Daniel L. Reger Scott R. Goode David W. Ball

http://academic.cengage.com/chemistry/reger

Chapter 21 Nuclear Chemistry

Review

• Most naturally occurring elements are mixtures of

isotopes

, which are represented by symbols of the form

Z A

X where

X

A

is the symbol of the element, = mass number, and

Z

= atomic number.

• •

Nuclide

is the nucleus of a specific isotope.

A

nucleon

is a proton or neutron.

Definitions

• • A

stable isotope

is one that does not spontaneously decompose into another nuclide.

An

radioactive nuclide

is one that spontaneously decomposes into another nuclide.

Stable Nuclide Characteristics

1 The number of neutrons is equal to or greater than the number of protons (except for 1 H and 3 He).

2 Up to

Z

= 20, the number of neutrons and protons are nearly equal; above 20 the ratio of n/p increases slowly to about 1.6:1.

3 Nuclear stability is greater for nuclides containing even numbers of protons, neutrons, or both.

Stable Nuclide Characteristics

4 Certain numbers of protons and neutrons (called

magic numbers

) confer unusual stability: 2, 8, 20, 26, 28, 50, 82, and 126.

5 The zone of stability contains all of the stable nuclides, but some nuclides in this band are unstable. 6 Tc (Z=43), Pm (Z=61), and all elements beyond Bi (

Z

=83) have no stable isotopes.

Radioactivity

• There are three major kinds of emissions from radioactive.

α particles are high-energy 4 He nuclei.

β particles are high-energy electrons that originate from the nucleus.

γ rays are very short wavelength (high energy) electromagnetic radiation.

Nuclear Equations

• A

nuclear equation

describes any process in which a nuclide undergoes change.

 In a balanced nuclear equation the sum of the mass numbers and atomic numbers on the reactant and product sides of the equation must be equal.

238 92 U  234 90 Th  2 4 

Z

: 92 = 90 + 2

A

: 238 = 234 + 4

Alpha Decay

• An alpha decay decreases the atomic number by 2 and the mass number by 4.

232 90 Th  228 99 Ra  4 2  • Alpha decay is seen in all elements heavier than bismuth (

Z

> 83).

Beta Decay

• Beta decay increases the atomic number by one, without changing the mass number.

14 6 C  14 7 N + 0 1  • The β particle does not exist in the nucleus, but is created at the instant of its emission.

• Beta emission is observed in nuclides that have too many neutrons to be stable.

Positron Emission

• • A

positron

is identical to an electron, except its charge is positive.

Positron emission decreases the atomic number by one, without changing the mass number.

40 19 K  40 18 Ar   0 1 β • • The symbol for the positron and beta particle is the same, except for the sign of

Z

.

Positron emission is seen in nuclides that have too many protons to be stable.

Electron Capture

• • • In

electron capture

an electron in a low energy orbital of the atom is capture by the nucleus and converts a proton to a neutron.

44 22 Ti   0 1 e  44 21 Sc X rays (

not

g rays) accompany electron capture, because the atom produced is in an excited electronic state.

• Electron capture and positron emission both decrease the atomic number by 1.

Both processes occur when the nuclide contains too many protons to be stable.

Predicting Modes of Decay

For radioactive elements • When

Z

> 83, an emission is often observed.

• If

A

> atomic mass decay occurs.

 • If

A

< atomic mass of element  0 1  decay or electron capture occurs.

Example Problem

• Predict the mode of decay, and write the nuclear equation for each radioactive nuclide: (a) 226 88

Ra

(b) 22 11

Na

(c) 130 I 53

Radioactive Series

• Among the heavier elements radioactive decay series are common.

Detecting Radiation

• • • • Radiation detection is based on the ionization caused by high energy particles and light and includes: Exposure of photographic film.

Geiger counters.

Scintillation counters.

Decay Rates

• Radioactive decays obey a first order rate law: rate =  

t N



kN

where N is the number of radioactive nuclei.

• Usually the half-life,

t

1/2 , is given rather than

k.

t

1/2  ln 2

k

 0 .

693

k

Nuclear Transmutation

• • A

nuclear transmutation

is a reaction in which two particles or nuclides produce nuclides that are different than the reactant species.

Two of the first nuclear transmutations observed were: 14 7 N  4 2   17 8 O  1 1 p 9 4 Be  4 2   12 6 C  0 1 n

Nuclear Reactions of Charged Particles • When both reactants in a nuclear transmutation are positively charged, very high energies are needed because of electrostatic repulsion.

• Devices used to produce these high energy particles include cyclotrons and linear accelerators.

Energy and Mass

• The energy equivalent of mass is calculated from

E

=

mc

2 where

E

is energy,

m

the speed of light.

is mass, and

c

is • When a nuclear change occurs a measurable difference in the mass of the products and reactants is observed.

