Nuclear ChemistryDHS Chemistry
Chapters 4 and 25
Little House on the Prarie
Review: Structure of an Atom
Structure of an Atom
An atom consists of three types of subatomic particles, protons, neutrons, and electrons.
Structure of an Atom
• Protons and neutrons are located in the nucleus
• Electrons are in a cloud surrounding the nucleus.
• The number of protons is equal to the atomic number.
Structure of an Atom
If the atom is neutral, the number of electrons is equal to the number of protons.
The mass number is equal to number of protons + number of neutrons.
Structure of an Atom
• Elements with the same number of protons, but have different numbers of neutrons is called an isotope.
Structure of an Atom
Protons Neutrons Electrons
Symbol P+ no e-
Charge + 0 -Location Nucleus Nucleus
Electron cloud
Relative mass
1 amu 1 amu negligible
What’s a chemical reaction?
Chemical Reactions
Why Call it Nuclear Chemistry?
See unitedstreaming.com
Chemical VS. Nuclear • Nuclear reactions are caused from
unstable nuclei becoming stable through radioactive decay.
•Releasing particles and high-energy waves
•Alters the number of nuclear particles (neutrons and protons).
•Nuclear reactions are very different from chemical reactions.
Chemical Changes
1. Occur when bonds are broken and formed.
2. Atoms remain unchanged, though they may be rearranged.
3. Involve only valence electrons.4. Associated with small energy
changes.5. Reaction rate is influenced by
temperature, pressure, concentration, and catalyst.
Nuclear Changes
1. Occur when nuclei emit particles and/or rays.
2. Atoms are often converted into atoms of another element. Thus their identity changes.
3. May involve protons, neutrons, and electrons.
4. Associated with large energy changes.5. Reaction rate is not normally affected
by temperature, pressure, or catalysts.
What is an isotope?
Isotope Examples
7
3 Li
7
3 Li
6
3 Li
6
3 Li
Lithium - 6 Lithium - 7
Mass Numbers
Atomic # = protons
Isotopes
• Isotopes are atoms of the same element that vary in their number of neutrons, thus they have different mass numbers.
•The convention for writing isotope names is first the element name dash and then the mass number.
For example: Carbon-14, carbon-12
carbon-13How many protons and neutrons does each of
the isotopes of carbon have?
• Carbon – 148 neutrons, 6 protons• Carbon – 126 neutrons, 6 protons• Carbon – 137 neutrons, 6 protons
Mass Number
(protons + neutrons)
More on Isotopes•When an isotope is unstable it is called a radioisotope.
•To gain a more stable configuration, the nuclei emit radiation.
•The resulting stable atom is called the daughter product.
•This is called radioactive decay.
Strong Nuclear Force• The dense nucleus has two different
kinds of nuclear particles (protons + neutrons) closed packed together.
•The protons are positively charged nucleons.
•The neutrons are neutral nucleons.•All of the protons in the nucleus repel each other and cause an electrostatic force that pushes the nucleus apart.
•However, there is a force holding the nucleus together.
•The strong nuclear force is a force that acts only on subatomic particles that are extremely close together.
•If the strong nuclear force overcomes the electrostatic force, the nucleus stays together.
Neutron to Proton Ratio
• The strong nuclear force is not always strong enough to overcome the electrostatic force. When this happens the nucleus breaks apart. The stability of the nucleus can be determined by the ratio of neutrons to protons.(n/p)
Neutron to Proton Ratio• Elements with low atomic numbers (<
20) are most stable when the neutron to proton ratio is 1:1.
• As the atomic number increases, more and more neutrons are needed to overcome the electrostatic force.• Thus, the stable ratio of neutron to
proton increases as the atomic number increases.
• 1.5:1 is the largest ratio for a stable nucleus.
Example: Determine the neutron to proton ratio for
Lead-206. Is it stable?
