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CHAPTER 4/25
Atomic Structure
Nuclear Chemistry
A Long, Long Time Ago…
• Greek Philosophers
4 elements are Earth, Water, Fire, and Air
• Aristotle- first recorded atomic thoughts
Matter is continuous (no atoms)
• Democritus (400 B.C.) - first recorded atomic theory
Atoms are smallest part of matter, each type of
matter has different atoms
A Long Time Ago…
• Antoine Lavoisier (1782)
Used experiments in closed containers to develop
the Law of Conservation of Matter
• Joseph Proust (1799)
Analyzed water to develop the Law of Definite
Proportions
• John Dalton (1803)
Compiled past research to develop the first useful
atomic theory
John Dalton
Dalton’s theory had 4 major tenets
1. All matter is composed of atoms
2. Atoms are indivisible
3. Atoms of 1 element are alike, but different
from those of other elements
4. Atoms combine in small, whole number ratios
to form compounds
A Little While Ago…
J.J. Thomson (1897)
discovers the electron
using a cathode ray tube
Proposes the “Plum
Pudding” Model of the
atom
Cathode Ray Tube Experiment
http://www.aip.org/history/electron/jjappara.htm
The Nucleus Exists!
Ernest Rutherford (1911)
uses the gold foil
experiment to “discover”
the nucleus
The Gold Foil Experiment
Results of Gold Foil Experiment…
Over 98% of the particles went straight through
About 2% of the particles went through but were
deflected by large angles
About 0.01% of the particles bounced off the gold foil
About that 0.01% of particles…..
…" It was quite the most incredible event that ever
happened to me in my life. It was almost as incredible
as if you fired a 15-inch shell at a piece of tissue paper
and it came back and hit you." E. Rutheford
Rutherford’s Conclusion
Rutherford's Nuclear Model
1. The atom contains a tiny dense center called the nucleus
the volume is about 1/10 trillionth the volume of the atom
2. The nucleus is essentially the entire mass of the atom
3. The nucleus is positively charged
the amount of positive charge of the nucleus balances the
negative charge of the electrons
4. The electrons move around in the empty space of the
atom surrounding the nucleus
Coworker James Chadwick later adds neutrons
Properties of Subatomic Particles
Particle Symbol
Electron
Proton
Neutron
ep+
n0
Location
Space surrounding
the nucleus
Nucleus
Nucleus
Relative
Electrical
Charge
Relative
Mass
Actual
Mass (g)
1-
1_
1840
9.11 x 10-28
1+
1
1.637 x 10-24
0
1
1.675 x 10-24
Understanding Periodic Blocks
•
• Atomic Number
• # of protons
• # of electrons
• Element Symbol
Mass Number
#protons + # neutrons
• Atomic Mass
The weighted average
atomic mass of isotopes
of an atom
Can 20.0983 Neutrons Exist???
No, 39.0983 is an average mass of all natural K atoms
• All K atoms MUST have 19 protons
• Some have 20 neutrons, some 19, some 21…
• Average is 20.0983 neutrons
• Individual isotopes are identified by the number
of neutrons
Identifying Protons, Neutrons, and Electrons
• Chlorine-35 (element-mass number)
• # of protons = 17
• # electrons = # protons
• # neutrons = mass number – protons
• 35-17 = 18
• Why no electrons in atomic mass?
• Electron has mass 1/1837 of Proton and Neutron
Isotopes
Nuclear Notation
or
Isotope Notation
# protons = ______
# neutrons = ______
# electrons = ______
Problems that can arise..
Charges and Ions – Only changes the number of electrons
Finding the mass number – use symbol, p+ and no or periodic table in
that order!
Atomic Mass Units
• Carbon-12
• 6 protons
• 6 neutrons
atomic mass unit is equal to one-twelfth of the mass of the
nucleus of a carbon-12 atom
Atomic Mass Problems
Boron has two naturally occurring isotopes: boron-10 (19.8%, 10.013 amu)
and boron 11 (80.2%, 11.009 amu). What is the atomic mass of boron?
Find the Abundance x Mass for each isotope and then add products together.
0.198 x 10.013 = 1.98
0.802 x 11.009 = 8.83
1.000
10.81 amu
Chemical vs. Nuclear Change
•
Chemical Change- produces new kinds of matter with new
properties
•
•
•
Involves breaking and forming BONDS
Accomplished by rearrangement of
ELECTRONS
Nuclear Change- produces a new nucleus that contains less
energy
•
•
Involves emission or capture of nuclear
particles
Accomplished by changing PROTONS and
NEUTRONS in nucleus
ALPHA (α)
BETA (β)
GAMMA (γ)
Symbol
4 He
2
0 e
-1
0 γ
0
Shielding
Easy
(skin/clothes)
Hard (2 cm
lead)
Energy
High
Medium
(aluminum
foil)
Med
Biological
Hazard
Actual
Hazard
High
Med
Low-Med
Low
Med
High
Low
Other Radiation
•
Ionizing Radiation- has significant energy to
change atoms and molecules into ions
•
•
•
Types: alpha, beta, gamma, x-rays
Effects to living organisms: changes in, DNA (cell
death/cancer)
Nonionizing radiation- does not have
significant energy to ionize atoms or
molecules (types: microwaves, visible light,
radiowaves)
Nuclear Stability
(Electrostatic Force vs. Nuclear Force)
Electrostatic force arises from the interaction between
two protons (repulsive force)
Nuclear force arises between protons and neutrons due
to their close proximity to one another
Neutron to Proton Ration and the Band of
Stability
1.5:1
1:1
As the atomic number
increases, more and more
neutrons are needed to
create a strong nuclear
force to oppose and
increasing electrostatic
force
Nuclear Reactions
•
Nuclear Decay
•
Radioactive Decay
Half-lives
• Half-life is the time taken for half of
the atoms of a radio-active
substance to decay.
Number of half-lives
TE/t1/2
5 variables to consider
1. Initial quantity
2. Final quantity
3. # of half-lives
4. Value of a half life
5. Time Elapsed
Sample Problem
A sample of radioactive iridium has a half life of 12 years.
In 60 years, how much iridium would remain from a 50g
n
sample?
Final
1
 
