Download File 15-16unit 6

Atomic Structure
1
Stepwise Timeline of Atomic
Theory
Dalton
1803
Thomson
1897
Rutherford
1911
Modern Theory
Bohr
1913
Chadwick
1932
Democritus
~ 460 B.C. to 360 B.C.
Who
Greek Philosopher
What
• Atoms cannot be
created, destroyed or
divided.
How
• Observing nature
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Dalton’s Atomic Theory
• Who
• John Dalton (1766-1844), an
English schoolteacher and
chemist
• What
• proposed his atomic
theory of matter in 1803.
– Although his theory has been
modified slightly to
accommodate new
discoveries, Dalton’s theory
was so insightful that it has
remained essentially intact up
to the present time.
Dalton
•
What ( write this outside the text box)
1) Cannot be created or destroyed.
2) All atoms of one element are exactly
alike, (same size, mass, properties) but
different from atoms of other elements
3) Combine in whole number ratios to form
compounds.
• How• work with gases
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J.J. Thomson -1903
•
•
•
•
Who
a British physicist.
What
Plum Pudding model
(or Chocolate Chip
Cookie model)
Discovered electrons
Model
– Atom was a positively
charged sphere with
negative electrons in it
like chips
POSITIVE CHARGE
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ELECTRONS
J. J. Thomson
• How
• discovered that
cathode rays are made
up of invisible,
negatively charged
particles referred to as
electrons.
• http://tinyurl.com/cathod
ert
Ernest Rutherford
• Who
• British chemist
and physicist.
• What
• Found nucleus (1911)
•
•
•
Occupies a small volume
of the atom
Contains almost all the
mass of the atom
Electrons orbit around
nucleus
• Discovered proton
Ernest Rutherford
• How
• Gold Foil Experiment
Alpha particles which are
positively charged pass
through unmolested most of
the time.
Occasionally they would
bounce off when they hit
something (proton) that was
also positively charged.
Niels Bohr - 1913
• Who
• Danish Physicist
• What
• Planetary Model
– Electrons (e-) have
definite path around
the nucleus (orbit)
– e- arranged around the
nucleus according to
energy level
– e- with lowest energy
level are closest to
nucleus
• How
• Spectral emission lines
Chadwick
Who
British Scientist
• What
• Discovered the
neutron in 1932
• How
• Used alpha
particles
Modern Atomic Theory
1. All matter is made up of very tiny particles called
atoms.
2. Atoms of the same element are chemically alike.
3. Individual atoms of an element may not all have
the same mass. However, the atoms of an
element have a definite average mass that is
characteristic of the element.
4. Atoms of different elements have different
average masses.
5. Atoms are not subdivided, created, or destroyed
in chemical reactions.
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Atom and Elements
• Element - a substance that is composed of
a single type of atom.
• Atom - the smallest particle of an element
that retains the properties of that element.
– The diameter of an atom is measured in
nanometers
– 1 nm = 1 x 10-9 m = 0.000000001 m
• Atoms are composed of sub-atomic
particles.
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Proton
• Discovered by Ernest Rutherford in early
1900’s
• Determines the identity of an atom
• Relative mass of 1 atomic mass unit
• Part of the nucleus of an atom
• Positive charge
If you change only the # of protons, you
change the element being described.
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Neutron
•
•
•
•
•
Discovered by James Chadwick in 1932
Determines the isotope of an atom
Relative mass of 1 atomic mass unit
Part of the nucleus of an atom
No charge (neutral)
If you change only the # of neutrons, you
have a new isotope (variety) of the element .
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Changing the number of
neutrons
• Creates ISOTOPES -
–Atoms of the same element but
with a different number of
neutrons.
• Isotopes of an element have nearly
identical chemical properties
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Electron
• Discovered by J. J. Thomson in 1903
• Determines the charge of an atom
(charged atoms are called ions)
• Relative mass of 0 (~1/1836) atomic
mass unit
• Make up the electron cloud of an atom
• Negative charge
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Changing the number of
electrons
• When an atom loses electrons, it
results in a net positive charge and
is called a CATION
•
ions are
I itive
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Example of a cation
Neutral potassium (K)
has 19 protons and 19
electrons.
If potassium (K) loses
an electron, it only has
18 electrons.
19 protons = +19
19 electrons = -19
0
19 protons = +19
18electrons = -18
+1
This is written as K+1
and is called a cation
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Changing the number of
electrons
• When an atom gains electrons, it
results in a net negative charge and
is called an ANION
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Example of an anion
Neutral bromine (Br)
has 35 protons and 35
electrons.
If bromine (Br) gains an
electron, it has 36
electrons.
35 protons = +35
35 electrons = -35
0
35 protons = +35
36 electrons = -36
-1
This is written as Br -1
and is called an anion
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cation
anion
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Describing an atom
• ATOMIC NUMBER
• Equals the number of protons in an element.
In a neutral atom, the atomic number also equals the number of electrons.
- All atoms of the same element have the same number of protons.
