Nuclear Chemistry
What I will learn in this unit:
Describe the characteristics of alpha, beta, and gamma radiation Describe the radioactive decay process in terms of balanced nuclear reactions Calculate half-life, and be able to use and interpret half-life diagrams Compare fission and fusion reactions Know the basic structure and function of a nuclear power plant, and know the benefits and hazards of nuclear power
Nuclear Chemistry (definition) The study of the atomic nucleus, its reactions, and radioactivity
What is Radioactivity?
Radioactivity • spontaneous emission of particles or energy during nuclear decay
Nuclear Decay • spontaneous disintegration of a nucleus • occurs when particles or energy escape from an unstable nucleus • releases large amounts of energy and radiation
Subatomic Particles Review
Proton () • positively charged particle found in the atomic nucleus
Neutron () • neutral particle found in the atomic nucleus
Electron () • negatively charged particle found in the electron cloud (outside the nucleus)
Nuclear Chemistry Page 1 of 18 Remember…
Atomic Number = # protons • Shown on periodic chart (the whole number!) • Adding or removing protons from an atom changes the atom to a different element!
Mass Number = #p + #n • NOT shown on periodic chart (the decimal number is the atomic mass!)
Charge = #p - #e
An atom is neutral • # protons = # electrons
An ion has a charge • More electrons than protons = negative charge • Fewer electrons than protons = positive charge
Isotopes • Same number of protons, different number of neutrons
Example #1: What is the identity of an atom that has 50 protons, 64 neutrons and 48 electrons? Sn
Example #2: What is the atomic number, mass number and charge of the element in Example #1?
Atomic number = 50 Mass number = 50 + 64 = 114 Charge = 50 – 48 = +2
Example #3: What is the identity of an atom that has a mass number of 70, a charge of +2, and an atomic number of 30?
Zn
Nuclear Chemistry Page 2 of 18 More About Isotopes
There are 2 different ways to identify isotopes:
Hyphen-Notation = element – mass number
Chemical Configuration
Example: Hyphen Notation Chemical Configuration
Carbon – 12
Carbon – 13
Carbon – 14
Some nuclei are unstable, and therefore radioactive.
Ex: 12C 13C 14C
stable stable unstable
Radioactive? no no yes
Example #4: Write the hyphen-notation and the chemical configuration for an iron atom that has 23 electrons and 32 neutrons.
58 3+ Iron-58; 26Fe
Example #5: Write the hyphen notation and determine the number of protons, neutrons and electrons for .
Phosphorus-32 Protons = 15 Neutrons = 17 Electrons = 15
Nuclear Chemistry Page 3 of 18 Commonly Used Radioactive Isotopes Radioactive isotopes are important in everyday life
Isotopes Use 11C, 13N, 15O PET scans Carbon-14 Carbon-dating; research Phosphorus-32 Cancer therapy 60Co, 137Cs Food preservation 99mTc Radiotracer for heart Iodine-131 Radiotracer for thyroid, brain Uranium-235 Create electrical power Americium-241 Fire alarms
Transuranium Elements • Anything with an atomic number higher than uranium • All are synthetic and radioactive • Many were synthesized between 1940-1961 by Glenn Seaborg
Example #6:List 2 transuranium elements.
(Answers will vary; anything with atomic number higher than 92)
Nuclear Chemistry Page 4 of 18 Radiation Exposure • We are constantly exposed to background radiation from both natural and man-made sources. • Total exposure is estimated to be about 0.360 rem/year, or 1/1700 of the lethal dosage. • Rem = the unit used to quantify the biological effects of radioactivity
Radiation Effects
Radiation Dosage Effect 0-25 rem No immediately observable effects 25-50 rem Small decrease in WBC (= white blood cell) count (decreased resistance to disease) 50-100 rem Larger decrease in WBC; lesions 100-200 rem Radiation sickness: nausea, vomiting, hair loss; blood cells die 200-300 rem Hemorrhaging, ulcers, death 300-500 rem 50% die within a few weeks >700 rem 100% die
Natural Sources of Radiation
Includes: • Cosmic rays • Radioactive isotopes in rocks, the air, soil, water and our bodies
Man-made Sources of Radiation
Includes: • Television • X-ray generators • Smoke alarms • Nuclear power plants • Nuclear weapons and fuel • Cigarettes
Radiation is produced during Nuclear Reactions
There are several types of radiation: • Alpha radiation • Beta radiation • Gamma radiation
Nuclear Chemistry Page 5 of 18 Properties of the Different Types of Radiation
Alpha Particle Beta-minus Beta-positive Gamma particle particle Radiation Symbol , α
Charge +2 -1 +1 n/a
What is it??? Helium nucleus Electron Positron (anti- High energy matter electron) electromagnetic radiation
Speed Largest and Faster than alpha Faster than alpha Speed of light slowest form of radiation
