Chemistry Regents
Mr. Markic
Page 1 of 7
Chapter 25 - Nuclear Chemistry
The study of ____________ ____________ and their uses in chemistry
Radiation • The penetrating rays & particles emitted by a radioactive source
Radioactivity • Process that occurs when nuclei change ____________
Radioisotopes • Atoms containing ____________ ____________
 Recall… What are isotopes?
o
How do nuclear reactions differ from chemical reactions?
Chemical Reactions
• Atoms become stable by ____________or
____________electrons
• Rate ____________changed by catalysts,
temperature, pressure, etc.
• Mass & charge ____________
Nuclear Reactions
• Nuclei become stable by undergoing changes
that ____________of energy
• Rate ____________be changed
• Charge conserved, mass ___________________
o A ____________small amount of mass
is converted into energy
Nuclear Stability
• Most nuclei are stable
• The stability of a nucleus depends on its neutron to proton
____________
• The stable nuclei are in the ‘________________________’
Radioactive Decay
• Occurs in atoms with too _______or too _______neutrons
• The nucleus will attempt to become more stable by releasing
energy through ______________
• Transmutation o when the unstable nucleus of one element is ____________
into a stable nucleus of a different element
o Occurs naturally and artificially
Types of Radioactive Decay
o Alpha particles,
o Beta particles,
o Positron emission
o Gamma rays,
Chemistry Regents
Mr. Markic
Page 2 of 7
Alpha Decay – α
• Gives off ____________ particles (helium nuclei)
• Occurs when the neutron: proton ratio is low
• A radioactive element gives off 2 protons and 2 neutrons
o The charge on the nucleus ________________________
• Alpha particles do not travel far & are not very penetrating due to their large mass and charge
o Sheet of paper or surface of skin stops them
240
o
o
o
o
94Pu
→ 23692U + 42He (α)
Atomic # # of protons Mass # # of neutrons -
Beta Decay – β
• Gives off ____________ particles (electrons)
• Occurs when the neutron: proton is high
• A neutron breaks apart into a proton, which stays in the nucleus, and an electron which is released
o 10n → 11H + 0-1e (β)
o Charge on the nucleus ________________________
• More penetrating than α particles
o Can pass through paper, but are stopped by aluminum foil or thin pieces of wood
228
o
o
o
o
88Ra
→ 22889Ac + 0-1e (β)
Atomic # # of protons Mass # # of neutrons -
Positron Emission
• Gives off a ____________ (positive electron)
• Occur when the neutron: proton is low
• A proton breaks apart into a neutron, and a unit of positive charge
o 11p → 10n + 0+1e (positron)
o Charge on the nucleus ________________________
22
o
o
o
o
Atomic # # of protons Mass # # of neutrons -
11Na
→ 2210Ne + 0+1e
Chemistry Regents
Mr. Markic
Page 3 of 7
Gamma Decay – γ
• Gives off high energy photons called ________________________
• _____________________________________________
• Very dangerous - extremely penetrating
o Pass easily through paper, wood, & the human body
o Some can be stopped by several meters of concrete or several centimeters of lead
240
o
o
o
o
o
94Pu
→ 24094Pu
Atomic # # of protons Mass # # of neutrons Nucleus changes only in its energy state
Summary of Radiation
Particle
Mass
Charge
Symbol
4
2He,
α
Penetrating
Power
Low
Alpha
4 amu
2+
Beta
Positron
0 amu
0 amu
11+
0 e, β
-1
0 e
+1
Moderate
Moderate
Gamma
Ray
0 amu
None
γ
High
Nuclear Equations
• Similar to chemical equations…
• Mass and charge must balance on both sides
•
14
•
By using the concept of conservation of charge and mass number, you can identify a missing particle in an
equation.
7N
+ 42He → 178O + 11H
Sample Exercises
1. What product is formed when radium-226 undergoes alpha decay?
2. What element undergoes alpha decay to form lead-208?
3. What product is formed when Mg-27 decays by beta emission?
Chemistry Regents
Mr. Markic
Page 4 of 7
1H
 _____ + 0-1e
6.
14
3Li
 94Be + _____
7.
241 Am
95
4.
3
5.
9
6C
 0-1e + _____
8.
16
7N
 166O + _____
 42He + _____
9. What forms when francium-220 decays?
10. What forms when potassium-37 decays?
11. What forms when potassium-42 decays?
+ 10n  42He + _____
12.
