While chopping onions or producing sawdust in a woodwork project, have you ever
wondered what effect making smaller and smaller pieces would have? Our science
courses teach us that the materials we see are made up of molecules formed from
combinations of atoms, and each different combination has different properties.
In the Golden Age of Greece, Leucippus, a student of Zeno in the school of Pythagoras,
proposed that the world consisted entirely of atoms with empty space in between.
Most Greek thinkers believed that each material was a particular blend of the four
elements earth, air, fire, and water. After Democritus went on to explain how
different atoms would provide different observed properties in materials, little
further expansion of the atomic theory of materials took place for over 2000 years
until John Dalton put forth his atomic theory at the start of the 19th century.
If we leap ahead to the start of the 20th Century, we find that the results observed
by the chemists like Dalton and Joseph Priestly over the previous two centuries,
plus new instruments available to physicists, began to divulge the properties that
an atom must exhibit. Ionized gases, such as those in a mercury or sodium arc lamp,
were seen to emit not a continuous spectrum of light, but rather a series of discrete,
well-defined colors of light. In 1911 Lord Rutherford observed that the positive
electrical charge is concentrated in a very small, relatively massive nucleus of an
atom, and this nucleus is surrounded by a cloud of light-weight, negatively-charged
electrons, the total charge being balanced in an atom. This 'solar system model' of
the atom was revolutionary for the time.
Niels Bohr, in collaboration with Rutherford, developed a model for an atom that
required that the electrons move in circular orbits around the nucleus, orbits that
have certain quantized amounts of momentum. Many such orbits exist, but the electrons
tend to fill the lowest-energy orbits, with only a few allowed to occupy each level.
If an electron is caused to undergo a transition from one orbit to another, it must
absorb or emit a wave having the energy difference of those orbits. These wave-like
emissions consist of visible light or X-ray radiation. Transitions between the
allowed orbits in an atom thus cause radiations having energies associated with
the energy differences of the allowed orbits. We see that the discrete colors of
light that are seen in gas arc lamps can be understood as the radiations corresponding
to electrons, previously excited by electric forces, undergoing transitions from one
higher-energy allowed orbit to another lower-energy one.
The success of Bohr's model for the atom in explaining the discrete colors emitted
by gaseous arc lamps brought immediate attention to the question why this structure
should exist and be stable. The theory of quantum mechanics was developed to describe
these phenomena. Not only has quantum theory been successful in describing atomic
structure and the interactions with radiations, but it has also been applied to
understand the structure of the nucleus and radiations and particles associated
with that part of an atom.
So we see that, just as you might wonder how small you can chop the onion and how
the properties would change as the pieces shrink, the scientists whose unexplained
observations of the light emissions from atoms were inspired to completely change
their way of thinking about matter. The concepts they developed for the very small
world of atoms, the quantum world, get "curiouser and curiouser." Learn more about
these concepts and the people who developed them by going to the web sites cited below.
Check back soon for the Complete Story!