.

The atoms in a CH₂ group or molecules like water can vibrate in six different ways: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting:

Symmetrical stretching	Antisymmetrical stretching	Scissoring
Rocking	Wagging	Twisting

symmetric stretching: when the two attached atoms move away and toward the central atom at the same time.

antisymmetric stretching: When the two attached atoms move away and toward the central atom at the same time.

scissoring: Just like the name says scissoring is when the two atoms move away and toward each other

Rocking: This motion is like a pendulum on a clock going back and forth only here an atom is the pendulum and there are two instead of one.

wagging: If a person holds up their hand in front of them and puts there two fingers in a "V" sign and bend there wrist toward and away from them. Here the tips of the fingers are the attached atoms and the wrist is the central atom.

twisting: This motion is like a person is walking on a treadmill where their waist is the central atom and their feet are the two attached atoms

Molecules with more than three atoms

Molecules with more than three atoms are even more complicated and have even more vibrations which are sometimes called "vibrational modes". Each new vibrational mode is basically a different combination of the six shown above. The more atoms in the molecule the more ways they can be combined.

Applications of vibrational motions:

When light of a certain frequency hits a molecule that has a vibration whose motion corresponds to the same frequency then the light gets absorbed into the molecule and the energy from the light causes the bonds to move in that specific vibrational motion. By checking for light that gets absorbed, scientists can tell if a certain kind of molecular bond is there and match it against a list of molecules that have that bond.

However, some molecules like helium and argon are only one atom and don't have any bonds. This means that they won't absorb any light in the same way a molecule with more than one molecule can.