Nuclear Binding Energy

•

Nuclear binding energy

is the amount of energy required to keep the protons and neutrons together in the nucleus.

• Some of the mass of the nucleons is converted into binding energy.

• The mass of a nuclide is always less than the masses of the neutrons and protons present.

Mass Defect

• The

mass defect

is the difference between the mass of the nucleons and the mass of the nuclide.

• The larger the mass defect, the greater is the nuclear binding energy.

Calculating Mass Defect

• The mass of a 7 Li atom is 7.016003 u. Calculate the mass defect (in u) given the following masses: 1 H 1.007825 u 1 o n 1.008665 u

Nuclear Binding Energy from Mass Defect • The 7 Li nuclide has a mass defect of 0.042132 u. Calculate the binding energy of this nuclide, in kJ/mol, using the equation D

E

= D

mc

2

Nuclear Fission

• •

Nuclear fission

forms two nuclei of roughly similar size from a single heavy nucleus.

Fission reactions release very large quantities of energy (“

exergonic”)

and produce several neutrons in addition to two nuclides.

236 92 U  141 56 Ba + 92 36 Kr + 3 1 0 n

Fission Yield Curve

• More than 370 nuclides, with

A

= 72 to 161, are found from the fission of 235 U.

Chain Reaction of

235

U

• 235 U absorbs a neutron which induces fission, and a chain reaction.

Critical Mass

• The

critical mass

is the minimum mass needed for a nuclear chain reaction to maintain a self-sustaining reaction.

Nuclear Power Reactor

• In a nuclear power reactor, conditions maintain criticality at a constant power level.

•

Nuclear Power and Safety

Long term storage of radioactive fission products and fear of disastrous accidents are the major deterrents to increased use of nuclear power.

• Current technology incorporates the radioactive waste in glass loaded into stainless steel containers, which are buried deep underground.

• Strict government regulations are enforced to assure the safe operation of reactors.

Reactor Designs

• •

Nuclear fusion

is the combination of two light nuclides to form a larger one.

The energy produced by the sun comes from fusion reactions, such as 1 1 H  1 1 H  2 1 H  0 1 n 

E

= -9.9

 10 7 kJ/mol 1 1 H  2 1 H  3 2 He 

E

= -5.2

 10 8 kJ/mol 3 2 He + 1 1 H  4 2 He +  0 1 β 

E

= -1.9

 10 9 kJ/mol

Reactor Designs

• • The possibility of fusion reactors is at least several decades away from reality.

An international consortium of the U.S., Japan, Russia, and the European Community are jointly designing a experimental thermonuclear power reactor.

Units of Radioactivity and Radiation Dose

Radioactivity

SI unit Common unit

Radiation Dose Name Abbrev Definition or conversion

Becquerel Bq Curie Ci 1 disintegration /second 3.7 10 10 disintegrations /second Quantity of radiation that transfers 1 10 -2 J of energy per kilogram of matter SI unit rad

Effective Radiation Dose

SI unit Common unit Sievert rem rad Sv rem 1 Sv = 100 rem 1 rem = 0.01 Sv

Most Radiation has a Natural Source

Radon

• • • 222 Ra is produced by the decay of natural 238 U found in rocks such as granite. Home radon test kits are sold because of the great public awareness and the potential dangers of radon accumulation. Since radon generally enters homes through the basement, one of the most effective means of eliminating the radiation danger is to add ventilation fans in the basement.

Nuclear Medicine

• • When a patient has an overactive thyroid gland, he or she is often given a dose of 131 I, which is a beta-emitter. Iodine concentrates in the thyroid gland, and the beta radiation from the 131 I isotope reduces the amount of hormone produced by the thyroid gland.

• • •

CT Scans

Radiopharmaceuticals are synthesized with isotopes that emit gamma radiation coupled to organic molecules that are taken up specifically in target organs.

Modern scanners use gamma ray cameras that take measurements at thousands of different locations.

Computerized tomography is often abbreviated CT and referred to as a CT scan.

• • • •

Gamma Radiation Scans -

99m

Tc

Technicium is a transition metal with a variety of stable oxidation states that makes it an excellent species to use to react with organic molecules. Heart imaging is done with a compound formed by technicium(I) and six isonitrile ligands.

Other compounds have been developed for mapping the brain, lungs, etc. Over 100 different nuclear medicine diagnoses are available to clinicians.

Positron Emission Tomography (PET) Scans • • • A fluorinated sugar (fluorodeoxyglucose) that contains 18 F is often used.

This sugar is taken up most by the organs that are subdividing most quickly. And cancer cells are among the fastest.

PET scans are particularly effective in diagnosing lung, head and neck, colorectal, esophageal, lymphoma, melanoma, breast, thyroid, cervical, pancreatic, and brain cancers.