6 Li
3
13.
27
13Al
14.
27
14Si
83Bi
 42He + _____
29Cu
 6630Zn + _____
15.
214
+ 42He  _____ + 01e
16.
66
 0-1e + _____
17.
235
92U
 9038Sr + _____ + 10n + 40-1e
Half-Life
• Time required for ____________ of the nuclei of a radioactive sample to decay into products
•
After each half-life, __________of the existing radioactive atoms have decayed into atoms of a new
element
Chemistry Regents
Mr. Markic
Page 5 of 7
Table N
• Each element has a characteristic half-life
•
Anywhere from a fraction of a second to billions of years
•
½ life is ________________________
Uses for Half-Life
• Dating
• Nuclear medicine
Half-Life Calculations
• No formula; make a ________________________ chart
• Always start with time = ____________
• If no mass is given, start with ________________________
Sample Problems
1. Most chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days.
a. What fraction of a sample of Cr-51 will remain after 168 days?
b. If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days?
2. How much was present originally in a sample of Cr-51 if 0.75mg remains after 168 days?
3. According to Reference Table N, how much of a 100. microgram (μg) sample of nitrogen-16 will remain
after 28.52 seconds of decay?
4. In 5.49 seconds, 1.20g of Ar-35 decay to leave only 0.15g. What is the half-life of Ar-35?
5. Na-24 has a half-life of 15 hours. How much Na-24 will remain in an 18.0g sample after 60 hours?
Chemistry Regents
Mr. Markic
Page 6 of 7
6. How many half-lives are required for a radioisotope to decay to 1/32 of its initial value?
7. What fraction of 226Ra will be left after 4797 years?
Artificial Transmutations
• Particle Accelerators o Collision of a proton or α particle with a target nucleus
o Uses magnetic or electrostatic fields to accelerate particles & overcome the repulsive forces
•
Neutron Collisions o Occurs when a neutron collides with a target nucleus
o Used to prepare radioactive nuclei from stable nuclei
▫ 23892U + 10n → 23992U
▫
59
27Co
▫
32
16S
+ 10n → 6027Co
+ 10n → 3215P + 11H
Nuclear Fission
• ____________ of the nucleus into smaller fragments
o Occurs when the nucleus is bombarded with
____________
o Uranium-235 and Plutonium-239 are the only
fissionable isotopes
________________________________________________
•
•
Releases huge amounts of ____________
Very exothermic
o 1 kg U-235 = 20,000 tons of dynamite
o Atomic bombs & nuclear reactors
Nuclear Reactors
• Uses controlled fission
• Energy from fission reaction heats the coolant
• Heated coolant is used to produce steam
• Steam turns a turbine
• Turbine drives a generator to produce electricity
Chemistry Regents
Mr. Markic
Page 7 of 7
Nuclear Fusion
• Light nuclei ____________ to produce a nucleus of greater mass
• Solar fusion – H nuclei fuse to make He nuclei
________________________________________________
Fusion as an Energy Source
• Produces ____________ energy than fission
• Occurs only at high temperatures (over 40,000,000ºC)
• More appealing than fission because:
o Availability/low cost of light isotopes
o Products are generally not ________________________
Detecting Radiation
• Radiation ____________ be seen, heard, felt, or smelled
• Ionizing radiation o radiation with enough energy to knock electrons off atoms of the bombarded substance to produce
ions
o Can be detected by Geiger counters, scintillation counters, and film badges
Using Radiation
• Medicine o Diagnostic tools
o Treatment for cancer
o Help to determine mechanisms for chemical
reactions
o Trace movements of atoms in biological
systems
•
Radiation in the body should:
o Have a ____________ half-life
o Be ____________ eliminated from the body
Other Uses of Radiation
• Tracers o Any radioisotope used to follow the
path of a substance
o Ex. Used in agriculture to test the
effects of herbicides, pesticides, and
fertilizers
•
Industrial Applications –
o Used to measure the thickness or
strength of a material based on
radioactive absorption
•
•
Food Irradiation –
o Kills insects, bacteria, & mold
o Prevents the ‘sprouting’ of fruits &
vegetables
Dating o Using half-lives, ‘age’ of objects can be
determined
o C-14 → C-12:
o U-238 → Pb-206: