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ATOMIC THEORY AND THE ATOM
Chapter 11
ATOMIC THEORY HISTORY
ATOMIC MODELS
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istry/atoms/
ATOMIC HISTORY
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Democritus- 440 B.C.
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He proposed that if you kept cutting something in half you
would eventually end up with an “uncutable” particle.
Which he called an atom
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Greek: Atomos - indivisible
Aristotle- 338-322 B.C
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He disagreed with Democritus. He believed you would
never end up with an indivisible particle.
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Dalton- 1808
In search of understanding why elements combine in
specific ratios he discovered through his experiments that
this happens because they are made of individual atoms.
 His theory states 3 things
1- Atoms are small and cannot be
created, divided or destroyed.
2- Atoms of the same element are all alike
3-Atoms join with other atoms to make
new substances
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Thomson discovered electrons. His plum-pudding
model described the atom as a lump of positively
charged material with negative electrons
scattered throughout.
Rutherford discovered that atoms contain a small,
dense, positively charged center called the nucleus.
• Bohr suggested that electrons move around the
nucleus at only certain distances.
According to the current atomic theory, electron
clouds are where electrons are most
likely to be in the space around the nucleus.
Rutherford
Gold foil
Experiment
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ATOMIC STRUCTURE
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THE ATOM
Atoms are extremely small.
 Atoms contain a nucleus.
 Atoms are made of subatomic particles
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Protons
 Neutrons
 Electrons
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PROTONS
 Positively
charged particles (+).
 Found in the nucleus.
 All protons are identical
 Each proton= 1 amu (atomic mass
unit)
 Every atom of the same element has
the same number of protons.
NEUTRONS
 Particles
that have no charge
 Found in nucleus
 All neutrons are identical
 Protons & Neutrons are the most
massive particles in the atom but
located in a very small area
(nucleus)
ELECTRONS
 Negatively
charged particles (e-)
 Located outside of the nucleus in rings or
levels called atomic clouds
 Their mass is so small that it is usually
considered zero.
It takes more than 1,800 electrons to
equal the mass of one proton.
However electrons occupy most of an
atoms volume.
PARTS OF THE ATOM
SOME THINGS TO REMEMBER:
 In
an atom, the number of protons
equals the number of electrons. As a
result, the positive charge from
protons equals the number of
electrons which makes the atom
neutral.
 All atoms of an element have a
different atomic number.
Tells us the number
of protons in an
atoms nucleus
Atomic mass unit =
Protons + Neutrons
ATOMIC MASS
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Atoms are too small to be measures in everyday
untis of mass (grams)
Scientist use units known as Atomic mass units
(amu)
A proton or a neutron has a mass equal to about
1/1000th
Atomic Mass is equal to the number of protons
and neutrons in an atom.
ISOTOPES
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mistry/isotopes/
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Isotopes are atoms that
have the same number
of protons but different
numbers of neutrons.
Each element has a
limited number of
isotopes that occur
naturally.
Some isotopes can be
unstable and change it’s
composition. We call
these radioactive.
ISOTOPES
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Isotopes of hydrogen and
uranium are shown as
examples of the isotope
notation method
Unlike other elements'
isotopes, the two heavy
isotopes of hydrogen have
their own names.
Isotopes are atoms
that have the same
number of protons but
have different number
of neutrons
ARRANGEMENT OF ELECTRONS
The first ring can only hold
2 electrons
 The second ring can hold
up to 8 electrons
 The third ring can hold up
to 8 electrons
Fourth can hold up to 18
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Principle quantum model
2, 8,18, 32. Electron
Capacity = 2n2
The variable n represents
the Principal Quantum
Number, the number of
the energy level in
question.
P=
N=
Valence Shell: last orbital where the “giving” and
“receiving” of electrons take place CHEMICAL REACTIONS
TALK ABOUT: IS IT AN ELEMENT OR ION?
IONS AND VALENCE ELECTRONS
• The four forces at
work in an atom are
gravity, the
electromagnetic force,
the strong
 force, and the weak
force.
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BOHR MODEL AND FLAME LAB