Thursday, 9 July 2009

Atomic Theory

Atomic Theory:
The ancient philosopher, Heraclitus, maintained that everything is in a state of flux.
Nothing escapes change of some sort (it is impossible to step into the same river). On
the other hand, Parmenides argued that everything is what it is, so that it cannot
become what is not (change is impossible because a substance would have to
transition through nothing to become something else, which is a logical contradiction).
Thus, change is incompatible with being so that only the permanent aspects of the
Universe could be considered real.
An ingenious escape was proposed in the fifth century B.C. by Democritus. He
hypothesized that all matter is composed of tiny indestructible units, called atoms. The
atoms themselves remain unchanged, but move about in space to combine in various
ways to form all macroscopic objects. Early atomic theory stated that the
characteristics of an object are determined by the shape of its atoms. So, for example,
sweet things are made of smooth atoms, bitter things are made of sharp atoms.
In this manner permanence and flux are reconciled and the field of atomic physics was
born. Although Democritus' ideas were to solve a philosophical dilemma, the fact that
there is some underlying, elemental substance to the Universe is a primary driver in
modern physics, the search for the ultimate subatomic particle.

It was John Dalton, in the early 1800's, who determined that each chemical element is
composed of a unique type of atom, and that the atoms differed by their masses. He
devised a system of chemical symbols and, having ascertained the relative weights of
atoms, arranged them into a table. In addition, he formulated the theory that a
chemical combination of different elements occurs in simple numerical ratios by
weight, which led to the development of the laws of definite and multiple proportions.

He then determined that compounds are made of molecules, and that molecules are
composed of atoms in definite proportions. Thus, atoms determine the composition of
matter, and compounds can be broken down into their individual elements.
The first estimates for the sizes of atoms and the number of atoms per unit volume
where made by Joesph Loschmidt in 1865. Using the ideas of kinetic theory, the idea
that the properties of a gas are due to the motion of the atoms that compose it,
Loschmidt calculated the mean free path of an atom based on diffusion rates. His
result was that there are 6.022x1023 atoms per 12 grams of carbon. And that the typical
diameters of an atom is 10-8 centimeters.
Matter exists in four states: solid, liquid, gas and plasma. Plasmas are only found in
the coronae and cores of stars. The state of matter is determined by the strength of the
bonds between the atoms that makes up matter. Thus, is proportional to the
temperature or the amount of energy contained by the matter.

The change from one state of matter to another is called a phase transition. For
example, ice (solid water) converts (melts) into liquid water as energy is added.
Continue adding energy and the water boils to steam (gaseous water) then, at several
million degrees, breaks down into its component atoms.

The key point to note about atomic theory is the relationship between the macroscopic
world (us) and the microscopic world of atoms. For example, the macroscopic world
deals with concepts such as temperature and pressure to describe matter. The
microscopic world of atomic theory deals with the kinetic motion of atoms to explain
macroscopic quantities.
Temperature is explained in atomic theory as the motion of the atoms (faster = hotter).
Pressure is explained as the momentum transfer of those moving atoms on the walls of
the container (faster atoms = higher temperature = more momentum/hits = higher

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