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How is radioactivity related to atomic structure?
Following the discovery of
radioactivity by Henri Becquerel
in 1896, many scientists were
keen to find out more about it and
understand where it came from.
As a result of this work, we now
know that radiation comes from
radioactive atoms.
In a radioactive atom, the nucleus is unstable and so it emits
particles or waves of radiation to form a more stable atom.
This process is called radioactivity or radioactive decay.
This is why early experiments with radioactivity lead to
important discoveries about the structure of the atom.
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What are atoms?
It is now known that all matter is made of atoms.
In some substances, all the atoms are the same,
which means that the substance is called an element.
For example, gold is
an element made up
of only gold atoms.
It is only relatively recently that we have had microscopes
powerful enough to ‘see’ individual atoms. Before that, the
idea that atoms existed was only a theory.
The first person to suggest the idea of atoms was the
Greek philosopher Democritus, in 450 BC.
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What did Dalton think atoms were like?
Ideas about atomic
structure have
changed over time.
In 1803, John Dalton
reintroduced the idea
that everything is made
of atoms. He said
atoms were solid
spheres of matter that
could not be split.
Dalton also suggested
that each element
contained identical
atoms.
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How did electrons spoil Dalton’s model?
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.
His discovery did not fit with Dalton’s model of the atom,
and so Thomson had to propose a new model.
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What is the plum pudding model?
Based on his discovery, Thomson
adapted Dalton’s model of the atom.
He suggested that an atom is a
positively-charged sphere with negative
electrons distributed throughout it.
Thomson’s 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
doubts about this model.
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What was Rutherford’s involvement?
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• http://www.youtube.com/watch?v=wzALbz
Tdnc8
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What did Geiger and Marsden do?
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What were Geiger and Marsden’s 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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How did Rutherford interpret the results?
Rutherford had expected all the alpha
radiation to pass through the gold foil.
He was surprised that some alpha
particles were deflected slightly or
bounced back.
The ‘plum pudding’ model could not explain these results,
so Rutherford proposed his ‘nuclear’ model of the atom.
He suggested that an atom is
mostly empty space with its
positive charge and most of its
mass in a tiny central nucleus.
Electrons orbited this
nucleus at a distance, like
planets around the Sun.
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How did Rutherford explain the results?
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Which model of the atom?
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What is the modern model of the atom?
Experiments showed that Rutherford’s atomic model (a tiny,
positively-charged nucleus orbited by electrons) was correct.
Further developments in understanding about atomic
structure followed, but Rutherford’s nuclear model still forms
the basis of the modern model of the atom.
The nucleus is where
most of the mass of the
atom is found. It contains
protons and neutrons.
The electrons orbit the
nucleus in layers called
shells.
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What are atoms made of?
Atoms are made of three basic building blocks called
protons, neutrons and electrons.
There are two properties of protons, neutrons and electrons
that are especially important: mass and charge.
Particle
Mass
Charge
proton
1
+1
neutron
1
0
electron
almost 0
-1
In any atom, the number of electrons is equal to the number
of protons and so the overall charge of an atom is zero.
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Particles in the modern model
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What makes a carbon atom carbon?
The atoms of any particular element always contains the
same number of protons.
Carbon atoms always have six protons. Atoms with different
numbers of protons must be other elements. For example:
 all atoms with 1 proton are hydrogen atoms;
 all atoms with 17 protons are chlorine atoms.
In the periodic table, there are two numbers found with each
element. What do these numbers represent?
Mass number is the number of
protons + the number of neutrons.
Atomic number (or proton
number) is the number of protons
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What are isotopes?
All carbon atoms have the same number of protons, but
not all carbon atoms are identical.
Although atoms of the same element always have the same
number of protons, they can have different numbers of
neutrons. Atoms that differ in this way are called isotopes.
For example, carbon exists as three different isotopes:
carbon-12, carbon-13 and carbon-14:
mass number
is different
atomic number
is the same
Potassium is another element that exists as three different
isotopes: potassium-39, potassium-40 and potassium-41.
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Atomic structure – key words
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