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Transcript
Lesson 1: Alchemy and Atomic Models
Key Points:
1. Understand how
theories of the
structure of the
atom were
developed.
2. Learn the parts of
the atom and their
characteristics.
Discovery and Development of the Atomic
Theory
Part I: Alchemy and the birth of Chemistry
Long before the science of chemistry existed, people made
use of chemical reactions to dye cloth, tan leather, and
prepare foods. Eventually people began to search for
explanations for the structure and behavior of matter.
Discovery and Development of the
Atomic Theory
One of the earliest “Atomists” was the Greek philosopher
Democritus, he proposed that matter was made of indivisible
particles called atoms. His ideas were overshadowed however
by a more famous philosopher, Aristotle, who proposed that all
matter was made of four elements, earth, air, water and fire. As
a result the idea of atoms was not discussed seriously for 2000
years.
Discovery and Development of the
Atomic Theory
During the middle ages
chemistry reappeared
because of the rise of a group
called the alchemists, whose
goal was to change common
substances into gold.
Although they were never
successful in this quest they did
make many important
discoveries that helped to
develop science. They
produced many experimental
procedures and laboratory
apparatus. Their experiments
yielded a wealth of knowledge
about the characteristics of
substances.
Discovery and Development of the Atomic
Theory
Later the contributions of Roger Bacon and Antione Lavoisier
changed chemistry from a trial and error process of observation
to a science of measurement, and paved the way for modern
chemistry and the atomic theory.
Discovery and Development of the Atomic Theory
Suppose we take a small cube of the element lead and cut it
into smaller and smaller pieces. As the soft grey metallic pieces
get smaller and smaller they still retain the properties of lead.
Eventually we would reach a point when the particle of lead
could no longer be divided and still retain its properties. This
particle is an atom the smallest particle of an element that
retains the properties of that element.
• It was not until the late 1700s that chemists were able to
relate chemical changes to events at the level of
individual atoms. At that time the English chemist John
Dalton first stated his atomic theory. Dalton’s Atomic
Theory included the following ideas:
1. All elements are composed of tiny indivisible particles
called atoms.
2. Atoms of the same element are identical. The atoms of
any one element are different from those of any other
element.
3. Atoms of different elements can combine with one
another in simple whole number ratios to form
compounds.
4. Chemical reactions occur when atoms are separated,
joined, or rearranged. However atoms of one element
are not changed into atoms of another by a chemical
reaction.
This theory has been generally accepted over the years
although we now know that an atom can be broken down into
smaller particles. Dalton believed in the “solid sphere model”
which said that an atom is nothing more than an infinitely small
solid sphere.
Later in 1897 J.J. Thomson discovered that atoms contained
small negatively charged particles called electrons. So he
modified Dalton’s model into what he called the “Plum
Pudding model” which said that an atom was a solid sphere
with small negatively charged particles embedded in it like
plum pudding had pieces of plum in it.
Shortly thereafter, Protons were discovered, this was due to
the fact that all chemists agreed that atoms had to be
electrically neutral, so if there was a small negatively charged
particle there had to be a positively charged particle as well,
this positively charged particle is many times larger than the
electron and is called the Proton.
In 1932 James Chadwick discovered that Protons were not
the only large particle in the atom. He found that the mass of
many elements was too large to be accounted for by only
protons and electrons. His experiments proved the existence
of the third and final subatomic particle the Neutron which
carries no charge or is neutral.
Further to this discovery a scientist named Ernest Rutherford
felt that atoms could not have all these particles floating
around randomly, and that the plum pudding and solid
sphere models were not accurately explaining the findings of
later experiments. So he conducted an experiment where
he bombarded a thin sheet of gold foil with alpha particles.
If the protons, neutrons and electrons were simply floating
about (which was the current theory) then the alpha
particles should pass through the foil unhindered.
To the shock of Rutherford and his associates many of the
alpha particles bounced back. Rutherford was so impressed
that he was quoted as saying “it was like firing a 15inch shell at
a piece of tissue paper and having it bounce back at you!”.
This led to the development of the Rutherford model of the
atom which said the atom was made of a dense central
nucleus where the heavy particles (protons and neutrons)
were located and a the electrons surrounded this nucleus.
Later experiments by Neils Bohr showed this to be mostly
correct except that the electrons are not positioned randomly
they can be found orbiting the nucleus at fixed distances or
energy levels.
Further to this Louis de Broglie surmised that electrons could
not be found in fixed positions, but did show evidence of
distinct energy levels, so he combined the Rutherford
nuclear modeland the Bohr model into what is known as the
“electron cloud model”.
In de Broglie’s model the electrons can jump or fall from one
level to another depending on the energy of the atom. This is
what allows for “glow in the dark” fluorescent type materials,
electrons in the atom gain energy from light and are excited to
a new level. When the light is turned off they slowly return to
their previous state, releasing their energy in the form of light.
The de Broglie model is currently the accepted model.
The structure of the atom, as we currently understand it is that
there is a dense positively charged nucleus that contains the
neutrons and protons, and there is a large volume of space
surrounding the nucleus in which the electrons can be found.
So in fact atoms are made up of a lot of empty space!
So if all atoms are made of protons neutrons and electrons,
how can the atoms of one element differ from another? The
answer is that all elements have different numbers of protons in
their nucleus, Carbon atoms, for example, have 6 protons
while lead atoms have 82 protons, and fluorine atoms have 9
protons. These differing numbers of subatomic particles
influence the properties of the different elements.