Transcript Slide 1

By Daniel Cooke
Presentation Outline
•Introduction
•Ancient Atomic theory
•The Modern Atomic Theory
•Rutherford's Experiment
•Bohr’s Model
•Quantum Theory of the Atom
Introduction
Atomic theory first originated with Greek
philosophers around 2500 years ago. This basic
theory remained unchanged until the 19th century
when it first became possible to test the theory with
more sophisticated experiments.
As science has rapidly advanced over the past few
centuries the atomic theory has been refined in
accordance with the accepted scientific principles
and theories of the time.
Advances in technology and theory that have
allowed experiments to be designed that enable us
to probe matter to microscopic scales.
Ancient Atomic Theory
The atomic theory of matter was first proposed by
Leucippus, a Greek philosopher who lived at
around 400BC. At this time the Greeks were trying
to understand the way matter is made.
According to Anassagora, it is possible to subdivide
matter in smaller and smaller parts, and he
proposed that this process can be continued with
no limit. In Anassagora's view, you can always
divide a bit of substance into two parts, and each of
these parts is also divisible into two parts, and so
on--no matter how small each part gets there is no
problem dividing it again into even smaller parts.
Leucippus
But according to Leucippus, eventually you arrive
at small particles which can not be further
subdivided. Leucippus called these indivisible
particles atoms (from the Greek word atomos,
meaning “indivisible”).
Ancient Atomic Theory continued..
Leucippus's atomic theory was further developed by his
disciple, Democritus who concluded that infinite
divisibility of a substance belongs only in the imaginary
world of mathematics.
Democritus
Democritus suggested the atomic theory, explaining
that all things are "composed of minute, invisible,
indestructible particles of pure matter which move
about eternally in infinite empty.". If a sample of a pure
element was divided into smaller and smaller parts,
eventually a point would be reached at which no further
cutting would be possible—this was the atom of that
element.
According to the ancient Greeks, atoms were all made of the same basic
material, but atoms of different elements had different sizes and shapes. The
sizes, shapes, and arrangements of a material’s atoms determined the
material’s properties. It was believed that there were four elements that all
thing were mare from; Earth, Air, Fire and Water.
The Modern Atomic Theory
For centuries scientists did not have the methods or technology to
test their theories about the basic structure of matter, so people
accepted the ancient Greek view.
In the 19th century John Dalton made inferences
that exhibited how atoms bond together in
definite proportions. Dalton was able to say that
atoms of different elements combine in whole
number ratios.
John Dalton
This theory, to go along with four other theories,
made up what Dalton called the "Modern Atomic
Theory.“ Included in these were two theories that
stated atoms could not be divided or destroyed, a
theory that stated different elements contain
different chemical properties, and atoms of the
same element contain the same chemical
properties
The Modern Atomic Theory
continued…
Dalton made two assertions about atoms: (1) atoms of each
element are all identical to one another but different from the
atoms of all other elements, and (2) atoms of different elements
can combine to form more complex substances.
Although the two theories that speculated atoms couldn't be
divided were false, Dalton contributed greatly to the advances of
atomic theory, and would greatly influence J.J. Thompson in his
own discoveries.
Expanding the Modern Atomic
Theory
J.J. Thompson
J.J. Thompson is the person who is credited for
discovering the electron. Thompson created a tube
that had a positively charged anode on one side and a
negatively charged cathode on the other side.
Thompson then applied a magnet to the middle of the
tube and discovered that negatively charged particles
were emanating towards the positive magnetic field.
From this, Thompson concluded that negatively
charged particles, called electrons, were present in
atoms.
Thompson then created the Plum
Pudding model, which suggested that
electrons and protons were randomly
placed throughout the atom.
The “Plum Pudding”
Rutherford's Experiment
In 1911 British scientist Ernest Rutherford
set out to test Thomson’s proposal by
firing a beam of charged particles at
atoms.
Alpha particles are heavy particles with
twice the positive charge of a proton.
Alpha particles are now known to be the
nuclei of helium atoms, which contain two
protons and two neutrons.
Ernest Rutherford
Ernest Rutherford's experiment was to
emit alpha particles towards a thin gold
sheet. Rutherford would then determine
where the deflections of the alpha particles
would go, and therefore be able to theorize
what kind of placement protons and
electrons had.
Rutherford's Experiment continued…
Rutherford's Experiment continued…
Rutherford observed that most of the alpha particles went strait
through the foil. However a large proportion were deflected through
small angles an some (though very few) deflected straight back.
Rutherford then theorized that there was something called a nucleus,
which contained a high density of positively charged particles.
Rutherford was able to say there was a nucleus because alpha
particles that deflected right back must have hit something more
massive and with a strong positive charge. This led Rutherford to
propose a very different model for the atom.
Instead of supposing that the positive charge and
mass were spread throughout the volume of the
atom, he theorized that it was concentrated in the
center of the atom. Rutherford called this
concentrated region of electric charge the nucleus
of the atom.
Bohr’s Model
Danish physicist Niels Bohr used new knowledge
about the radiation emitted from atoms to develop
a model of the atom significantly different from
Rutherford’s model.
Niels Bohr
Scientists of the 19th century discovered that when
an electrical discharge passes through a small
quantity of a gas in a glass tube, the atoms in the
gas emit light. This radiation occurs only at certain
discrete wavelengths, and different elements and
compounds emit different wavelengths.
Bohr developed a theory by which he could predict the same
wavelengths scientists had measured radiating from atoms with a
single electron. He concluded that because atoms emit light only at
discrete wavelengths, electrons could only orbit at certain designated
radii, and light could be emitted only when an electron jumped from
one of these designated orbits to another.
Quantum Theory of the Atom
To make his theory work, Bohr had to
propose special rules that violated the rules
of classical physics. He concluded that, on
the atomic scale, certain preferred states of
motion were especially stable. In these
states of motion an orbiting electron
(contrary to the laws of electromagnetism)
would not radiate energy.
The quantum mechanical view of atomic structure is that the nucleus
is at the center of the atom and provides the electrical attraction that
binds the electrons to the atom. Contrary to Bohr’s theory, however,
the electrons do not circulate in definite planet-like orbits. Due to the
wavelike character of electrons and provides the framework for viewing
the electrons as fuzzy clouds of negative charge.
Continued…
Electrons still have assigned states of motion, but these states of
motion do not correspond to fixed orbits. Instead, they tell us
something about the geometry of the electron cloud—its size and
shape and whether it is spherical or bunched in lobes like a figure
eight. Physicists called these states of motion orbitals.
The way electrons fill up orbitals determines the number of
electrons that end up in the atom’s valence shell. This in turn
determines an atom’s chemical and physical properties, such as
how it reacts with other atoms and how well it conducts
electricity.
It is through today’s
understanding of the
nature of matter that
advancement of technology
is able to continue at an
increasing rate.
References
•encarta.msn.com/encyclopedia
•www.funsci.com/fun3_en/democritus/democritus
•www.aare.edu.au
•www.absoluteastronomy.com/encyclopedia