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Kinetic Molecular Theory

The Molecular Theory of Matter states that the matter is composed of a large number a small particle, individual atoms or molecules which are in constant motion.
This theory is also termed as the Kinetic Molecular Theory of Matter and the Kinetic Theory.

By making some assumptions like "all matters comprise particles that are placed far apart and are also in constant motion", this theory helps to explain the behavior of matter. Two important areas are there to explain the flow or transfer of heat and the relationship between pressure, temperature and volume properties of gases.

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Kinetic Molecular Theory Definition

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When we define kinetic molecular theory we need to make use of the following gaseous laws as well. The basic gases theories like Boyle’s Law, Charles Law and Gay-Lussac law are derived from the observable quantity of the gas, like pressure, volume and temperature. These properties are called as the macroscopic property of the gas.

The same law can be explained by considering the atomic theory of the gas, which considers that the whole bulk of gas is composed of a large number of small particles which are called atoms or molecules. In fact the properties of liquids and solids can also be explained by the atom and molecular theory. The Kinetic Molecular theory is based on the behavior of the atoms or the molecules of gases.

All the observable macroscopic properties of the gases like volume, temperature and pressure can be explained by the behavior of molecules and atoms.
The basic definition of the kinetic theory of gas may be given as follows ‘The theory which explains the macroscopic properties of the gases by means of motion and the proximity of the particles (atoms or molecules) of gases.’

Kinetic Molecular Theory of Matter

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The kinetic theory of matter was used by the scientist to explain the behavior of the atoms and molecules in solids, liquids and gases.
  1. The Kinetic molecular theory considers that atoms and molecules which make up the matter are always in constant motion.
  2. This theory was able to explain the forces that interact between the molecules or particles in all the three state of matter.
  3. The kinetic molecular theory of matter also takes the temperature of the substance and is basically due to the energy of the atoms or molecules which constitute the matter.
  4. When the temperature of the matter is increased then the energy possessed by the molecules increases and hence they move faster, that is the kinetic energy increases.
  5. It can be said that the temperature is used to measure the energy of the movement between the molecules.

Kinetic Theory of Gases Postulates

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The basic concept of the kinetic theory of gases was given by Daniel Bernoulli. After that many authors have contributed to the theory.Clausius and Maxwell are pioneers in the development of the modern kinetic theory of gases.

The kinetic molecular theory is explained by the following postulates which describe the behavior of gas particles (that is the behavior of the molecules and atoms of gases).
  1. Gases are considered as consisting of a large number of particles which are called as atoms or molecules. Each atoms or molecules behave like a perfectly identical elastic sphere. Basically this postulate considers atoms of gases as similar with no difference in the shape or ise and as elastic sphere.
  2. The molecules or particles of the gas are in continuous motion. The motion of the molecules or atoms of the gases are also considered to be random and they move in all directions. If the gas molecules do not move then they take up a fixed position and they will have a fixed shape. But it is known that gases take up the shape of the container.
  3. The size of the each molecule constituting the gas bulk is considered to be very small; the distance between the molecules of the gas is very high compared to the volume of the molecule. Hence the actual volume occupied by the molecules of the gas is very negligible in comparison to the space in between the molecules of the gases. The gas molecules are considered to be point mass. That is their volume is very less compared to the observable volume. So the volume of the gas molecules is considered to be zero. This postulate of the gas explains its high compressibility.
  4. The molecules of gas do not attract another molecule. That is the forces of attraction or forces of repulsion between the gas molecules are very less. The molecules are not attracted towards the walls of the container. This postulate explains why gases can expand, since the molecule has no attraction between them they can expand when the volume of the container is increased and occupy the space which is available.
  5. Gases are considered to move in all directions along a straight line. During their motion they come across one another. They colloid with each other and the collision of the particles of the gas are considered to be perfectly elastic. They also colloid against the wall of the container. The collision with each other and with the walls is perfectly elastic. Their momentum and kinetic energy does not change during the collision, although the momentum and the kinetic energy can be transferred from one molecule to another.
  6. Molecules move along the straight line during the collision. The average distance traveled by the molecules during the two successive collisions is known as the mean free path of the colliding molecules.
  7. The pressure of the gas is due to the collision of the particles in the walls of the container. The pressure is described as the force per unit area. When the gas molecules occupy a smaller volume then the surface area would be low and the force per unit area would be large. So the pressure is high.

Kinetic Molecular Theory of Gases

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The Molecular Theory of Matter is a prediction that how matter should behave, based on some assesments and approximations.

The theoretical assesments are nothing but observations recorded from experiments.
There are examples where we assess that all materials consist of small molecules or atoms.
Approximations are made to keep these kind of theoretical explanations very simple.

Matter consists of small particles

The first of all assumption is that all matter contains of large number of minute particles and are mostly individual atoms or molecules.

Large separation between particles

The next assumption is concerned about the separation of the particles.
In a gaseous medium, the space between the particles are very large when compared to their size, resulting in the absence of any attractive or repulsive forces between the molecules.

In a liquid medium, although the distance between the particles is large, and yet they are close enough to allow the attractive forces to confine the material to a certain shape of its container.
In a solid, the particles are packed very close and the forces of attraction are confined to the material itself and this helps in maintaining a specific shape.

Particles in constant motion

The next assumption is that each particle is in constant motion.
In gaseous medium, the particle movement is found to be very random and with no specific or pre-destined direction.
In liquid medium, the particle movement is restricted to a large extent by the volume of the liquid.

In solids, the particles movement is completely or almost restricted within a small area, which helps the solid to maintain its shape.
The kinetic energy is basically determined by the velocity of each participating particle.

Kinetic Theory of Gases Equation

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The Kinetic theory gases of an equation is given as

The Kinetic energy of the gas = $(\frac {1} {2})$ mv2

Where m is the mass of the gas molecule and v is the velocity of the gas.

The kinetic energy of the gas is also given by the equation

Kinetic energy of the gas = $(\frac {3} {2})$ kT

Where k is the Boltzmann constant and T is the temperature in K

Kinetic Molecular Model

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The Molecular Theory of Matter states that "matter comprises of a large number of small particles which are in constant motion". This theory also assumes that particles are small and widely separated. They collide with each other and exchange energy.

The theory helps in explaining the flow or transfer of heat and the relationship between pressure, temperature and the volume properties of gases.

Kinetic Molecular Theory and Temperature

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All molecules of a gas do not possess the same speed, and this could be explained by the fact that during the collision of molecules, the speed or momentum changes. If initially the speed of a molecule is the same, it changes after the molecular collision.

So it is seen that particles have different speeds, which are changing constantly. But although individual speed changes, the distribution of speed remains constant. That is the total velocity that does not change. But if the temperature of a gas is increased, then the average kinetic energy also changes.

Because the molecules of the gas moves with greater speed, the energy of the molecule changes as a consequence of the heat applied to the gas system. So the average kinetic energy increases and the speed distribution increases.

More topics in Kinetic Molecular Theory
Boltzmann Distribution Ideal Gas Equation
Internal Combustion Engine Brownian Motion
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