Transcript Atomic Theories during history

The Atomic Models
From the Ancient Greeks to the actual
models…all you need to know.
1
The Greek Beginning
The concept of the smallest particle was
conceived in the 5th century BC by
Leucippus of Miletus. His pupil, Democritus
of Abdera (picture) 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. "Democritus of Abdera said that there is no
end to the universe, since it was not created by any outside
power."
2
The 5 points of their
conclusion are as follows...
1. All matter is made up of undividable
particles called atoms.
2. There is a void, which is empty space
between atoms.
3. Atoms are completely solid.
4. Atoms are homogeneous, with no internal
structure.
5. Atoms vary in:
a. Size
b. Shape
c. Weight
3
John Dalton’s Model
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. In
1830, he stated his most well-known quote 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.
However, only 3 or 4 pages in the third chapter
discussed the atomic theory he proposed. In this
theory, there are four basic ideas...
1) chemical elements are made of atoms.
2) the atoms of an element are identical in their
masses
3) atoms of different elements have different
masses
4) 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
Interesting Fact: The
unit for atomic weight
was called a "dalton"
for many years.
Today, it is used in
biochemical circles,
(e.g. "The atomic
weight of that protein
is 35,000 daltons.") 4
Joseph John Thompson
On April 30, 1897, Joseph John Thomson
announced that cathode rays were
negatively charged particles which he
called 'corpuscles.' He also announced
that they had a mass about 1000 times
smaller than a hydrogen atom, and he
claimed that these corpuscles were the
things from which atoms were built up.
This hypothesis was not generally accepted until 1899 when
an Irish physicist named George Francis FitzGerald
proposed that the "corpuscles" Thomson spoke of were
actually free electron particles. Thomson designed the
famous "plum pudding model" shown below.
The Plum Pudding
Model
5
Ernest Rutherford
• New Zealander, studied under J. J.
Thomson at the Cavendish
Laboratory in England. He was the
first scientist to propose the theory
of the nuclear atom. His experiment,
shown to the right, proves that
"uranium emanations were shown to
consist of three types of rays, alpha
(helium nuclei) of low penetrating
power, beta (electrons), and
gamma, of exceedingly short
wavelength and great energy."
Basically, he proved that the atom
had a nucleus.
6
Niels Bohr’s model
Bohr’s model of the Carbon atom
• Niels Bohr came up with
the Bohr Model in 1915.
It is referred to as the
"planetary model". In the
picture, the red and
green balls symbolize
the neutrons and
protons that form the
nucleus. The electrons
orbit the nucleus like the
planets orbit the Sun.
7