The invisible motion of still objects

Even though most objects around us seem to be stationary, their internal structures beg to differ. Let’s zoom into them and learn about their atomic structures.

You must’ve heard of this definition of Matter- “Anything and everything that occupies space and has mass is made up of matter”. If matter constitutes everything, then what is Matter made up of? Diving deep into Matter, atoms were found to be in constant, regularized and orderly motion. So, even if the objects around us are still, internally everything is in constant flux.

What kind of movement do molecules undergo?

Atoms bond together to form molecules. For example, two Hydrogen and one Oxygen atoms come together under the covalent bond to form a water molecule.

Now, these molecules so formed undergo movement in three ways- Rotation, Translation and Vibration.

  1. During Rotation and Translation, the distance between the respective atoms making up a molecule stays the same. This kind of movement simply moves the molecule in space.
  2. Whereas, Vibration causes the distance between these respective atoms to change and therefore alters the molecule’s shape as well.

Movements within a molecule

Any molecule’s nature can be studied by taking its molecular structure and the movements it experiences into count. To understand these movements better, the three-dimensional pace is divided into three axes- x, y and z.

The freedom of degree enacted by a molecule depends on its movements. During Translation, molecules can move in the direction of either of these three axes, thereby making it three degrees of freedom. Furthermore, adding the Rotational movement to this increases their freedom to six degrees.

However, a linear molecule like that of Carbon di-oxide experiences rotation in such a manner that it only spins around its own axis. This doesn’t induce any change in the positions of the atoms.

Understanding Vibration is quite complex and it varies for every different kind of molecule. For example, a Hydrogen molecule that has two hydrogen atoms linked linearly responds to vibration as if the atoms were a spring.

This leads to the observation that the number of atoms and bonds of a molecule is directly proportional to how much vibration it experiences. So, a water molecule having two bonds and three atoms exhibits three modes of vibration- symmetric and asymmetric stretching and bending. This gets even more complicated as the bonds keeping a molecule together increase and get complex.

Calculating the vibrational modes

Yes! Having the knowledge of the number of atoms within the molecule is necessary to estimate the number of vibrations it shows.

We start with the degrees of freedom exhibited by a molecule. This can be calculated by a simple formula: 3 X no. of atoms of a molecule (N).

Having calculated the degrees of freedom, now we move on to the vibrational modes. These can be calculated using the formula: 3N-6 (or 3N-5, for linear molecules)

These movements within a molecule can be attributed to the absorption of energy in the form of heat or electromagnetic radiation. As the rate of these three kinds of movements experienced by the molecules increases, it also increases the kinetic energy. This further leads to an increase in temperature and thermal energy. Microwave heating uses this very phenomenon as its basis.

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