Nivaldo J. Tro

http://www.cengage.com/chemistry/tro

Mark Erickson • Hartwick College

Chapter 8Nuclear Chemistry

Radiation• The emission of energetic

particles

• The study of radiation and the processes that produce it is called nuclear chemistry.

• Unlike the chemistry we have studied to this point, nuclear chemistry often results in one element changing into another one.

Tragedy• April 26, 1986, 1:24 am

• V.I. Lenin nuclear power plant

• Chernobyl, USSR

• Explosions in reactor 4

• 31 immediate deaths, 230 hospitalizations, countless exposures to high-level radiation

• The aftermath continues to this day.

Chemistry• Chemistry as studied up to this point

– Atomic and molecular changes involve electrons.– Atoms react to achieve a stable octet electron

configuration.

• Nuclear chemistry– Atomic changes involve the nuclei.– Nuclei emit energetic particles we call radiation.

Becquerel• Discovered that his

paper-wrapped photographic plate was exposed by uranium-containing crystals.

• This disproved his hypothesis linking exposure to UV light with phosphorescence.

• But it revealed a brand-new phenomenon that he called the emission of uranic rays.

Marie and Pierre Curie• Searched for the elements that produced the uranic rays• Discovered two new emitters of uranic rays; one was a new

element (polonium)• Radioactivity not the result of a chemical reaction• Since the rays were not unique to uranium, a new term was

proposed: “radioactivity”• Discovered radium as a result of its “extreme radioactivity”

Radioactivity• Characterized by Rutherford• The result of nuclear instability

Alpha Radiation• Composed of particles consisting of two protons and

two neutrons

• Represented by the symbol for a helium nucleus

• High ionizing power

• Low penetrating power

Writing Nuclear Reaction Equations

• Identify the type of nuclear reaction and the particle(s) involved.

• The sum of mass numbers and the sum of the atomic numbers must balance on both sides.

atomic numbers on each side must balance:

atomic mass numbers on each side must balance:

2 + 90 = 92

4 + 234 = 238

daughter nucleus

92238U

24He +

90234 Th

Concept Check 8.1

• Identify the daughter nucleus from the alpha decay of I-131

Concept Check 8.1 Solution

Alpha decay of I-131 results in the loss of a helium nucleus and the formation of Sb-127 as the daughter nucleus.

53131I

24He +

51127Sb

atomic numbers on each side must balance:

atomic mass numbers on each side must balance:

51 + 2 = 53

127 + 4 = 131

daughter nucleus

Beta Radiation• Composed of particles consisting of energetic electrons

represented by the symbol β.

• Smaller than alpha particles, so more penetrating

• But this also means less ionizing power

• In beta decay, a neutron converts to a proton, emitting an electron and increasing the atomic number by 1.

Concept Check 8.2

• Identify the daughter nucleus from the beta decay of C-14

Concept Check 8.2 Solution

• Beta decay of C-14 involves the loss of an electron from the nucleus. Like alpha decay, the atomic numbers and atomic mass numbers from each side must balance.

614C

-10e +

714N

0 + 14 = 14

-1 + 7 = 6 daughter nucleus

Gamma Radiation• An energetic photon

emitted by an atomic nucleus

• Represented by the symbol

• Gamma rays are electromagnetic radiation, not matter.

• Highest penetrating power, lowest ionizing power

Concept Check 8.3

Considering the three type of radiation, alpha, beta, and gamma,

a)Order them according to increasing penetrating power.

b)Order them in according to increasing ionizing power.

Concept Check 8.3 Solution

Alpha particles are large and carry a 2+ charge, therefore do not penetrate very far but have tremendous ionizing power. Beta particles are high energy electrons with 1− charge, therefore smaller and have greater penetrating power and moderate ionizing ability. Gamma rays are electromagnetic radiation and not matter, therefore, have no mass or charge and possess great penetrating power but low ionizing ability.

a) Penetrating power: alpha < beta < gamma

b) Ionizing power: gamma < beta < alpha

Half-Life• The time required for half

of the nuclei in a sample to decay

Concept Check 8.4• Radon-222 decays via alpha emission to Po-218 with a half-

life of 3.82 days. If a house initially contains 800. mg of radon-222, and no new radon enters the house, how much will be left in 15.3 days? How many alpha emissions would have occurred within the house?

