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Transcript
CHAPTER 3
ATOMIC STRUCTURE
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All matter is made of atoms – a tiny particle
that makes up all elements.
Democritus – a Greek philosopher – was the
first to suggest the existence of atoms and
thought they were indivisible and
indestructible.
The modern process of discovery regarding
atoms began with John Dalton
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All atoms are composed of tiny indivisible
particles.
Atoms of one element are identical and are
different from atoms of a different element.
Atoms of different elements can mix together
or chemically combine in simple whole number
ratios to form compounds.
In chemical reactions atoms are separated,
joined or are rearranged, but never change into
atoms of another element.
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Atoms are very small. A penny contains about
2.4 x 1022 atoms. There are only 6 x 109 people
on earth.
The radii of most atoms falls between 5 x 10-11
m and 2 x 10-10 m.
Despite their small size, atoms can now be seen
with scanning tunneling microscopes.
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Subatomic particles
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Electron – discovered by J.J. Thomson in 1897
He performed experiments passing electric current
through a gas at low pressure.
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Because the beam was attracted to the positive
plate, Thomson concluded the electrons were
negative in charge.
Further experiments by Robert A Millikan
determined the mass of an electron: 1/1840 the
mass of a hydrogen atom.
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The Plum Pudding model of the atom
electron
Positively
charged
mass
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Protons
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further studies with cathode ray tubes lead to the
discovery of the proton by Eugene Goldstein in 1886.
They have a positive charge and a relative mass of 1
Neutrons
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James Chadwick discovered the neutron in 1932.
They have no charge and a mass equal to a proton.
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The gold foil experiment
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In 1911, Ernest Rutherford performed an experiment
where he bombarded a piece of gold foil with
positively charged helium nuclei known as alpha
particles. He predicted that all the alpha particles
would pass through the gold foil while some would
be deflected if they passed close to an electron.
Something very unexpected happened.
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http://micro.magnet.fsu.edu/electromag/java
/rutherford/
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Atomic Number – the number of protons in an
atom. This determines the identity of the atom.
It also tells us the number of electrons because
atoms are electrically neutral.
P+ N
N
hydrogen
(tritium)
P+ N
P+ N
helium
P+ N
P+ N
lithium
P+
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Mass Number – this is the number of protons
and neutrons added together in the nucleus of
the atom.
If you know the atomic number and the mass
number you can determine the number of all
the particles in an atom.
Manganese has an atomic number of 25 and a
mass number of 55. How many protons,
electrons and neutrons does manganese have?
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This is called a nuclear symbol and shows the
mass number on top and the atomic number on
the bottom.
# of protons
and neutrons
A
X
Z
# of protons
Represents
element
60
27
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Isotopes – atoms of the same kind with a
different number of neutrons and a different
mass.
P+
hydrogen
P+ N
deuterium
P+ N
N
tritium
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Hyphen notation is another way to express
isotopes of an element.
Co - 60
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Measured in atomic mass units
The mass of an atom is a relative mass.
The masses of all atoms are determined by
their relative mass to one atom of Carbon-12.
A hydrogen atom has a mass of 1/12 the mass
of a carbon atom so its mass is 1.0 amu
A magnesium atom is twice as heavy as a
carbon atom so its mass is 24 amu
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Most atoms have at least one isotope – an atom
of the same kind having a different number of
neutrons.
The mass of an atom is an average of the
masses of all the isotopes of that atom.
Mass of each isotope x % of their occurrence
added together.
ex. Cu-63 (69.17%) and Cu-65 (30.83%)
Average atomic mass = ?
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(63 x .6917) + (65 x .3083) =
43.5771 + 20.0395 = 63.6166
Round the number to the 0.01 place
The average atomic mass of copper is 63.62
amu