The atomic and kinetic theories of matter
The Atomic Theory - chemical properties The Kinetic Theory - physical properties
Combining the two theories

The Atomic Theory - chemical properties
The Modern Atomic Theory is used to describe the chemical behavior of a substance or how elements and compounds react to form new substances. The theory states that matter is made up of extremely small and numerous particles called atoms. This theory has evolved over the past two millennia from early Greek theories of matter, through the dawn of the modern scientific era, in the early 1800s, to our present mathematical descriptions of matter.

Click for larger imageToday, scientists believe that the many and varied properties of matter can be explained by the presence of atoms and the combinations of atoms Click for larger imageto form molecules and ionic crystals. Only in the past few years have modern electron microscopes been able to detect individual atoms. The atomic theory is one of the most tested and accepted theories in science and is able to predict the behavior of matter to a high degree.

The term "atom" was derived from the original Greek hypothesis put forward by Democritus, that all matter was made up of tiny unseen particles he called "atomos", the indivisible.

The Kinetic Theory - physical properties

This theory of matter describes how atoms and molecules interact to form the four physical states of matter: solids, liquids, gases and plasma. The Kinetic Theory explains the properties of the physical phases and the processes involved in changes of state, like melting and boiling, in terms of the movement of particles and the forces of attraction between particles. The Kinetic Theory can be used independently of the Atomic Theory because it does not stipulate the type of particle in terms of atoms or molecules.

Click for larger image The Kinetic or Particle Theory has five postulates describing the physical behavior of matter. The postulates are:

  1. Matter is made of tiny unseen particles.
  2. There is space between the particles. In solids and liquids, this space is quite small. In gases there is far more empty space than particles.
  3. There are attractive and repulsive forces between the particles. These forces become weaker with distance between the particles and vary from one type of particle to another.
  4. The particles are in constant motion in the form of vibrations, rotation and translation movement.
  5. The particles move faster as the temperature increases.

Combining the two theories
The combination of Atomic Theory, to explain chemical behavior of matter, and the Kinetic theory, to explain the physical properties and transformations of matter, gives scientists two very powerful tools with which to describe matter. See Atomic theory - early models, Atomic theory - modern models and Changes in state for more on these theories.

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  How small are atoms?
Were there other theories of matter?
  Matter and light
Atomic theory - early models
Atomic theory - modern models
Of atoms and elements
From atoms to molecules
Making molecules - the rules of covalency
  Question 1
Question 2
Question 3
Question 4
  States of Matter