Concept Check 8.4a Solution• Initial amount of radon-222 (half-life of 3.82 days): 800. mg

(a) How much will be left after 15.3 days?(b) How many alpha emissions occurred?

• First determine how many half-lives have passed by dividing the total time passed by the half-life:

15.3 days

3.82 days /half-life = 4.00 half-lives

800. mg 1st half-life 400. mg

400. mg 2nd half-life 200. mg

200. mg 3rd half-life 100. mg

100. mg 4th half-life 50.0 mg left at the end of 15.3 days

Concept Check 8.4b Solution(b) How many alpha emissions occurred in 15.3 days?

• 800. mg – 50.0 mg (left) = 750. mg of radon-222 decayed.

• Each decay of an atom of radon-222 is an alpha decay.

750. mg Rn 1 g Rn

1000 mg Rn

1 mol Rn

222 g Rn

6.02 1023 atoms Rn

mol Rn

= 2.03 1021 atoms Rn decayed and also the number of alpha emissions

Nuclear Fission• General idea: If nuclei emit

particles to form lighter elements, they might also absorb particles to form heavier elements.

• The result would be a synthetic element.

Nuclear Fission• Fermi hoped to make a synthetic element with atomic

number 93.

• He detected beta emission following his neutron bombardment of uranium.

• Subsequent experiments by Hahn, Meitner, and Strassman seemed to confirm Fermi’s work.

Nuclear Fission• Just before the outbreak of WWII,

Hahn, Meitner, and Strassman reported that no heavier element was detected; rather two lighter elements were formed.

• Previous nuclear processes had always been incremental.

• Contradicting all previous experiments in nuclear physics, they proposed a model for the fission of uranium atoms based on absorption of neutrons.

• Large amounts of energy were also emitted during fission.

Nuclear Fission• Weeks later, U-235 fission was proposed as the basis for

both a chain reaction and a bomb of inconceivable power.

Enrico Fermi and Leo Szilard• Enrico Fermi and Leo

Szilard constructed the first nuclear reactor at the University of Chicago; they achieved a self-sustaining, controlled fission reaction lasting 4.5 minutes.

The Manhattan Project• The largest scientific endeavor of its time, the race to beat

Germany to the atomic bomb was code-named “Manhattan Project.”

• Collection and synthesis of fissionable fuel (U-235 and Pu-239) were pursued at Oak Ridge, TN and Hanford, WA. J. Robert Oppenheimer directed bomb design at Los Alamos, NM.

Critical Mass: Fissionable Fuel• Lesser masses of fissionable material will not undergo self-

sustaining fission; too many neutrons are lost to the surroundings instead of being absorbed by other U-235 nuclei.

• After the successful controlled reaction, the goal became the construction of a device where fission would spiral out of control.

Atomic Bomb: Fat Man and Little Boy

• Two designs were constructed and a successful test carried out on July 16, 1945. Two atomic bombs (one uranium and one plutonium) were dropped on Japan only weeks later.

• Little Boy– Uranium– Cannon-like barrel

• Fat Man– Plutonium– Squeezed by implosion

Nuclear Power• Nuclear reactors are

designed to produce a controlled fission reaction.– Uranium rods are

interspersed with control rods of neutron-absorbing material, usually boron or cadmium.

• Heat of fission boils water to produce steam, which turns the turbine to produce electricity.

Nuclear vs. Coal-burning Power Plants

• Nuclear

– Uses 100 lb. of fuel per day

– Produces enough electricity for a city of 1 million people

– Does not produce air pollution, greenhouse gases, or acid rain

– Problems include waste disposal and accidents

• Coal-burning– Uses 5 million lb. of

fuel to produce an equivalent amount of energy

Waste Disposal• Uranium oxide pellet fuel

assemblies are replaced with fresh fuel every 18 months.

• Most spent fuel is currently stored on-site.

• 1982 Nuclear Waste Policy Act– Established a program to

build an underground nuclear waste repository

• Yucca Mountain, NV is the controversial site of this much-delayed project.

Nuclear Accidents• Nuclear power plants cannot detonate like nuclear explosions.

– Enriched uranium at 3% U-235 vs. 90% U-235

• Three Mile Island: March 28, 1979

• Chernobyl: April 26, 1986

• Fukushima Daiichi Nuclear Power Plant, March 11, 2011

• Superior power plant design in the U.S. has meant no accidental nuclear deaths; nevertheless public support for nuclear power is chilly.

