Molecular Motion

Introduction

Matter can exist in three states- solid, liquid and gas. These states of matter can be differentiated on the basis of the position or motion of their constituent particles. In solid-state, particles cannot move from their position. They can only vibrate at their mean position. On the contrary, in gaseous state particles can move randomly due to their high kinetic energy. They do not have a fixed position but show random movement.

The liquid state can be considered as an intermediate state of matter in which particles can be moved and do not have a fixed position but their kinetic energy is less than gaseous particles and more than solid particles. They acquire the shape of the container in which they are.

So we can say that all particles show motion more or less. They are in constant motion and also different from molecules such as liquid molecules have more freedom of movement compared to solid molecules. At the same time, they show less movement compared to gaseous molecules. In other words, we can say that gaseous molecules have the greatest degree of motion.

Table of Contents

• • Molecular Motion Definition
  • Types of Molecular Motion
  • Different Types of Motion
  • Random Molecular Motion
  • Measure of Molecular Motion

Molecular Motion Definition

Molecular motion is defined as the movement of constituent particles or molecules in a certain direction. The molecular motions are affected by heat and temperature. This is because temperature is the measurement of the average kinetic energy of the molecules and represents the motion of molecules.

Similarly, heat transfers energy among constituent molecules that increase the kinetic energy of molecules. The mathematical relation between kinetic energy and temperature can be shown below;

E = kT

where, E=Energy, k=boltzmann constant, T=temperature

Types of Molecular Motion

The different types of molecular motions are

• • • 1. Translational motion: In such kind of motion, molecules can move from one place to another in the same or different direction but always remains on the same axis.
      2. Rotational motion: In this type of motion, the molecule can rotate in and around the axis.
      3. Vibrational motion: In this type of motion, molecules can vibrate at their mean position. These motions are very common in solid state.
      4. Electronic motion: In this type of motion, electrons can move from place to place and orbital to orbital. Electronic motions cause a change in the colour of substances.

Different Types of Motion

Unlike the solid and liquid states, molecules in the gaseous state show random motion. That is the reason; gases take the shape of the container and spread quickly in space. The random motion of molecules in the gaseous state is due to high kinetic energy in molecules. They have weak intermolecular interactions between them.

Three States of Matter

The intermolecular space between gaseous molecules is very large. They can show all the three types of molecular motion, vibrational, rotational and translational motion.

• • • • In vibrational motion, molecules move back and forth whereas in rotational motion the molecule rotates in space. In translational motion, molecules move in certain directions. Molecules of solid-state are capable of vibrational motion due to strong intermolecular forces.
      • Therefore, they show the least random molecular motions. Like solids, liquids are capable of vibrational motion but at the same time, they can also show rotational and translational motions due to weak intermolecular forces between molecules. Hence, liquids can show random molecular motions but are less random compared to gas molecules.
      • So we can say that random motion is related to temperatures, intermolecular forces of attractions, the kinetic energy of molecules and heat transfer.

Random Molecular Motion

• • • • Unlike solid and liquid states, molecules in a gaseous state show random motion. That is the reason, gases take the shape of container and spread quickly in space.
      • The random motion of molecules in the gaseous state is due to high kinetic energy in molecules. They have weak intermolecular interactions between them.
      • The intermolecular space between gaseous molecules is very large. They can show all three types of molecular motion: vibrational, rotational and translational motion.
      • In vibrational motion, molecules move back and forth whereas in rotational motion the molecule rotates in space.
      • In translational motion, molecules move in certain directions. Molecules of solid-state are capable of vibrational motion due to strong intermolecular forces.
      • Therefore, they show the least random molecular motions. Like solids, liquids are capable of vibrational motion but at the same time they can also show rotational and translational motions due to weak intermolecular forces between molecules.
      • Hence, liquids can show random molecular motions but less random compared to gas molecules.
      • So we can say that random motion is related to temperatures, intermolecular forces of attractions, the kinetic energy of molecules and heat transfer.

Measure of Molecular Motion

• • • All atoms or molecules require different amounts of energy for different types of molecular motion. The light absorbed during motion can be used to measure molecular motion.
    • We can measure the frequencies of absorbed light and correlate them with some bonding arrangements which are present in the molecules.
    • Various types of spectroscopy methods like NMR spectroscopy, UV-spectroscopy, Mass spectroscopy, and Infrared spectrums can be used to measure the molecular motion. These methods also provide sufficient information about the structure of molecules.
    • Each type of absorption indicates a certain type of bond-like absorption frequency of the C-H bond is different from H-H or O-H bond.