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CHAPTER 4:
Matter is Made up of Atoms
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ATOMS & THEIR STRUCTURE
Aristotle thought matter was
made of air, earth, fire and
water.
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Democritus (250 B.C.)-
Said the world is made of empty space
& tiny particles called atoms
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DEVELOPMENT OF THE ATOMIC THEORY
(3 SCIENTISTS INVOLVED)
1. Lavoisier (1743-1794)-
Law of Conservation of Matter
The Father of Modern Chemistry
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Law of definite proportions-elements that make up compounds are found in fixed proportions
2. PROUST (1799)
Joseph Proust
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3. DALTON’S ATOMIC THEORY
1. All matter is made of
atoms.
2. Atoms are indivisible
3. All atoms of 1 element 3. All atoms of 1 element
are alike, but are
different than atoms of
other elements
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Was he right?
1. All matter is made of atoms.
2. Atoms are indivisible
3. All atoms of 1 element 3. All atoms of 1 element are alike, but are different than atoms of other elements
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DISCOVERY OF ATOMIC STRUCTURE
4 SCIENTISTSJJ Thomson (1897)
1. Discovered the electron during his vacuum
tube/cathode ray experiments.
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2. JJ’s Plum Pudding model
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3. When studying Ne he discovered
isotopes-
atoms of an element that are alike chemically,
but differ in mass.
*Same # protons, different # neutrons
4. His work led to the discovery of the neutron
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NAGAOKA
Saturnian model or
early planetary
model
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RUTHERFORD
A. Gold foil experiment
1. shot positive particles through a
thin sheet of Au.
2. Most went straight through, but
others were deflected.others were deflected.
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WHY?
http://www-outreach.phy.cam.ac.uk/camphy/nucleus/nucleus6_1.htm
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CONCLUSIONS DRAWN FROM THE GOLD
FOIL EXPERIMENT
1. Most of the atom is empty space.
2. Discovery of the nucleus- small,
dense, positively charged central
core of the atomcore of the atom
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NEILS BOHR (1913)1. Given credit for the planetary model of the atom.
2. Was more specific than Nagaoka because he Nagaoka because he concluded that electrons are arranged in E levels.
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BOHR’S STATEMENTS ABOUT
ELECTRONS
1. Said that electrons absorb E &
move to higher states.
2. These “excited” electrons give
off that E as light waves & fall off that E as light waves & fall
back to a lower E level.
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Neils Bohr’s Atomic Model
An excited electron jumps
to an outer E level.
It falls back, releasing light It falls back, releasing light
waves.
The “packet of light”
released is called a
photon.
+
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Nucleus
Electron
Orbit
The atom is not flat!
Orbit
Energy Levels
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Our View of Electrons Today
�E levels are not neat orbits
�E levels are spaces around the
nucleus where electrons are
most likely to be foundmost likely to be found
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DaltonNagaokaJJ Thomson
Rutherford
BohrToday’s model
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Subatomic particle Mass in µ Charge
Protons 1.01 Positive
Neutrons 1.01 Neutral
Subatomic Particles
Mass of atoms is measured in atomic mass units (µ, or amu)
Neutrons 1.01 Neutral
Electrons 0.0005 Negative
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Determining Atomic Structure
1. Atomic number-
the # of protons in the nucleus & the #
of electrons if the atom is neutral
2. Atomic mass/Mass number-2. Atomic mass/Mass number-
the sum of protons + neutrons in the
nucleus
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Carbon12.011
C6
Atomic number
Atomic mass
Atomic number
# protons = 6
# electrons = 6
# neutrons = 12-6= 6
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You try itHow many protons, neutrons, and
electrons are present in nitrogen?
p = 7
e = 7
n = 14-7=7n = 14-7=7
Try bromine
p = 35
e = 35
n = 45
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Why isn’t atomic mass a whole
number?
� It is a weighted average of all isotopes of an
element.
� The isotope most common in nature is the one
that has a mass closest to the one found on the
periodic table.periodic table.
Ex:
Most C has a mass of 12.
The most common isotope of
carbon is therefore C-12
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Electrons in Energy Levels
�1st E level can hold a max of 2
electrons
�2nd E level can hold 8
�3rd can hold 18
�4th can hold 32
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Drawing atoms
Draw an atom of beryllium
#p= 4
#n=9-4=5
#e=4
# valence electrons = 2
Beryllium9.012
Be4
Valence electrons = #
electrons in outermost E
level
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Try to draw an atom of phosporus on
your own
P=15
Valence = 5
P=15
N=16
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This is P-31.
P=15P=15
N=16
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This is P-33.
P=15
The only thing
different is the
# of neutronsP=15
N=18
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Drawing ions
P=15
To draw a +1
ion, take away
1 electron
This is a neutral
atom of P-31
Now the P=15
N=16 atom has a
+1 charge
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Drawing ions
P=15
To draw a -1
ion, add 1
electron
Now the P=15
N=16 atom has a
-1 charge
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Drawing ions
P=15
What do you do
to make the
atom have a -3
charge?
P=15
N=16
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IN SUMMARY
You have learned about:
�The development of the atomic
theory
� The discovery of atomic structure� The discovery of atomic structure
� How to construct a model of an
atom of any element using the
periodic table
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The End!!!!!!!!!