Can be stopped Piece of paper Plastic, aluminum Plastic, aluminum Thick lead or by… foil foil concrete
Nuclear Reactions
• Nuclear reactions typically result in a change in the identity of an element • Transmutation = the process of an atom changing from one type of element to a different type of element
Nuclear Chemistry Page 6 of 18 Chemical Reaction Nuclear Reaction Forms New substances New isotope or element
Energy changes Small; energy from Large; energy from binding breaking/forming chemical bonds energy of nucleus Involves Only valence electrons Change in number of protons or neutrons
When writing nuclear reactions… • Reactants (starting elements and particles) are on the left side of the arrow; products (the particles and elements that are formed) are on the right side of the arrow. • You must balance both the mass (top) numbers and the atomic (bottom) numbers • Use the atomic number to identify the element • You must memorize the symbols for the particles, such as the electron
Example: +
Particle Symbols You Need to Know
Proton Alpha particle
Neutron Electron (beta-minus) ,
Positron (beta-positive) ,
Gamma rays
Types of Nuclear Reactions
Alpha Emission
• A helium nucleus (2 protons and 2 neutrons) is emitted from the nucleus
• Example: à +
Example #7: Write the nuclear equation for alpha decay of the following:
Nuclear Chemistry Page 7 of 18 235 4 231 Uranium-235 92U 2He + 90Th
222 4 218 Radon-222 86Rn 2He + 84Po
Nuclear Chemistry Page 8 of 18 Beta Emission • A neutron in the nucleus is converted into a proton and electron, then the electron (β- particle) is emitted
• Example: à +
Example #8: Write the nuclear equation for beta decay of the following:
131 0 131 Iodine-131 53I -1β + 54Xe
27 0 27 Magnesium-27 12Mg -1β + 13Al
Positron Emission • A proton is converted into a neutron and positron, and the positron is emitted from the nucleus
• Example: à +
Example #9:Write the nuclear equation for positron emission from the following:
40 0 40 Potassium-40 19N 1β + 18Ar
15 0 15 Oxygen-15 8O 1β + 7N
Electron Capture • The nucleus captures an electron, combines it with a proton and forms a neutron
• Example: + à
Example #10: Write the nuclear equation for electron capture by the following:
125 0 125 Iodine-125 53I + -1e 52Te
22 0 22 Sodium-22 11Na + -1e 10Ne
Gamma Emission • High energy gamma rays are emitted from the nucleus, either alone or with other types of radiation • Gamma rays do NOT change the mass number or the atomic number
Nuclear Chemistry Page 9 of 18 Decay Series A series of nuclear reactions that occur until a stable nucleus is formed
Example #11: Write the first 4 nuclear reactions in the uranium- 238 decay series as shown in the graph.
238 4 U 2He + 234Th
234 0 Th -1β + 234Pa
234 0 Pa -1β + 234U
234 4 U 2He + 230Th
Fission vs. Fusion
Definitions:
Fission: heavier nuclei split apart to form light nuclei
Fusion: light nuclei combine (fuse) together to form heavier nuclei
Fission • Fission can occur at normal temperatures • Fission of uranium-235 is used in nuclear power plants to produce electricity. • Some nuclear bombs are uncontrolled fission reactions. • Fission of uranium produces about 26 million times more energy than combustion of methane.
• Example of a Fission Reaction:
Nuclear Chemistry Page 10 of 18 + à + + 3
*Notice that neutrons are on both the reactant side and product side. The neutrons produced can hit other 235U atoms to start a new fission reaction. This is called a chain reaction.
Nuclear Chemistry Page 11 of 18 Nuclear Power: Pros and Cons
Pros Cons • No air pollution • Expensive to build and maintain • No greenhouse gas emissions • Risk of accidents • Low cost fuel because very little is • Security needed • Thermal pollution (warm water into streams and rivers) • Disposal of nuclear waste (must be buried for possibly thousands of years)
Nuclear Power Plant
Function of the Parts of the Power Plant:
A. Containment structure – thick layers of concrete and steel to prevent radiation leakage
B. Control Rods – controls the rate of reaction; can be used to shut reaction down
C. Reactor – where the nuclear reactions take place
D. Steam generator – nuclear reactions produce heat energy which is used to boil water (produces steam)
H. Turbine – steam runs the turbine, which produces electricity
K. Fuel Rods – usually contain uranium-235; the fuel for the nuclear fission reaction
I. Condenser – sends cool water to the cooling tower and the reactor; vital to keep reactor from overheating
Nuclear Chemistry Page 12 of 18 Fusion
• Fusion of hydrogen nuclei to helium nuclei powers our sun. • Uncontrolled fusion of heavy hydrogen results in an explosion (hydrogen bomb).
