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Greek Beginnings-Democritus
The concept of the smallest particle was born in the 5th century BC by Leucippus of
Miletus. His pupil, Democritus of Abdera developed five major points that their theory
was based upon. Historians have discovered this from the quotations of other Greeks
(most of the original documents by Leucippus and Democritus have been lost). In the
4th century BC, the well known philosopher Aristotle strongly argued that the atomic
theory was completely incorrect and was therefore dismissed by scientists for many
decades. In fact, the Catholic Church agreed with Aristotle's position and announced
that atomistic ideas were equivalent to those of Godlessness.
The 5 points of their conclusion are as follows...
Point
Point
Point
Point
Point
#1
#2
#3
#4
#5
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All matter is made up of undividable particles called atoms.
There is empty space between atoms.
Atoms are completely solid.
Atoms are homogeneous, with no internal structure.
Atoms vary in
1) Size
2) Shape.
3) Weight
John Dalton
John Dalton is now called the father of modern atomic theory for his efforts. His atomic
theories were introduced in 19th century England. In September of 1803, John Dalton
wrote his first table of atomic weights in his daily logbook. Two years after he developed
his atomic weights, he published them in a book called "A New System of Chemical
Philosophy”. In it he was the first to propose that elements be identified with symbols.
Also in this book, Dalton discussed the atomic theory he proposed. In this theory, there
are four basic ideas...
1) elements are made of atoms.
2) the atoms of an element are identical in their masses, atoms of different
elements have different masses
3) atoms only combine in small, whole number ratios such as 1:1, 1:2, or 2:3
4) atoms can be neither created nor destroyed
J. J. Thomson
In 1897, J.J. Thomson discovered the electron, the first subatomic particle (particle
smaller than an atom). He also was the first to attempt to say that the electron was part
of an atom’s structure. For thousands of years before Thomson, scientists had thought
that the atom was solid and had no smaller parts.
Thomson was experimenting by applying high voltages to gases at low pressure creating
what are known as cathode rays. He noticed an interesting effect. The cathode rays
generated in his experiment were attracted to positively charged electric plates.
Thomson correctly concluded that the cathode rays must consist of particles that are
negatively charged themselves. Further experiments showed that these particles were
about 2000 time lighter than hydrogen atoms. Thus he had discovered something
smaller than the smallest atom! These tiny, negatively charged particles were called
electrons.
But where had these tiny particles come from? Since they were so small, Thomson
reasoned that they could only have come from inside atoms. So Dalton's idea of the
indivisible atom had to be revised.
Thomson proposed a different model for the atom. He said that the tiny negatively
charged electrons must be embedded in a cloud of positive charge (after all, atoms
themselves carry no overall charge, so the charges must balance out). Thomson
imagined the electrons as chocolate chips spread through a cookie dough.
Earnest Rutherford
The next development came about 10 years later. Two of Ernest Rutherford's students,
Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University
with radiation. They were using radioactive materials to generate dense, heavy alpha
particles aimed at a very thin piece of gold foil. They expected the particles to barge
their way straight through the gold atoms without being affected by the positive charge
spread throughout the atom that Thomson's model described. However, they got a big
surprise. They discovered that most of the particles passed right through the gold foil
unaffected, but some were slightly deflected and some others completely bounced
back off the foil! Ernest Rutherford interpreted these results and suggested a new
model for the atom in 1911. He concluded that Thomson's model could not be right. The
positive charge in an atom must be concentrated in a tiny volume at the center of the
atom, otherwise the heavy alpha particles fired at the foil could never be repelled back
towards their source. This discovery gave rise to the concept of a nucleus. In
Rutherford’s model of the atom, the electrons (discovered by Thomson) orbited through
largely empty space around the dense, positively charged nucleus.
Niels Bohr
The next important development came in 1914 when Danish physicist Niels Bohr revised
the model again. It had been known for some time that the light given out when atoms
were heated always had specific amounts of energy – the atomic spectra, but no one
had been able to explain this. Bohr suggested that the electrons must be orbiting the
nucleus in certain fixed energy levels (or shells). If this were the case, atomic spectra
could be explained from the energy given off as electrons jumped from one fixed energy
level to another. The energy must be given out when 'excited' or energized electrons fall
from a high energy level to a low one. The Bohr model of the atom is sometimes
referred to as the “planetary model” in which electrons orbiting the nucleus are similar
to planets orbiting the Sun.
Quantum Model
The Quantum model includes a nucleus containing neutral particles known as neutrons
and positive particles called protons. Electrons are no longer considered to have fixed
planet-like orbits, however the specific quantities of energy possessed by electrons is
very well defined as described by Niels Bohr and atomic spectra evidence. The specific
location of electrons can not be known, but rather certain probabilities for their location
exist based on the quantities of energy that are possible.
The quantum mechanical model is based on quantum theory, which says matter also
has properties associated with waves. According to quantum theory, it’s impossible to
know the exact position and momentum of an electron at the same time. This is known
as the Uncertainty Principle.
The quantum mechanical model of the atom uses volumes of space in which there is
likely to be an electron (called orbitals). So, this model is based on probability rather
than certainty.