Mass Defect• Mass defect is the difference between the experimentally

measured mass of an atom and the sum of the masses of individually measured protons, neutrons, and electrons.

• The missing mass was converted to energy when elements form from constituent protons and neutrons.

• This energy is related to the mass defect by Einstein’s equation E = mc2.

Nuclear Binding Energy• Einstein’s equation E = mc2 represents the energy

that holds a nucleus together.

• The highest values for this binding energy are for elements with mass numbers close to 56.

Nuclear Binding Energy

Nuclear Binding Energy: Fission

• The products have higher binding energy than the reactants; it follows that the products weigh less.

• The missing mass is converted to energy according to

E = mc2

• This difference in binding energy is the source of the energy liberated in fission.

Fusion• In fusion, the nuclei of lighter elements are fused into heavier ones.

• Like fission reactions, the products of fusion have higher nuclear binding energies, so energy is released.

• Fusion releases ten times more energy per gram than fission.

• Fusion is responsible for the sun’s energy and is the basis of modern nuclear weapons.

Controlled Fusion• Advantages

– Potential for an almost limitless source of energy for society

– Less radioactive waste products– Naturally occurring deuterium in water is a reactant and

abundant.

• Disadvantages/obstacles– High temperatures required and a lack of materials

available for containment– Current production methods consume more power than

they produce.

Radiation and Human Life• Radiation can destroy biological molecules.

• Low-level alpha emitters present little danger externally, but once ingested have access to internal organs.

• Danger is usually overstated by the popular press.

• rem: most common unit for measuring human exposure

• Exposure, on average, per year, is 1/3 rem.

Possible Effects• The human body can repair itself

and suffer no adverse effects.

• Abnormal growth can begin that leads to cancerous tumors.

• Damage of intestinal lining leads to radiation sickness, hampering the intake of nutrients and water.

• Damage to the immune system allows infection to go unchecked.

• Genetic defects in offspring have occurred in laboratory animals.

Radon• Radon is the major source of human radiation exposure.

• Naturally occurring uranium deposits in the earth lead to the collection of radon in residential basements.

• Significance of radon as a health threat is controversial.

Carbon Dating• Carbon-14 is created in the

upper atmosphere and becomes incorporated in living tissue at a constant level, then drops after death.

• The half-life of C-14 is 5730 years.

• Levels of C-14 in carbon-based artifacts are compared to modern levels as an age signature.

• The Shroud of Turin analysis determined the age of the materials in the shroud.

Concept Check 8.5

• A fossil has a carbon-14 content that is 12% of that found in living organisms. Estimate the age of the fossil.

Concept Check 8.5 Solution

• A fossil has a carbon-14 content that is 12% of that found in living organisms.

• After 3 half-lives (17190 years), 12.5% of the C-14 in the sample remains, therefore the sample is slightly older than 17190 years old.

100% 1st half-life 50% 5730 years

50% 2nd half-life 25% 11460 years

25% 3rd half-life 12.5% 17190 years

The Age of the Earth• U-238 is used to measure longer periods of time.

• It decays to lead with a half-life of 4.5 × 109 years.

• Lead levels in artifacts are used as an age signature.

• Uranium to lead dating

Concept Check 8.6

• A moon rock is found to contain 50% uranium and 50% lead. How old is the moon rock?

Concept Check 8.6 Solution

• A moon rock is found to contain 50% uranium and 50% lead. How old is the moon rock?

• After 1 half-life, the original amount of uranium will be reduced to 50%, therefore, the rock is about 4.5 million years old.

100% 1st half-life 50% 4.5 109 years

50% 2nd half-life 25% 9.0 109 years

25% 3rd half-life 12.5% 1.35 109 years

Nuclear Medicine• Diagnosis

– Radioactive elements concentrate in specific areas of interest in the body.

– Gamma emitters will expose photographic film, allowing images of organs to be recorded.

• Therapy– Radiation can destroy

cancerous tumors.– Minimizing exposure of healthy

tissue is a challenge.

Chapter Summary

Molecular Concept

• Radioactivity

• Alpha radiation

• Beta radiation

• Gamma radiation

• Half-life

• Fission

• Carbon dating

Societal Impact

• The discovery of radiation ultimately led to the creation of the first atomic bomb.

• Nuclear power is used widely throughout the United States.