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Atomic Theory
Throughout history man has
attempted to explain the nature of
matter.
Democritus, an early Greek,
proposed that matter was
composed of tiny indivisible
particles called atoms, meaning
indivisible.
It was not until 1807 that
the atomic theory was
thoroughly revised by John
Dalton.
Dalton’s theory stated
• All matter is made up of tiny particles
called atoms that cannot be split into
smaller particles (wrong).
• Atoms cannot be created or destroyed
• All atoms of the same element have the
same properties and atoms of different
elements have different properties.
• Atoms of different elements can combine to
form new substances.
His ideas marked the
beginning of the
modern atomic theory.
The work of later scientists such as
JJ Thomson, Lord Rutherford and
Neils Bohr established a workable
model of the atom.
Thomson
Rutherford
Bohr
Dalton stated that the atom is a
hard, dense sphere.
Thomson in 1903 stated that the
atom is a mass of positive charge
and contained negative electrons.
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eep+ p+ eeep+ eep+ e-
e- p+
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Rutherford’s Model in 1911
described the atom as having a
dense nucleus surrounded by
electrons.
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Bohr developed a model in 1913 that
had a dense nucleus and electrons that
move in fixed orbits (shells) around the
nucleus in a “cloud” of activity
http://micro.magnet.fsu.edu/electroma
g/java/rutherford/
http://particleadventure.or
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James Chadwick (1885 - 1962)
developed a new model of the
atom in 1932. Similar in many
ways to other models, his showed
the nucleus made up of particles
called neutrons.
10,000,000 atoms placed side by
side would measure only 1 cm.
The particles inside
atoms are called
subatomic particles.
In the middle of every
atom is its nucleus.
The nucleus is made of
the proton and neutron.
A third type of
subatomic particle spins
around the nucleus, it is
called the electron.
Proton
Neutron
Electron
Proton
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The electron is by far
the smallest of these
subatomic particles.
Their mass is only
1/1840th the mass of a
proton.
Even though protons
are much larger than
electrons, they both
have the same amount
of electrical charge.
Protons have a positive charge (+).
Electrons have a negative charge (-).
Neutrons have no charge (0).
Particles with opposite
electrical charges are
attracted to each other, in the
same way that magnets with
opposite poles stick together.
Got a problem, ask a chemist,
they have solutions.
A small piece of sodium which
lived in a test tube fell in love
with a bunsen burner. “Oh
bunsen, my flame, I melt
whenever I see you” the sodium
said.
“It’s just a phase you’re going
through” replied the Bunsen
burner.
Particles with the same
electrical charges repel each
other, in the same way that
magnets with the same poles
push apart.
Atoms have the same
number of protons as
electrons, so their
electrical charges
cancel each other out,
and are called neutral.
Atoms are normally
neutral and have no
overall electrical
charge. This is not
the case with ions.
Ions are charged atoms that have
gained or lost electrons. When
atoms are unbalanced, it causes
them to combine or react.
Electrons travel in shells. The
first shell contains 2 electrons,
the second 8, third 8, the fourth
18 and so on.
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If an atom gains an
electron it becomes
negatively charged. If
an atom loses an
electron it becomes
positively charged.
The number of neutrons
of an atom can vary.
The different atoms
caused by varying
numbers of neutrons is
called an isotope.
Different electrons
can contain different
amounts of energy.
The farther an
electron is from the
nucleus, the more
energy it contains.
The number of protons an
atom has determines the
type of element. If the
number of protons change,
the type of atom changes.