• Fusion reaction that occurs in the sun: + à +
Fusion Pros and Cons
Pros Cons • Produces even more energy per gram • Does not sustain a chain reaction. of fuel than fission. • Requires extremely high temperatures • Produces less nuclear waste than (108 - 109 °C) and pressures. fission. • We do not have the technology to • Fusion fuel is easy to get. (Heavy efficiently harness the energy hydrogen is found in water.) produced by fusion or to contain a fusion reaction.
Half-Life (t1/2) The time it takes for ½ of the atoms in a sample to undergo decay The atoms do NOT suddenly decay all at the same time; this process occurs over time
à à à
1 t1/2 2 t1/2 3 t1/2
Mass 100 g 50 g 25 g 12.5 g
% 100% 50% 25%
12.5%
1 Fraction 1 ½ ¼ /8
Nuclear Chemistry Page 13 of 18 The WHITE part of the circle represents the radioactive isotope you are starting with. The BLACK part of the circle represents the element that it turns into after it decays (undergoes a nuclear reaction).
Remember: The isotope undergoing decay does NOT just disappear – it is turning into a different element!
Nuclear Chemistry Page 14 of 18 Example #12: 11C undergoes positron emission to form 11B. The half-life of 11C is 20.3 minutes. Based on this information, write the reaction that occurs, then fill out the chart below.
11 11 0 Reaction: C B + 1β
# of half-lives Total time passed Fraction 11C Fraction 11B passed (minutes) (stable isotope) 0 0 1 0 1 20.3 ½ ½ 2 40.6 ¼ ¾ 3 60.9 1/8 7/8
Half-life Diagram Just a way to graph the half-life information.
Rate of Decay # of Time Amount (mg) of Half-lives20 (years) radium-226 left
) 18 0 0 20 g
m 16 (
6
2 14 1 1599 10 2 - 12 m u i 10 d
a 2 3198 5 r 8 f o
t 6 n 3 4797 2.5 u
o 4 m
A 2 0 4 6396 1.25 0 1000 2000 3000 4000 5000 6000 7000 Tim e (years)
Example #13: What is the half-life of radium-226, based on the chart?
1599 years
Example #14: Describe the difference between the half-life and the number of half-lives.
The half-life is the amount of time that is needed for half of the radioactive sample to decay. The number of half-lives describes how many times that much time has passed.
Nuclear Chemistry Page 15 of 18 Nuclear Chemistry Page 16 of 18 Half-Life Problems
To calculate the remaining or original amount of radioactive sample…
1. Determine the number of half-lives that have passed by the equation:
2. Reduce the original amount by half for every half-life that passed OR double the final amount for every half-life that passed.
Example #15: Phosphorus-32 has a half-life of 14 days. If you begin with 100. atoms, how many atoms of phosphorus-32 will remain after 28 days?
100. atoms × ½ × ½ = 25.0 atoms
OR: Time (days) Amount (atoms) 0 100. 14 50.0 28 25.0 atoms
Example #16: The half-life of is 4.46×109 years. If, after 2.23×1010 years passed, you have a final amount of 7.50×1022 atoms, how many atoms of uranium-238 did you start with?
7.50×1022 atoms × 2 × 2 × 2 × 2 × 2 = 2.40×1024 atoms
OR: Time (years) Amount (atoms) 0 2.40×1024 4.46×109 1.20×1024 8.92×109 6.00×1023 1.34×1010 3.00×1023 1.78×1010 1.50×1023 2.23×1010 7.50×1022 atoms
Nuclear Chemistry Page 17 of 18 Carbon-14
• Carbon-14 is formed when cosmic rays strike atmospheric nitrogen • Carbon-14 (like other carbon isotopes) reacts with oxygen to form CO2, which is taken up first by plants during photosynthesis, then by the animals that eat the plants
Carbon-14 : Carbon-12 Ratio • Assuming a constant rate of cosmic radiation and equal distribution of CO2 around the world, we can assume a specific 14C:12C ratio (1:1012) for all living organisms • The carbon-14 undergoes beta decay with a half-life of about 5715 years
Carbon-14 Dating • When an organism dies, no new 14C is taken in. The 14C already present continues to decay. • To estimate how long ago an organism died, scientists measure the decrease in 14C : 12C, then apply half-life calculations to determine how much time has passed.
Nuclear Chemistry Page 18 of 18