Lead – 20682 protons
124 neutrons (206 – 82)
124 = 1.51 stable 82 1 ratio
Nuclear StabilityNeutron : Protons
Stable if: • Smaller than Fe
~1 1• Between Fe & Pb
1-1.5 1• Bigger than Pb
1.5 1
Extra Practice
• Calculate the neutron to proton ratio, and determine if the isotope is stable.
1.
2. U- 293
3. Lead-206
2:2 1:1
201:92 2.18:1
124:82 1.5:1
stable
stableunstable
• The graph on the right plots the neutrons versus protons. The band created is called the band of stability. Anything that falls outside of that band is radioactive.
• All elements with an atomic number greater than 83 are radioactive.
The Band of Stability
III. Radioactivity
Radioactive Substances
• Bananas• Atomic fire ball candies• Colored gemstones (blue
topaz)• Fiesta Ware
• Table ware that contained unsafe amounts of radioactive Uranium
• Uranium for color glaze. Up to 14% can be Uranium
Geiger Counter
Types of Radiation
The experiment
Types of Decay
• Nuclear reactions change an atom of one element to an atom of another element. This process is called transmutation. In a nuclear reaction there are three common types of radiation that are emitted: alpha, beta, and gamma. The first two are involved in transmutation, changing the identity of the atom.
Properties of Alpha, Beta, and Gamma Radiation
Property Alpha(α) Beta (β) Gamma (γ)
CompositionAlpha
particlesBeta
particles
High-energy electromagneti
c radiation
Description of Radiation
Helium Nuclei
Electrons Photons
Charge 2+ 1- 0
Relative Mass heaviest lightest 0
Relative Penetrating
Power
Blocked by paper
Blocked by metal foil
Not completely blocked by lead
or concrete
• Emit means to give off or release
Alpha Particles• Alpha particles contain the same
composition as a helium nucleus. Out of all of the radiation particles, alpha particles move the slowest and are the least penetrating. As a result of alpha decay, the mass number decreases by 4 and the atomic number decreases by 2.
He42 42or
Alpha decay
Beta Particles• Beta particles are similar to an electron
except they come from the unstable nucleus of the atom. Beta particles are formed and ejected when a neutron decays to a proton and an electron. The proton stays in the nucleus and the electron is the beta particle. Beta emission is a constant flow of quick moving electrons that can be stopped by a metal foil. As a result of beta decay the atomic number increases by one. The mass number does not change.
• Gamma rays are short wavelengths (photons) that move the quickest of all the types of decay. They are very high-energy electromagnetic radiation. These rays often are released at the same time as an alpha or beta particle. The ray is the energy lost in the reaction. Gamma emission does not affect the atomic number or the mass number of the isotope.
Gamma Rays
Gamma Rays
Are very dangerous!!
(write that down)
C. Gamma ( ) Emission
• usually occurs along with other forms of radiation.
• Gamma particle is emitted• No change in mass number• No change in atomic number
Practice Problems
1. Why is radiation given off?
2. What is the most penetrating particle?
3. What is a main difference between a nuclear reaction and a chemical reaction?
4. Is Carbon-14 radioactive? Why or Why not?
III. Balancing Nuclear Reactions
Balancing Nuclear Reactions
• In nuclear reaction equations we account for all of the changes in the mass number and atomic mass that occur through the decay of the nucleus.
• To verify this, we include the mass number and atomic mass of every particle or atom involved in the reaction.
Mass number
Atomic number
24He
Mass number
Atomic number
10
Solving Problems• When solving/balancing a nuclear
reaction, •Look to find the difference of the mass numbers and atomic numbers between the reactants and the products.
•This will indicate the particle that was released or the atom that was formed.
•Make sure you have the same total mass number and atomic number on both sides of the equation.
