Particle model

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The particle model is one of the simplest models for the structure of matter . In contrast to the continuum model , it is based on the basic assumption that extended bodies consist of many individual particles , which only through their interaction give rise to the properties that are shown in the macroscopic states and processes. This assumption that a domain consists of the smallest, fundamental, indivisible or reducible elements, is called atomism . Atoms were identified as (almost) invariable particles of this type . Even with small macroscopic bodies, their number easily exceeds the order of magnitude of a 23-digit number, i.e. about one mole (approx. 6 * 10 23 ).

In the context of the particle model, it is possible to describe in a simple way how solid, liquid and gaseous substances are formed from atoms, what different mechanical properties these aggregate states have and how they transform into one another. A brief overview is given below. For the atoms themselves, in turn, there are other particle models , as shown in the list of atomic models , in which their properties are explained by their structure from even smaller particles, ultimately the elementary particles .

Explanations with the help of the particle model

Particle model of a crystalline solid

The particle model assumes that the particles of a pure substance are all identical to one another. But they differ from the particles of other substances, for example in their size, shape or mass . No statement is made about the internal structure of the particles.

In the simplest approach, the particles are represented as hard spheres, which is approximately correct for atoms in many contexts. This particle model is already suitable for describing noble gases in their gaseous state. In the next stage of the model, particles are assumed that can stably combine with other particles according to fixed rules. This corresponds to the chemical bond between atoms, which can thus form molecules of different sizes, masses and shapes. The particle model can thus interpret chemical transformations and the diversity of the materials surrounding us. The molecules of a chemically pure substance are all identical to one another. If they only consist of a few atoms, in many applications of the particle model it is sufficient to assume them again as spheres of the same type.

Finally, the particle model is extended by the fact that the particles can exert repulsive forces when approaching each other and attracting forces when they are in the middle. The latter are much weaker than the chemical bond, but determine the macroscopic appearance of the matter decisively. Among other things, the following observations can be explained in the context of this particle model:

  • The mechanical strength of solid bodies (see fig.) And the easy deformability of liquids and gases : In the crystalline solid body, the particles hold each other almost immovably on grid positions, in liquids only relatively weakly, and in gases not at all.
  • The heat energy and temperature : the particles are constantly in motion; the higher the temperature of a substance, the faster its particles move on average (thermal movement).
  • The physical states that are determined by the attraction of the particles to one another in conjunction with their more or less violent thermal movement.
  • The equations of state of gases, i.e. the relationship between pressure , density and temperature. This includes B. the following point:
  • The compressibility of gases: If you exert pressure on gas that is in a closed container, the volume is reduced. This is possible because the large distance between the particles is reduced. Most gases can even be liquefied at particularly high pressure. With liquids and solids, the volume can hardly be reduced at all because the particles are already close to one another.
  • The Brownian movement : A speck of dust in water seems to move irregularly in a zigzag on its own under the microscope, because the molecules of the water hit the speck of dust irregularly due to their own (thermal) movement.
  • The diffusion : without involvement of a flow itself of the particles dispersed by the thermal movement of a gas even in another gas (or vacuum); The same thing is done, for example, by the dye molecules of an ink drop in water.
  • The pressure : The particles enclosed in a volume hit the walls due to their thermal movement and thus generate an outwardly directed force that remains constant on average.
  • The heat transfer , in particular heat conduction : heating an object at a location, the particles located there fall into greater movement. They pass this on to the neighboring particles through impacts, whereby the faster movement gradually spreads throughout the object.
  • The absolute zero : Upon cooling, the thermal motion of the particles becomes slower. At −273.15 ° C the point is reached at which the substance cannot cool down any further.

"The most important finding in physics"

In order to properly appreciate the importance of the particle model, the great physicist Richard Feynman asked in his textbooks Lectures on Physics , which was published in the 1960s , which knowledge of physics is worth passing on to posterity if one only has the opportunity to do one Sentence would have. His answer:

"All things are made up of atoms - small particles that move forever, attract each other when they are some distance away, but repel when they are pressed against each other."

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