Nuclear Chemistry

Nine Mile

Oswego, NY

Radioisotope – an isotope that is Radioisotope – an isotope that is radioactiveradioactive Example: Carbon-14Example: Carbon-14

Radioactive isotopes can be naturally Radioactive isotopes can be naturally occurring, or they can be produced by occurring, or they can be produced by bombarding stable isotopes with high bombarding stable isotopes with high speed particlesspeed particles

StabilityStability

All nuclei with atomic numbers greater All nuclei with atomic numbers greater than 83 are unstablethan 83 are unstable They are all radioactiveThey are all radioactive

Stability is also dependant upon the ratio Stability is also dependant upon the ratio of protons to neutronsof protons to neutrons The closer an isotope is to a 1:1 ratio the The closer an isotope is to a 1:1 ratio the

more stable it ismore stable it is

Transmutation Any change in the nucleus, which causes

the element to change into a new element (change of atomic number)

Can occur naturally or artifically

Natural Transmutation Occurs naturally Single nucleus undergoes decay

Example: 3719K → 37

18Ar + 0+1e

Artificial Transmutation If the change is brought about by

bombarding the nuclei by high energy particles

Two reactants – a fast moving particle and the target material

Example: 3216S + 1

0n→ 3215P + 1

1H

Equations Mass must be conserved Atomic mass and atomic number must be

the same on both sides of the equation

Remember 4

2He 4 = superscript = mass number (atomic mass) =

protons + neutrons 2 = subscript = atomic number = protons

Equation Examples

1. What is X? 6

3Li + 10n → 4

2He + X

2. What is X? 14

6C → X + 0-1e

Types of Radiation – Table O

Alpha particles – helium nucleus, 2 protons, 2 neutrons

Beta particles – an electron, negative charge, no mass

Positron – electron with a positive charge, no mass

Gamma radiation (γ) – similar, but more energy than X-rays, no mass, no charge

Particle Mass (amu)

Charge Symbol Penetrating Power

Shielding

Alpha 4 +2 α, 42He Low Paper,

clothing

Beta 0 -1 β-, 0-1e Moderate Metal foil

Positron 0 +1 β+, 0+1e Moderate Metal foil

Gamma 0 0 γ High Lead, concrete

Charges of Decay Particles

•Negative particles will be attracted to positive charges

•Positive charges will be attracted to negative charges

•Non charged particles are not affected by charges

Alpha Decay – unstable nucleus emits an alpha particleExample: 226

88Ra → 22286Rn + 4

2He Beta Decay – unstable nucleus emits a

beta particleExample: 214

82Pb → 21483Bi + 0

-1e Positron Emission – unstable nucleus

emits a positronExample: 37

19K → 3718Ar + 0

+1e

Conversion of Mass to EnergyConversion of Mass to Energy

E = mcE = mc22

– E = energy (J)E = energy (J)– m = mass (kg)m = mass (kg)– c = velocity (speed) of light = 3.0x10c = velocity (speed) of light = 3.0x1088m/sm/s

Example: How many joules of energy are Example: How many joules of energy are released if 1.0g is converted to energy? released if 1.0g is converted to energy?

Mass DefectMass DefectThe actual atomic mass of an atom is less The actual atomic mass of an atom is less than what we would predict based upon the than what we would predict based upon the mass of individual protons and neutronsmass of individual protons and neutrons

The difference is because energy is The difference is because energy is released when the protons and neutrons released when the protons and neutrons combinecombine

The larger the mass defect, the more The larger the mass defect, the more energy is released upon formation, and the energy is released upon formation, and the more stable the particle ismore stable the particle is

Examples

1. Calculate the predicted mass of He-4. 1 proton = 1.00728, 1 neutron = 1.00867

• The actual mass of He-4 = 4.00150

• The mass defect is =

2. Convert the mass defect to energy (using E = mc2)

FissionFission

Splitting of a heavy nucleus to produce lighter Splitting of a heavy nucleus to produce lighter nucleinuclei

Nuclear Power PlantsNuclear Power Plants Neutron joins with a nucleus of a heavy Neutron joins with a nucleus of a heavy

elementelement Intermediate product is very unstableIntermediate product is very unstable Splits apart producingSplits apart producing

Two mid weight nucleiTwo mid weight nuclei At least one neutronAt least one neutron A great amount of energyA great amount of energy