Nuclear Equations
Nuclear Reactions
Ex 1: Write a balanced equation for the alpha decay of polonium-210
210
84 Po 206
82 Pb + 4
2 He
210
84 Po 206
82 Pb + 4
2 He
Ex 2: Write a balanced equation for the beta decay
of carbon-14
Practice
Fill in the blank with the proper radiation particle or isotope
1)
2)
3)
4)
120 55
Cs 12056 Ba + ____
120 55
Cs 12056 Ba + ____
15064Gd 146
62 Sm + _____
15064Gd 146
62 Sm + _____
24195 Am _____ + 4
2 He
24195 Am _____ + 4
2 He
_____ 21081 Tl + 4
2 He
_____ 21081 Tl + 4
2 He
Examples
• Ex 1. Write an equation for the alpha decay of Protactinium-231
• Ex. 2
83211Bi ____ 81
207Tl
III. Radioactive Decay Rates
Radioactive Decay Rates
• A. Half-Lives• We measure radioactive decay in
terms of half- lives. • A half life is the time it takes
for half of a radioactive sample to decay.
Candy Bar Bandits
• There is a candy bar left in the teacher’s lounge. Every 5 minutes a teacher walks in, looks at the candy bar, breaks the candy bar in half and eats it. If the candy bar originally had a mass of 20 grams, how much is left after 4 teachers have a taste?
Keeping Track
A T012345678
20g10g5g
2.5g1.25g
05 min10 min15 min20 min
Candy Bar
Keeping Track
A T012345678
20g10g5g
2.5g1.25g
05 min10 min15 min20 min
Example
• The half life of Carbon-14 is approximately 5730 years. If you had 12g of Carbon-14 today, in 5730 years you would only have 6g. The missing 6g decayed and turned into Nitrogen 14. And, in another 5730 years you will have 3g of Carbon-14 left and then in 5730 more years you would only have 1.5g of Carbon-14 left. Every 5730 years your sample is cut in half.
Practice
• What is the half-life of the sample in the graph?
~11 billion years
Half-Lives# of HLs Remaining mass
1 Original Mass x (1/2) Org. Mass x (.5)1
0.5
2 Original Mass x (1/2) x (1/2) Org. Mass x (.5)2
0.25
3 Original Mass x (1/2) x (1/2) x (1/2)
Org. Mass x (.5)3
0.125
4 Original Mass x (1/2) x (1/2) x (1/2) x (1/2)
Org. Mass x (.5)4
0.0625
5 Original Mass x (1/2) x (1/2) x (1/2) x (1/2) x (1/2)
Org. Mass x (.5)5
0.03125
6 Original Mass x (1/2) x (1/2) x (1/2) (1/2) x (1/2) x (1/2)
Org. Mass x (.5)6
0.015626
Ratio of remaining mass to original mass
Remaining mass
In the box
Amount Remaining = initial amount (1/2)HL
= initial amount (0.5HL)
HL = # of half lives
Total Time Passed = # of Half Lives
Time of one HL
Ex. 1 If gallium-68 has a half-life of 68.3 minutes, how much of a 10.0 mg sample is left after 342 minutes?
0.3125 mg
HL A T0123456
10mg5mg
2.5mg1.25mg.625mg0.3125
068.3136.6204.9273.2341.5
Ex. 1 If gallium-68 has a half-life of 68.3 minutes, how much of a 10.0 mg sample is left after 342 minutes?
Ans = 0.3125 mg
Amount Remaining = (Initial amount) (0.5) n
n = # of half lives that have passed 342 total minutes / 68.3 minutes = half lives
More Examples
Ex. 2 If the passing of 116 years leaves 25.00 mg of an original 400 mg sample of Strontium-90, what is the half life of Strontium-90?116/4 = 29 years
HL A T012345678
40020010050
25mg
0
116y
Don’t get it? Here it is in words.
• If you are trying to solve for the amount of element left, divide the time passed from the half life and that will give you the amount of half lives your sample has had. You can then divide your original mass of sample by 2 as many times as you have half lives.
• If you are trying to solve for the half life of your sample, take the remaining mass and count how many times you have to multiple it by 2 to get your original mass. That will tell you how many half lives have elapsed. Take that number and divide it by the total time that has elapsed to get the length of just one half life.
