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December 18, 2002
A: FROM MENTOR ROSE CLARK
IN PA
John Dalton proposed the atomic structure of materials and
proposed that
all materials are composed of extremely small particles that
are
indivisible. Atoms of an element are all the same and that
atoms of
different elements are different and have different properties.
The
discovery of atomic structure came later. Experiments by J.J.
Thomson
(mid-1800's) with a cathode-ray tube (generates a stream of
electrons
and emits light) showed that there were negatively charged
particles in
the atom, electrons. Experiments with radioactivity by Henri
Becquerel
(1852-1908) and Marie Curie showed that the atom contained
positive as
well as negatively charged particles. Ernest Rutherford (early
1900's,
1910-1911) put all of the information together along with
his
experiments using gold foil. Rutherford used alpha particles
(positive
particles) and passed them through a piece of gold foil (~100
atoms
thick). He found that most of the alpha particles just passed
through
the gold foil. He proposed that most of the atom is just empty
space.
A few of the alpha particles were repelled. When an alpha
particle
collides with the nucleus that is also positively charged,
it is
strongly repelled. Subsequent experiments led to the discovery
of
protons and neutrons in the nucleus. Protons were discovered
in 1919 by
Rutherford and neutrons in 1932 by James Chadwick. If you
want to read
about this in more detail, the text "Chemistry-The central
science" by
Brown, LeMay and Bursten gives a good summary of the developments.
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December 18, 2002
A: FROM MENTOR JOAN LUSK IN
RI
Many chemistry textbooks will explain this; here's a site
that does a good job.
http://dl.clackamas.cc.or.us/ch104-04/dalton's.htm
I think it's interesting to recognize that careful measurement
-
painstaking, not exciting in itself - was vital to Dalton's
developing atomic theory, which was an exciting, revolutionary
idea.
Careful measurements showed that the elements combined to
make
compounds in fixed ratios by weight, and sometimes in simple
multiples of a basic ratio. For example, 12 g of carbon could
combine with 16 g of oxygen (to make what we now know is CO)
or 12 g
of carbon could combine with 32 g of oxygen, exactly twice
as much
(and we now know that makes C02). Why should we find these
constant
ratios, in small multiples? Because the elements that combine
are
made up of particles, atoms, each with a fixed weight, and
the ration
of the weight of an atom of C to an atom of O is 12:16.
But even more interesting - those careful measurements were
just
careful enough, not so precise as to be confusing! If you
look at
the periodic table and the atomic weights, you'll see that
the atomic
weight of C is 12.0107 and O is 15.9994. (I used
http://www.webelements.com/ - being too lazy to pick up a
book. That
site gives you a wealth of info about each element.) Maybe
you
already know that elements can have several isotopes with
different
numbers of neutrons, and the atomic weight we measure is the
weight
of a mixture of these isotopes. If the early 19th century
analytical
methods had been good enough to find a ration of 12.0107 to
15.9994
and not 12 to 16, that could have been confusing indeed!
I enjoy noticing many examples of science advancing from a
simple
theory to explain most of the cases (or less precise measurments)
to
a more sophisticated but complicated theory that explains
more of the
cases (or more precise measurements.) Part of the "art"
to doing is
science is to know what level of precision is required to
advance
from the current state of our knowledge. Goldilocks would
have been
good at this - knowing when it's "just right."
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