the atomic modeltodhigh.com/clickandbuilds/wordpress/wp-content/uplo… · · 2018-03-02although...
TRANSCRIPT
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Introduction to atoms
Everything is made up of atoms. Atoms are the smallest
unit of any element and cannot be seen.
Atoms of different elements can combine to make new
substances. Like this, we can make
all of the materials on the planet!
Atoms have a unique structure.
They are made up of a core
nucleus which contains
protons and neutrons.
The nucleus is surrounded
by electrons which are in shells.
nucleus
electron shell
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How big are atoms?
Atoms have a diameter of about 1 × 10–10 metres (m), or
0.1 nanometres (nm), which is far too small to be seen with
the naked eye.
Within atoms are smaller, subatomic particles called
neutrons, protons and electrons.
The nucleus, which
contains neutrons and
protons, is 1/10,000 the
size of the atom and has
a diameter of about
0.00001nm.
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Charge of atoms
If we can’t see atoms, how do we know so much about them?
Protons are positively charged. The number of protons an
atom has can be referred to as its atomic number. You can
see that on the periodic table, elements are arranged in
order of their atomic number.
Neutrons are neutral and
do not have a charge.
If the number of protons and electrons are the same, there
will be no overall charge for the atom. It is said to be neutral.
electron
proton
neutron
Electrons are
negatively charged.
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John Dalton
John Dalton was an English chemist who lived in 19th century.
Dalton did a lot of scientific experiments involving gases.
His experiments led to him to think about what gases are
made up of.
The ancient Greeks had already come up with a theory that
everything is made of very small units, called atoms, which
cannot be broken down any further.
Dalton went on to do many experiments which showed that
when atoms of different elements combine to form chemical
compounds, they are always in whole numbers.
For example 2 oxygen atoms will
always react with 1 carbon atom
to make carbon dioxide, CO2.
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Dalton’s theory
Although there were some errors in Dalton’s work, which
were later corrected, most of his theory was correct.
Dalton theorised that:
Dalton also discovered colour blindness!
elements are made of extremely small particles, called
atoms, so small in fact that we cannot see them
atoms of a given element are identical in size and mass,
and atoms of different elements differ in size and mass
atoms cannot be subdivided, created, or destroyed
atoms of different elements combine to form
chemical compounds.
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JJ Thomson
Atoms were then thought to be solid,
indivisible spheres, but Thomson
found that the electron had a very
small mass. He thought that about
2,000 electrons must be needed to
make one hydrogen atom.
At the end of the 19th century, English physicist Sir Joseph
John Thomson, carried out an experiment where he
passed a high voltage beam through a low pressure gas.
He found that the beam would always
be attracted to a positive charge, and
therefore must be negative itself.
He had discovered the electron!
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Plum pudding model
Thomson then concluded that
there must be fewer electrons,
which are spread out and
mixed with positive charges,
like plums in a pudding. This
new model of the atom was
therefore called the plum
pudding model.
Thomson’s work earned him the Nobel Prize in 1906.
A hydrogen atom made entirely of electrons would be
negatively charged. As this was not observed, he knew that
that could not be correct.
negatively-
charged
plums
positively-charged
pudding
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Ernest Rutherford
Geiger and Marsden fired
positively-charged alpha
particles at a sheet of gold
foil, which is made up of
gold atoms. They expected
the alpha particles to pass
through the gold atoms.
Ernest Rutherford was a student of JJ Thomson.
Rutherford designed an experiment to test out the
plum pudding model theory.
The experiment was conducted by his
assistants, Geiger and Marsden.
gold
nucleus
alpha particle
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Naming the proton
They concluded that, for
this to occur, there must be
a small core of positive
charge that repels the
alpha particles. They had
discovered the nucleus!
Why is there no
deflection here?
What actually happened in Rutherford’s scattering
experiment was that some of the alpha particles were
deflected and some were reflected straight back .
gold
nucleus
alpha
particle
alpha particle
deflected
Rutherford named the
positive charges “protons”,
from the Greek word
“protos” meaning “first”.
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Nuclear model
New experimental evidence may lead to a scientific
model being changed or replaced. Rutherford’s scattering
experiment led to a new model of the atom.
Rutherford replaced the plum
pudding model with a new
model of the atom as a core
of positive charge in the
nucleus surrounded by
negatively-charged electrons.
He concluded that the nucleus
was very small.
This new model of the atom was called the nuclear model.
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Niels Bohr
Other scientists thought Bohr’s idea was unacceptable,
until later experimental data supported his claim.
Bohr won the Nobel prize for his work in 1922.
In 1914, Danish scientist Niels Bohr further adapted the
nuclear model.
Bohr suggested that electrons
are not just orbiting the nucleus
in a random fashion, but are in
fixed energy levels.
This can be compared to planets
orbiting the sun. They have a fixed
path that they follow, just like the
electrons around the nucleus.
electron
energy levels
(shells) electron
nucleus
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Sir James Chadwick
Chadwick won the Nobel prize
in 1935 for his discovery.
A few years later in 1932, English physicist, James Chadwick
carried out a series of experiments involving radiation.
Chadwick discovered that the
nucleus not only contained
positively-charged protons,
but also small particles with
the same mass as protons
but no charge. He called
these particles neutrons.