More on Half-lives•Because the half-life is constant, radioisotopes can be used to date objects.
•Radiochemical dating is the process of determining the age of an object based on the amount of a particular radioisotope is remaining in the object.
Carbon Dating
• Carbon dating is a specific type of radiochemical dating
• All living organisms have the same ratio of carbon-14, carbon-13, and carbon-12 as the in atmosphere.
• However, once an organism dies and there is no new carbon intake, the unstable carbon-14 starts to break down.
Practice1. Iron-59 is used in
medicine to diagnose blood circulation disorders. The half-life of iron-59 is 44.5 days. How much of a 2000 mg sample will remain after 133.5 days?
HL A T012345678
20001000500250
044.589
133.5
250 mg of the sample
Practice
• After 2 years, 1.99 g of a radioisotope remains from the sample that had an original mass of 2.00 g. What is the half-life of this isotope?
200 years
More on Carbon dating• Scientists calculate the ratio of
carbon-12 and carbon-13 to carbon-14 in dead organisms.
• Then they compare the ratio to the atmospheres ratio and determine how many half-lives have passed.
• With that information they can calculate how long the organism has been dead.
More Practice Solving Half-Life Problems1. Strontium’s half life of the
radioisotope strontium-90 is 29 years. If you had 10.0g of strontium-90 today, how much would you have left after 87 years? 1.25g
2. If you start off with 64 grams of some substance, how much will you have after 3 half lives? 8g
3. Iodine-131 has a half life of 8 days. What fraction of the original sample would remain at the end of 32 days? 1/16
1) Strontium’s half life of the radioisotope strontium-90 is 29 years. If you had 10.0g
of strontium-90 today, how much would you have left after 87 years?
Sr-90 half-life = 29 years
How many half-lives is 87 years? 87years ÷ 29 years = 3 half-lives 1 half-life
How much is left after 3 half-lives?10.0g 5.0g 2.5g 1.25g start after 1 half-life after 2 half-lives after 3 half-lives
1) Strontium’s half life of the radioisotope strontium-90 is 29 years. If you had 10.0g of strontium-90 today, how much would
you have left after 87 years?
HL A T
0123456
0295887
10g5g
2.5g1.25g
2) If you start off with 64 grams of some substance, how much will you
have after 3 half lives?
HL A T
0123456
64g32g16g8g
3) Iodine-131 has a half life of 8 days. What fraction of the original sample
would remain at the end of 32 days?
HL A T
0123456
1/11/21/41/81/16
08162432
IV. Nuclear Energy
A. Fission
Fission
• Fission means to break apart. Nuclear fission occurs when a nucleus splits apart into different fragments.
• This generally occurs with atoms that have a mass number heavier than 60.
• The nuclei do not always split the same way. Scientists have found 200 different products from the fission reaction of Uranium-235.
More on Fission• Another important factor of fission
reactions is that they cause a chain of reactions.
• The products of the initial reaction can collide with other molecules and cause a new fission reaction to occur.
• This domino affect could go on for a long time. This is how an atomic bomb works.
Fission Reaction
Nuclear Power• Nuclear power plants harness the
energy released in fission reactions and turn it into electricity.
• One of the main issues the power plant has to deal with is keeping the chain reactions going, but not letting them speed out of control. To this date there have been two large nuclear accidents.
Nuclear Power• Nuclear power plants have to be very
precise in their regulations of the reactions. Some of the products of the fission reaction are extremely radioactive.
• To ensure safety of all living things, the waste must be properly stored.
• It can take up to twenty half-lives for such radioactivity to reach levels safe enough for exposure.
• For some waste products this can be thousands of years.
Fusion
Fusion
• Fusion means to come together. • Nuclear fusion is the combining
of atomic nuclei. • In fusion reactions, scientists
bring together nuclei of atoms that have mass numbers less than 60.
Fusion
• This kind of a reaction also releases large amounts of energy.
• It useful to know, that the sun is powered by fusion reactions.
