Corpuscular theory of matter

The corpuscular model of matter is a model of the microscopic structure of matter, which seeks to explain the properties and behavior in each state of aggregation.

The fundamental postulate of the model is that matter is made up of small particles, which can be atoms, ions or molecules, always in continuous agitation.

In this model, the laws of classical mechanics are applied to the particles, although due to the large number present, they are not studied individually but by means of a statistical treatment. In this way, the average values ​​of the macroscopic quantities of interest are obtained, such as pressure, temperature and volume .

The corpuscular model also explains properties such as viscosity, hardness, flexibility and density of materials, as well as thermal expansion, among other phenomena.

Ideas about the corpuscular nature of matter date back to at least the 5th century BC In ancient Greece, Leucippus (450 BC – 370 BC) and Democritus (460 to 370 BC), his disciple, they had speculated about the organization of matter at the microscopic level.

These philosophers proposed that matter is composed of tiny indivisible particles, named atoms , a word with a Greek root that precisely means “indivisible.”

The theory was discarded by Aristotle, the most influential sage of his time. However, it was not until the seventeenth century that corpuscular theory began to gain prominence again, and in the nineteenth century, the secrets of matter finally began to unravel.

In 1803 the English chemist John Dalton (1766-1844) proposed again that matter was made up of corpuscles called atoms, which combined in certain proportions to form the molecules of a substance.

Dalton’s atoms were indivisible, but a strong theoretical structure soon developed, largely thanks to James C. Maxwell (1831-1979) and Ludwig Boltzmann (1844-1906). Thus the foundations of statistical mechanics and solid state physics were established .

The main postulates of the model are:

1. Matter, regardless of its state of aggregation, is made up of microscopic particles, which can be individual atoms or in combination to form molecules.
2. Different substances differ from each other because they are made up of different particles.
3. According to the state of aggregation of matter, there are empty spaces between the particles, larger in gases and smaller in solids.
4. The particles that make up matter are in perpetual turmoil. The greater the agitation, the greater the kinetic energy they possess, and if heat is added, the movement of the particles increases.
5. Likewise, in liquids and solids, the particles experience attraction of different intensity, due to intermolecular forces. In gases, the molecules are widely separated and move rapidly, so these forces do not have a major impact. But in liquids, whose particles have more cohesion, these forces are stronger, and in solids it is even higher.

Kinetic theory of gases

The corpuscular model was applied first to gases, as it is the simplest state of aggregation and the cohesion forces between the molecules are minimal. Furthermore, the properties of gases were well known through the experimental works of the English chemist Robert Boyle and the Frenchmen Joseph Gay Lussac and Jacques Charles.

The ideal gas model, the simplest, contemplates that:

1. The size of the particles is much smaller than the distances between them, and the dimensions of the container. They are point masses, that is, they lack dimensions and do not occupy a volume.
2. The particles are in permanent agitation, their movement being only translational.
3. Occasionally the particles collide elastically, with each other and with the walls of the container. In fully elastic collisions the kinetic energy is conserved.
4. There are no attractive forces between molecules. The only interactions are due to collisions that occur from time to time and for a very short time. Of the rest, each molecule acts independently of the others.
5. If there are no external forces acting on the particle system, they are distributed evenly throughout the available volume.

Although this is the simplest model, it explains well the behavior of any thin gas at high temperatures and low pressures. Scientists have developed other models better adjusted to the behavior of real gases, for this they consider that:

• The molecules have a measurable size.
• Intermolecular forces are not canceled out.

Ideal gas equation

Thanks to the ideal gas postulates, an expression arises that relates the macroscopic magnitudes of the pressure P, the volume V and the temperature T:

P ∙ V = nRT

Where n is the number of moles of gas and R is the universal gas constant, whose value in International System units is 8.314 J / mol ∙ K.

Kinetic energy and temperature

In an ideal gas, all internal energy manifests as kinetic energy. The mean kinetic energy K of the n molecules of an ideal gas is directly proportional to its temperature in kelvin:

Corpuscular model in liquids and solids

In reality, all matter can be found in the three simplest states: as a gas, liquid, or solid, depending on pressure and temperature. There are other states of aggregation like plasma, but not near the surface of the Earth.

These states are due to the way in which intermolecular attractive forces act, which are short-range, that is, they do not act if the distance between molecules is very great and the particles move at high speed. This is the case of gases, which allows the phenomenon of diffusion to occur more quickly. This explains that when uncovering a perfume the aroma spreads quickly.

On the other hand, when the particles are closer and their movement is slower, as in liquids and solids, these intermolecular forces have the opportunity to exert their action.

Thus, when a gas passes to the liquid state its volume decreases, because the distance between its particles decreases, and even more when it becomes a solid.

Matter in liquid state

In the liquid state, the intermolecular forces are not negligible, but they act on a smaller scale than in the solid state. The particles form small groups, which continuously disintegrate and regroup, providing mobility to the material, while maintaining a fixed volume.

This mobility allows the liquids to adapt to the shape of the container and gives them the ability to flow, in addition to being able to mix more easily. Diffusion can occur, but more slowly than in gases.

Still, the particles have enough cohesion that the liquid remains incompressible over a wide range of temperatures.

Surface tension

Molecules remain on the surface of the liquid, acted upon by unbalanced intermolecular forces, resulting in a net upward force. This net force is minimized when the liquid has the smallest surface possible.

Thanks to this force, pins, clips, insects and other small objects manage to stay on the still surface of the water without sinking.

Solid state matter

In the solid state the particles are very close to each other, and intermolecular forces cause the particles to stay in fixed positions. Although they can vibrate around this position, solids take on a defined shape and maintain their constant volume.

As heat flows, the amplitude of the vibratory motion increases and the temperature increases. This causes the dimensions of the body to increase with temperature, a phenomenon called thermal expansion . If enough heat flows, the solid can even go into the liquid phase.