Fission

• Example: 235

92U + 10n 92

36Kr + 14156Ba + 3 1

0n + energy

• The three neutrons given off can be reabsorbed by other U-235 nuclei to continue fission as a chain reaction

• A tiny bit of mass is lost (mass defect) and converted into a huge amount of energy

• See Fission

Chain Reaction The neutrons that are emitted can become

reactants causing more nuclei to undergo fission and release more energy

The reaction can be controlled by limiting the number of interactions between neutrons and nuclei

Nuclear Power Plants

Main Components Fuel – Uranium or Plutonium Control Rods - absorb neutrons to control the rate

of the reaction Containment Structure – building that houses the

reactor

Main Components Coolant – Water, cools

the reaction Cooling Tower – cools

the discharge water, releases water vapor

Nuclear Power Advantages:

Cleaner than conventional fossil fuels – no greenhouse gases or acid rain

More efficient, cleaner Disadvantages:

Many of the bi-products of the reactions are radioactive (unstable) and have long half-lives, making the storage and disposal of these wastes dangerous

FusionFusion

Combining of light nuclei Combining of light nuclei

to produce a heavier nucleusto produce a heavier nucleus The Sun, Hydrogen BombThe Sun, Hydrogen Bomb

Example: Example: 2211H + H + 22

11H H → → 4422He + energyHe + energy

See FusionSee Fusion

FusionFusion

AdvantagesAdvantages Products are not highly radioactiveProducts are not highly radioactive Produces a lot of energyProduces a lot of energy

Disadvantages Disadvantages Requires extremely high temperatures and Requires extremely high temperatures and

pressures, therefore not yet available to pressures, therefore not yet available to produce energy on Earthproduce energy on Earth

Half-Life

Time it takes for half of the atoms in a given sample of an isotope to decay

Each isotope has its own half-life (Table N) The shorter the half-life of an isotope, the less

stable it is Half-life is a constant factor, it is not affected

by temperature or pressure Geiger counter can be used to record the

decay of an isotope

•Use Table N for Half-Life and Decay Modes

1. Calculate the mass of I-131 that 1. Calculate the mass of I-131 that remains after 32.28 days, if the remains after 32.28 days, if the mass of the original sample was mass of the original sample was 100.0g. 100.0g.

2. If 50.0g of a radioactive isotope 2. If 50.0g of a radioactive isotope decays to 6.25g in 60.0 days, what decays to 6.25g in 60.0 days, what is the isotope’s half-life? is the isotope’s half-life?

3.3. What fraction of a phosphorus-32 What fraction of a phosphorus-32 sample will remain after 28.6 days? sample will remain after 28.6 days?

4.4. 50.0g of cobalt-60 decays for 21 50.0g of cobalt-60 decays for 21 years. How many grams remain years. How many grams remain after this time? after this time?

5.5. After 14.4 seconds, 3.00g of After 14.4 seconds, 3.00g of nitrogen-16 remains. What was the nitrogen-16 remains. What was the mass of the original nitrogen-16 mass of the original nitrogen-16 sample? sample?

DatingDating

Each radioactive substance is presently Each radioactive substance is presently decaying at the same rate as when the decaying at the same rate as when the substance substance

By comparing the amount of By comparing the amount of 1414C that C that remains to the amount of remains to the amount of 1212C that is C that is present, the amount of present, the amount of 1414C (and the age) C (and the age) can be calculatedcan be calculated

Chemical TracersChemical Tracers

A radioisotope is used to follow the path of A radioisotope is used to follow the path of a chemical processa chemical process

Example: C-14 is used to follow the path of Example: C-14 is used to follow the path of carbon in organic reactionscarbon in organic reactions

Medical Applications

• Some radioisotopes have the ability to kill living tissue

• Any radioisotope used in medicine must have a short half-life, making it quickly eliminated by the body

Medical Examples• Cancer: Cobalt-60 emits large amounts of

gamma radiation which can be used to kill tumor cells

• Thyroid: Iodine-131 is used in the detection and treatment of thyroid conditions

• Gamma Radiation: meats are irradiated to kill bacteria, producing a longer shelf life

• Anthrax: Cobalt-60 and Cesium-137 are two sources of gamma radiation that can be used to destroy anthrax

Radiation Risks Can damage normal cells High doses can cause illness, death Can cause mutations that can be passed onto

offspring