Transcript History of the discovery of atomic structure

History of the discovery of
atomic structure
Beginning
Man has always wanted to know, to
understand how nature works.
It is in his nature to ask questions
and then to seek the answers.
One such question is “What is the
nature of matter that makes up
the physical universe?”
Start of the Quest for Answers
Democritus – a Greek
natural philosopher
In 400 B.C. he did a mind
experiment in an
attempt to explain
the nature of matter.
His question was “If I take
an object and
repeatedly cut it in
half, will there be a
point that it cannot
be cut any smaller?”
His Answer
YES!
He decided
there was such
a point.
He called that
final particle
“an atom”.
Democritus’s Atomic Theory
1. All matter consists of invisible particles
called atoms (from the Greek
“Atomos” for indivisible).
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass,
position, and arrangement.
John Dalton
Brought Democritus’s Atomic Theory into
the realm of scientific knowledge,
instead of being relegated to the
whims of the human mind.
Presented his theory to the world in 1803.
John Dalton described atoms as tiny balls
of material. He said the atoms of a
particular element are all identical.
These are original Dalton model atoms
He used small wooden balls
to model atoms and showed
how they could combine to
produce compounds
John Dalton’s Theory
1. All matter is made up of atoms
2. All atoms of the same element are identical.
All atoms of different elements are
different
3. Atoms can not be created nor destroyed only
rearranged. (law of conservation of
matter)
4. Atoms combine in fixed ratios to form
compounds. (law of definite proportions)
5. Atoms combine in different ratios to form
different compounds. (law of multiple
proportions)
JJ Thomson
In 1897, whilst studying cathode rays, JJ Thomson
discovered tiny particles with a negative charge.
These negative particles were
given out by atoms and were
much smaller than atoms.
Thomson had discovered
the existence of electrons.
This discovery contradicted
Dalton’s theory that atoms
were solid spheres of matter.
This led Thomson to propose a new model of the atom.
J.J. Thomson’s Experiment
Using a cathode ray
tube, he
projected a
beam of
particles
through a
magnetic
field and
observed how
they were
deflected.
The plum pudding model
Thomson suggested that an atom is a positively-charged
sphere with negative electrons distributed throughout it.
This model became known as
the plum pudding model,
because the electrons in the
atom were thought to be like
raisins in a plum pudding.
Electrons had been proved
to exist but there were still
doubts about this model.
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In 1911 Ernest Rutherford, (a New Zealand
scientist working in Britain) carried out an
experiment that proved that atoms were
not solid lumps of material as thought by
Thomson but were in fact mostly empty
space with a very small solid centre called
the nucleus.
He came up with a new model which
had a solid, positively charged
centre and the negative electrons in
‘orbits’ around it.
Rutherford’s Atomic Model
Geiger and Marsden’s experiment
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The results
The results of Geiger and Marsden’s experiment were:
2. Some alpha particles
were slightly deflected
by the gold foil.
3. A few alpha
particles were
bounced back
from the gold foil.
1. Most alpha particles
went straight through
the gold foil, without
any deflection.
The experiment was carried out in a vacuum, so deflection
of the alpha particles must have been due to the gold foil.
How can these results be explained in terms of atoms?
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It was almost as incredible as if
you fired a 15-inch shell at a piece
of tissue paper and it came back
and hit you.
Rutherford’s explanation
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www.youtube.com/watch?v=XBqHkraf8iE
Niels Bohr
In 1913, Niels Bohr proposed an atom
was like a tiny solar system (the
nucleus was like the sun and the
electrons like the planets).
Niels Bohr
Electrons travel in discrete orbits
defined by their angular
momentum.
This made it possible to calculate
the energy levels of electrons
and the frequency of the light
emitted when electrons returned
to the home state.
In 1920 Rutherford came up with the idea that atoms must
contain a third particle. He thought this because the masses of
atoms that were being measured were heavier than you would
get from just the masses of protons and electrons added
together. He said that this particle would have no charge and
that this would make it very difficult to detect.
Following work in Germany and
France the Neutron was finally
identified by James Chadwick
working in Cambridge. He
realised that an unusual
radiation that had been
observed in some experiments
was in fact a neutral, sub-atomic
particle.
James Chadwick
Along with other scientists,
Chadwick realized the atomic
mass of an element, was often
twice what it
should be.
This led to Chadwick conducting
an experiment that proved the
existence of neutrons.
James Chadwick
In 1932, he proved
the existence
of the neutron,
a particle
having the
same mass as a
proton and no
charge.