Chapter 4

Nuclear Chemistry: The Heart of Matter

Radioisotopes

• Radioactive decay

– Many isotopes are unstable

– Nuclei that undergo radioactive decay

– May produce one or more types of radiation

Natural Radioactivity

• Background radiation– What occurs from

natural sources– Accounts for >80%

of radioactivity exposure

Types of Radiation

• Ionizing radiation – knocks electrons out of

atoms or groups of atoms– Produces charged species – ions– Charged species that cause damage

Alpha Decay

• Nucleus loses particle– Mass decreases by 4 and atomic number decreases by 2

He42

Beta Decay• Nucleus loses particle

– No change in mass but atomic number increases

e01

Positron Emission• Loses a positron

– Equal mass but opposite charge of an electron– Decrease in atomic number and no change in mass +

Electron Capture• Nucleus absorbs an

electron and then releases an X-ray

• Mass number stays the same and atomic number decreases

Gamma Radiation

• Release of high-energy photon

• Typically occurs after another radioactive decay• No change in mass number or atomic number

Nuclear Equations

• Elements may change in nuclear reactions• Total mass and sum of atomic numbers must be

the same• MUST specify isotope

Po He Rn 21884

42

22286

Differences Between Chemical and Nuclear Reactions

Example 4.1  Balancing Nuclear Equations

Write balanced nuclear equations for each of the following processes. In each case, indicate what new element is formed.a. Plutonium-239 emits an alpha particle when it decays.b. Protactinium-234 undergoes beta decay.c. Carbon-11 emits a positron when it decays.d. Carbon-11 undergoes electron capture.

Solutiona. We start by writing the symbol for plutonium-239 and a partial equation showing that one of the products is an alpha particle (helium nucleus):

23994

Pu42

He + ?

Mass and charge are conserved. The new element must have a mass of 239 – 4 = 235 and a charge of 94 – 2 = 92. The nuclear charge (atomic number) of 92 identifies the element as uranium (U):

23994

Pu42

He +235

92U

Half-Life

• Period for one-half of the original elements to undergo radioactive decay

• Characteristic for each isotope

• Fraction remaining =

n = number of half-lives

n2

1

You obtain a new sample of cobalt-60, half-life 5.25 years, with a mass of 400 mg. How much cobalt-60 remains after 15.75 years (three half-lives)?

Example 4.2  Half-Lives

SolutionThe fraction remaining after three half-lives is

1

2n

1

23

1

2 x 2 x 2

1

8===

The amount of cobalt-60 remaining is ( ) (400 mg) = 50 mg.1

8

You have 1.224 mg of freshly prepared gold-189, half-life 30 min. How much of the gold-189 sample remains after five half-lives?

Exercise 4.2A

What percentage of the original radioactivity remains after five half-lives?

Exercise 4.2B

You obtain a 20.0-mg sample of mercury-190, half-life 20 min. How much of the mercury-190 sample remains after 2 hr?

Example 4.3  

There are 120 min in 2 hr. There are ( ) = 6 half-lives in 2 hr. The fraction remainingafter six half-lives is

The amount of mercury-190 remaining is ( ) (20.0 mg) = 0.313 mg.

Solution

1

2n

1

26

1

2 x 2 x 2 x 2 x 2 x 2

1

64===

1

64

12020

A sample of 16.0 mg of nickel-57, half-life 36.0 hr, is produced in a nuclear reactor. How much of the nickel-57 sample remains after 7.5 days?

Exercise 4.3A

Tc-99 decays to Ru-99 with a half-life of 210,000 years. Starting with 1.0 mg of Tc-99, how long will it take for 0.75 mg of Ru-99 to form?

Exercise 4.3B

Radioisotopic Dating• Use certain isotopes to estimate the age of various

items• 235U half-life = 4.5 billion years

– Determine age of rock• 3H half-life = 12.3 years

– Used to date aged wines

Carbon-14 Dating• 98.9% 12C• Produce 14C in upper atmosphere• Half-life of 5730 years• ~50,000 y maximum age for dating

H Cn N 11

146

10

147

A piece of fossilized wood has carbon-14 activity one-eighth that of new wood. How old is the artifact? The half-life of carbon-14 is 5730 years.