- The smaller of the two numbers in the periodic table square, always a
whole number
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Describing an atom
• ATOMIC MASS
– A weighted average of the mass of all
the isotopes (varieties) of an atom
– Each element has only one atomic mass
– Also called “average atomic mass”
– The larger of the two numbers in the periodic table
square
– Always a decimal number
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Describing an atom
• MASS NUMBER
Equals the # protons + # neutrons in
an atom
– Not always the same for atoms of an element
 isotopes
– Not listed on the periodic table
– Always a whole number
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APE MAN
A = Atomic Number
Always the
P = Number of Protons
same number in
E = Number of Electrons a neutral
atom
M= Mass number
A = Atomic Number (again)
N = Number of Neutrons
Mass Number minus
Atomic number
equals Number of
neutrons
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Isotope Notation
mass number
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C
element
symbol
atomic number
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Isotope Name
• name of the element dash mass number
• Example: Carbon -14 is the isotope name
for a carbon atom with a mass number of 14
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Isotope Notation
mass number
14
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C
element
symbol
atomic number
number of neutrons = mass number – atomic number.
How many protons and neutrons in this isotope?
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Practice
Isotope
notation
Isotope
name
Silicon -
Atomic
number
Mass #
# of
p+
# of
no
14
18
# of e-
Helium - 4
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Determining (Average)Atomic Mass
• To determine the atomic mass you must know
what percent of each isotope of the element is
found in nature. This is called the relative
abundance.
• Example: There are 2 common isotopes of Chlorine.
25% is chlorine - 37
75% is chlorine - 35
Calculate the average atomic mass of chlorine.
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Average Atomic Mass
• Neon in nature is 90.5% Neon-20,
0.3% Neon-21, and 9.2% Neon-22.
What is the average atomic mass of Neon?
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REACTIONS
CHEMICAL
• involve the transfer or
sharing of electrons
NUCLEAR
• involve the absorption or
emission of particles by
the nucleus of an atom
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Nuclear Chemistry Vocabulary
• Nuclide- General name
given to the nucleus of
an atom
Parent nuclide- initial
nucleus
Daughter nuclide- the
nucleus after the decay
has occurred
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Nuclear Chemistry Vocabulary
• Radiation - energy that is emitted from a source
and travels through space.
– Ionizing Radiation- Has enough energy to
change atoms and molecules into ions;
examples: X-rays and gamma rays.
– Nonionizing Radiation- Does not have enough
energy to ionize matter; examples: radio waves,
microwaves
– Accidentally discovered by Henri Becquerel in
1896 when he was performing a lab with
fluorescent screens.
– Radioactivity is the spontaneous emission of
radiation from the nucleus of an atom.
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Types of Ionizing Radiation
Symbol
Alpha
Beta
4
2
0
1
positive
He
e
formed
when a
neutron
splits
-1
Gamma

Charge
or 
4
2
or
(deflected by a
magnet)
Penetrating Ability
Limited ability to pass
through matter.
Neutral
(is not deflected
by a magnet)
Big
(can be stopped by paper)
negative Can penetrate better
than alpha
(deflected by a
magnet)
Size
Small
(can be stopped by a few mm of Al)
Penetrates the farthest.
nothing
(several cm of lead or a larger layer
of concrete will block )
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penetrating ability
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Why decay happens
– To become more stable.
– Large atoms are stable when the
neutron: proton ratio is 1.5:1
– Decay happens when the neutron:
proton ratio is too high.
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Alpha decay
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Alpha Decay
• Occurs when an alpha particle leaves the
nucleus
- alpha particle = Helium nucleus
– Parent  daughter: mass decreases by 4
and atomic number decreases by 2
Example: Thorium-230 undergoes alpha
decay. Write the decay reaction.
230
90 Th
------>
4
2
He +
226
88 Ra
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Alpha decay practice
Write the decay reaction for alpha decay of Uranium-238.
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Beta decay
Ac +
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Beta Decay
0
e
1
• occurs when a beta particle
is emitted
from the nucleus
• Parent  daughter: equal mass but
atomic number increases by 1.
• a neutron becomes a proton.
Example: Carbon-14 undergoes beta decay.
Write the decay reaction.
14
6C
----->
0
1e
14
+ 7N
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Beta decay practice
Write the decay reaction showing beta
decay of Thorium-234.
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Fission and Fusion
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Fission Reaction
•
•
•
•
•
•
•
Nuclear reaction
Splitting an atom’s nucleus
Releases energy
Alpha, beta are examples
Used in nuclear reactors
Causes a chain reaction
Problem: produce radioactive waste;
storage of fuel is dangerous
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chain reaction
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Fusion
• Nuclear reaction
• Two light nuclei are combined to form one
heavier more stable nuclei
• Energy is released
• this is how stars are fueled
• Problem with using on Earth: requires
EXTREMELY high temps and high pressure
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fusion in the sun
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How come the protons hang out
with each other?
The charge of a proton is positive. It is
repelled by other protons. So, how do the
protons stay in the nucleus? Shouldn’t
they want to avoid each other?
The answer is that a Strong Nuclear Force
exists, which is a very strong, but short
range, force between quarks that keep the
nucleus together by overcoming the
repulsion between the protons.
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