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:
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 pressure).
Ideal Gas Law:
Macroscopic properties of matter are governed by the Ideal
Gas Law of chemistry.
An ideal gas is a gas that conforms, in physical behavior, to a particular,
idealized relation between pressure, volume, and temperature. The ideal gas law
states that for a specified quantity of gas, the product of the volume, V, and
pressure, P, is proportional to the absolute temperature T and the number or
density of particles, n,; i.e., in equation form, PV = nkT, in which k is a
constant. Such a relation for a substance is called its equation of state and is
sufficient to describe its gross behavior.
Although no gas is perfectly described by the above law, the behavior of
real gases is described quite closely by the ideal gas law at sufficiently high
temperatures and low pressures (such as air pressure at sea level), when
relatively large distances between molecules and their high speeds overcome any
interaction. A gas does not obey the equation when conditions are such that the
gas, or any of the component gases in a mixture, is near its triple point.
The ideal gas law can be derived from the kinetic theory of gases and relies
on the assumptions that (1) the gas consists of a large number of molecules,
which are in random motion and obey Newton's deterministic laws of motion; (2)
the volume of the molecules is negligibly small compared to the volume occupied
by the gas; and (3) no forces act on the molecules except during elastic
collisions of negligible duration.
Thermodynamics:
The study of the relationship between heat, work, temperature, and energy,
encompassing the general behavior of physical system is called thermodynamics.
The first law of thermodynamics is often called the law of the conservation
of energy (actually mass-energy) because it says, in effect, that when a system
undergoes a process, the sum of all the energy transferred across the system
boundary--either as heat or as work--is equal to the net change in the energy of
the system. For example, if you perform physical work on a system (e.g. stir
some water), some of the energy goes into motion, the rest goes into raising the
temperature of the system.
The second law of thermodynamics states that, in a closed system,
the entropy
increases. Cars rust, dead trees decay, buildings collapse; all these things are
examples of entropy in action, the spontaneous movement from order to disorder. 
Classical or Newtonian physics is incomplete because it does not include
irreversible processes associated with the increase of entropy. The entropy of
the whole Universe always increased with time. We are simply a local spot of low
entropy and our destiny is linked to the unstoppable increase of disorder in our
world => stars will burn out, civilizations will die from lack of power.
The approach to equilibrium is therefore an irreversible process. The
tendency toward equilibrium is so fundamental to physics that the second law is
probably the most universal regulator of natural activity known to science.
The concept of temperature enters into thermodynamics as a precise
mathematical quantity that relates heat to entropy. The interplay of these three
quantities is further constrained by the third law of thermodynamics, which
deals with the absolute zero of temperature and its theoretical unattainability.
Absolute zero (approximately -273 C) would correspond to a condition in
which a system had achieved its lowest energy state. The third law states that,
as this minimum temperature is approached, the further extraction of energy
becomes more and more difficult.
Rutherford Atom:
Ernest Rutherford is considered the father of nuclear physics. Indeed, it
could be said that Rutherford invented the very language to describe the
theoretical concepts of the atom and the phenomenon of radioactivity. Particles
named and characterized by him include the alpha particle, beta particle and
proton. Rutherford overturned Thomson's atom model in 1911 with his well-known
gold foil experiment in which he demonstrated that the atom has a tiny, massive
nucleus.
His results can best explained by a model for the atom as a tiny, dense,
positively charged core called a nucleus, in which nearly all the mass is
concentrated, around which the light, negative constituents, called electrons,
circulate at some distance, much like planets revolving around the Sun.
The Rutherford atomic model has been alternatively called the nuclear atom,
or the planetary model of the atom.
A lesson plan for atomic theory - SOURCE
ATOMIC THEORY:
INTERNET BASED LESSON PLAN
Goal:
The objective of this lesson is for students to become familiar with the
historic and scientific development of atomic theory. They will recognize the
beginnings of atomic theory with the ancient Greeks, and follow its changes
through to modern wave theory. Studentswill also work in cooperative learning
groups, and further develop their computer and internet skills.
Overview:
The class will discuss as a large group, how advances in technology require
previous levels of discovery. Students will be informed that in their studies of
atomic theory, they will discover that scientific theory also requires previous
levels of theorization. In small groups, students will study atomic theory
through use of the Internet. Each group will then select the one stage or
scientist of atomic theory development it feels was the most significant in
reaching current wave theory. Groups will then prepare a written report that
defends their selection. Further studies can then be done on quantum studies,
the role of the electron in bonding, radioactivity, atomic bombs, fusion, or
fission.
Materials:
Computers with Internet Access.
INVITATION
Students will be engaged in a teacher led discussion regarding the
development of technology. The discussion should focus on how technology relies
on established levels of scientific discovery in order to advance. For example,
students should be asked: "Why weren't cellular phones invented when
regular phones were invented?", "Why weren't cd's around when 8-tracks
were?", etc. Students will quickly contribute to these and other scenarios.
Students should then be informed that the concept of cumulative development will
then be applied to scientific theory as they study atomic theory. They will
learn how reaching current atomic wave theory required the development of other
scientific theories and discoveries.
EXPLORATION
Students will be divided into small groups in which they will work on this
Internet based activity. Each group should proceed to following sites to learn
about atomic theory development. Students should be encouraged to take notes and
record the most significant information they encounter.
Each group should visit at least 3 of the following websites in order to
research atomic theory. These sites contain summaries of atomic theory that
provide excellent general perspectives on atomic theory. Groups should all
answer the review questions included in the first website listed below.
Websites on Atomic Theory :
EXPLANATION
Having visited the sites on atomic theory, each group should select the one
stage/ individual of atomic theory development it feels was the most significant
in reaching current wave theory. Groups are then to conduct further research on
the individual they have selected, by utilizing the respective websites listed
below. All groups should visit the website on Modern/Wave Atomic Theory in order
to complement their research. A written report is then to be prepared by each
group. The report should contain a general summary of atomic theory, it should
name the individual they have selected as the most significant, and should
clearly state reason(s) that defend their selection. Reports should be turned in
to the teacher, and as an time dependent option, groups can briefly present
their report verbally before the class.
Democritus
Dalton
Thomson
Rutherford
Bohr
Modern / Wave Atomic Theory
TAKE ACTION
Further studies can be done on quantum studies, the role of the electron in
bonding, radioactivity, atomic bombs, fusion, or fission. Also, the reports
written by the groups could be united to create a study guide of atomic theory
for younger grades or future use.
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