Example 4.4  

SolutionThe carbon-14 has gone through three half-lives:

It is therefore about 3 x 5730 = 17,190 years old.

1

8==

1

2

1

2x

1

2x( )1

2

3

How old is a piece of cloth that has carbon-14 activity that of new cloth fibers? The half-life of carbon-14 is 5730 years.

Exercise 4.41

16

Shroud of Turin• Alleged burial shroud of Jesus Christ

– Contains faint human likeness– First documented in Middle Ages

• Carbon-14 dating done in 1988– Three separate labs– Shroud ~800 years old– Unlikely to be burial shroud

Artificial Transmutation• Transmutation changes one element into another

– Middle Ages: change lead to gold• In 1919 Rutherford established protons as fundamental particles

– Basic building blocks of nuclei

H O He N 11

178

42

147

Uses of Radioisotopes• Tracers

– Easy to detect– Different isotopes have similar chemical and physical properties– Physical, chemical, or biological processes

• Agriculture– Induce heritable genetic alterations – mutations– Preservative

– Destroys microorganisms with little change to taste or appearance of the food

Nuclear Medicine• Used for two purposes• Therapeutic – treat or cure disease using radiation• Diagnostic – obtain information about patient’s health

Radiation Therapy• Radiation most lethal to dividing cells• Makes some forms of cancer susceptible• Try to destroy cancer cells before too much damage to healthy cells

– Direct radiation at cancer cells– Gives rise to side effects

Diagnostic Uses

• Many different isotopes used– See Table 4.6

• Can measure specific things– Iodine-131 to locate tumors in thyroid– Selenium-75 to look at pancreas– Gadolinium-153 to determine bone mineralization

Imaging• Positron emission tomography (PET)• Uses an isotope that emits a positron• Observe amount of radiation released

e B C 01

115

116

Penetrating Power of Radiation• The more mass the particle has, the less penetrating it is• The faster the particle is, the more penetrating it is

Prevent Radiation Damage• To minimize

damage – Stay a distance

from radioactive sources

– Use shielding; need more with more penetrating forms of radiation

Example 4.6

One of the isotopes used for PET scans is oxygen-15, a positron emitter. What new element is formed when oxygen-15 decays?

Phosphorus-30 is a positron-emitting radioisotope suitable for use in PET scans. What new element is formed when phosphorus-30 decays?

Exercise 4.6

SolutionFirst write the nuclear equation

0+1

e + ?15

8O

The nucleon number does not change, but the atomic number becomes 8 – 1, or 7; and sothe new product is nitrogen-15:

0+1

e +15

8O

157

N

Energy from Nucleus

• E = mc2

• Lose mass, gain energy– For chemical

reactions, mass changes are not measurable

– For nuclear reactions, mass changes may be measurable

Binding Energy• Holds protons and neutrons together in

the nucleus

• The higher the binding energy, the more stable the element

Nuclear Fission• “Splitting the atom”

• Break a large nucleus into smaller nuclei

Nuclear Chain Reaction• Neutrons from one

fission event split further atoms

• Only certain isotopes, fissile isotopes, undergo nuclear chain reactions

Manhattan Project• How to sustain the nuclear reaction?• How to enrich uranium to >90% 235U?

– Only 0.7% natural abundance• How to make 239Pu (another fissile isotope)?• How to make a nuclear fission bomb?

Radioactive Fallout

• Nuclear bomb detonated; radioactive materials may rain down miles away and days later– Some may be unreacted U or Pu– Radioactive isotopes produced during the explosion

Nuclear Power Plants• Provide ~20% U.S. electricity

– France >70% • Slow controlled release of energy• Need 2.5–3.5% 235U• Problem with disposal of radioactive waste

Nuclear Fusion

• Reaction takes smaller nuclei and builds larger ones– Also called thermonuclear reactions

e2 He H4 01

42

11

• Releases tremendous amounts of energy

–1 g of H would release same as 20 tons of coal