 
Initial  2 
Initial= 50g
Final  1 
 
50
2
Final= ?
Time Elapsed=
60 years
# of half lives= 60/12 = 5
Value of 1 half life= 12 years
n
Final  1 
 
50
2
5
Final 1

50
32
32 x  50
x  50 / 32  1.6 g
Half-life
• Half-lives can range
from a millionth of a
second to millions of
years
Radioactive Dating
•
Uses carbon-14 to tell age of fossils
•
•
•
•
C-14 is present constantly in atmosphere
15.3 decays/min in living organism
decays/min decreases by ½ every 5370 years an organism is
dead.
Only useful to 60,000 yrs ago
Transmutation
• All nuclear reactions are:
transmutation reactions
o Some transmutation reactions are induced
o All transuranium elements
(atomic #93 and greater) have
been produced through induced
transmutation.
Conversion of an
element into an
atom of another
element
Writing Induced Transmutation Reactions
Write the balanced nuclear equation for the induced
transmutation of aluminum-27 into sodium-24 by
neutron bombardment. An alpha particle is released in
the reaction.
27
13
Al  n Na He
1
0
24
11
4
2
Write the balanced nuclear equation for the alpha
particle bombardment of Pu-239. One of the reaction
products is a neutron.
239
94
Pu  He  Cm n
4
2
242
96
1
0
Nuclear Reactions for Energy
•
Fission- nucleus
broken into 2
smaller nuclei
• Fusion- smaller nuclei
join to form a larger,
more stable nuclei
Reactants and Products
•
•
Nuclear Powergenerated by a
controlled fission
chain reaction
Control rods- absorb
neutrons to slow the
chain reaction
•
Made of cadmium
•
Inserted or withdrawn
to keep temp of
reaction steady
Moderators- slow
neutrons down so
they DO hit uranium
fuel rods
•
Made of water,
beryllium, or graphite
•
Intended to allow
neutrons to be
absorbed by uranium
Cooling and Shielding
•
Water- acts as a coolant and transfers heat between
reactor and turbines that produce electricity
•
Steel & concrete- surround core and protect personnel
by absorbing radiation