History of Atomic Theory
Greek Beginnings
The concept of the smallest particle was conceived 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 vehemently 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
-
All matter is made up of undividable particles called atoms.
There is a void, which 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)
2)
3)
4)
chemical elements are made of atoms.
the atoms of an element are identical in their masses
atoms of different elements have different masses
atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and
so on.
5) atoms can be neither created nor destroyed
J. J. Thomson
In 1897, J.J. Thomson discovered the electron, the first subatomic particle. He also was the
first to attempt to incorporate the electron into a structure for the atom. The internal structure of
the atom had been a source of speculation for thousands of years. The Greeks taught that the atom
was solid, as did Dalton.
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 times 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 the bits of plum in a plum
pudding (rather like 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
unimpeded by the diffuse 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 fell back to a lower energy level from a
higher one. This means that the farther away from the nucleus the electron has, the more
energy it has…kind of like an elevator for electrons…the farther from the “ground floor” it is
the more GPE an elevator has. 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 a “planetary model” in which electrons orbiting the nucleus are
similar to planets orbiting the Sun.
Quantum theory
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 region around the nucleus that describes the possible locations of electrons
is referred to as an electron cloud. The electron cloud is divided into specific regions called
orbitals. It is the orbitals where the electrons can be found. Each orbital has a specific
name, shape and maximum number of electrons it can hold. The first orbital is the s
orbital. It is shaped like a sphere and holds a maximum of 2 electrons. The next orbital is
the p orbital. It is shape like a dumbbell and holds a maximum of 6 electrons. The shapes
get confusing after that, but the next orbital (d orbital) holds a maximum of 10 electrons
and the f orbital holds a maximum of 14 electrons.