Nuclear Chemistry

Bettelheim, Brown, Campbell and Farrell

Chapter 9

Nuclear Chemistry

• Background

• Types of radiation

• Nuclear Equations

• Half-Lives

• Units

• Uses– Medical– Other

Nuclear Reactions

• Involve changes in nuclei– Protons and Neutrons----NOT Electrons

Nuclear Chemistry- the study of the

properties and reactions of atomic nuclei

IntroductionNuclear radiation:Nuclear radiation: radiation emitted from a nucleus during nuclear decay– Results from an unstable nuclei– alpha particle (alpha particle ():): a helium nucleus, He2+; contains

two protons and two neutrons, has mass of 4 amu, and atomic number 2

– beta particle (beta particle ():): an electron; has a charge of -1, and a mass of 0.00055 amu

– gamma ray (gamma ray ():): high-energy electromagnetic radiation;

– positron (positron ():): has the mass of an electron but a charge of +1+1

Representing Isotopes12C6

14C6

14N + 0e7 -1

Example of a nuclear equation

Nuclear Radiation

• There are more than 300 naturally occurring isotopes– 264 are stable

• More than 1000 artificial isotopes have been made in the laboratory; all are radioactive

Alpha Emission

– in alpha emission, the new element formed has an atomic number two units lower and a mass number four units lower

24

92238 +Th90

234U He

24

84210 +Pb82

206Po +He

Beta Emission

• beta emission:beta emission: decomposition of a neutron to a proton and an electron

• emission of a beta particle transforms the element into a new element with the same mass number but an atomic number one unit greater

• Problem:Problem: carbon-14 is a beta emitter. When it undergoes beta emission, into what element is it converted?

e-10n0

1 H11 +

e-10

1532 S16

32 +P

Positron Emission

• positron emission:positron emission: decomposition of a proton in the nucleus to a give a “positive electron” (and a neutron)– in positron emission, the new element formed

has an atomic number one unit lower but the same mass number

e+10

611 +B5

11C

Electron Capture

electron capture:electron capture: electron near the nucleus is “captured” and combines with a proton to form a neutron)– in electron capture, the new element

formed has an atomic number one unit lower but the same mass number

12553I + 0

-1e → 12552Te

Gamma Emission

• In pure gamma emission, there is no change in either the atomic number or the mass number of the element– a nucleus in a higher-energy state emits

gamma radiation as it returns to its ground state (its most stable energy state)

– Usually accompanies or emission

611m +B 6

11B

60Co27

Half-Life• half-life of a radioisotope, thalf-life of a radioisotope, t1/21/2:: the time it

takes one half of a sample of a radioisotope to decay

Half-Life = 8 days

Amount of radioactive material left is given by

Rt = (1/2n)Ri

where Ri is initial amount of radioactivity, Rt is the amount of radioactivity at time t, and n is the number of half-lives

Amount left

• If you start with 50 Curies of P-32, how much is left after 28.6 days? (t½ = 14.3 days)

Hydrogen-3 (tritium)Carbon-14Phosphorus-28Phosphorus-32Potassium-40Scandium-42Cobalt-60Strontium-90Technetium-99m

Indium-116

Iodine-131Mercury-197

Radon-205Radon-222Uranium-235

Name Half-life Radiation

12.26 y5730 y0.28 s14.3 d

1.28 x 109 y0.68 s5.2 y28.1 y6.0 h

14 s

8 d65 h

Polonium-210 138 d2.8 m3.8 d

4 x 109 y

BetaBetaPositronBetaBeta + gammaPositronGammaBetaGamma

Beta

Beta + gammaGammaAlphaAlphaAlphaAlpha

Two factors determine how dangerous different kinds of

radiation are

• Ionizing power: Ability to cause damage

• Penetrating power: How far radiation will travel into the body

Ionizing Power

• Ionizing power is the ability to knock off electrons and thus cause damage

• Alpha particles have highest ionizing power

• Beta particles have moderate ionizing power

• Gamma rays have least ionizing power

Ability to penetrate sample

Comparison of Radiation Types

Radiation Ionizing Power

(do damage)

How far will it penetrate?

Alpha High Stopped by piece of paper

Beta Medium Stopped by thin sheet of metal or plexiglass

Gamma Low Pass through tissue easily

Radiation Dosimetry– Curie (Ci)Curie (Ci) or millicurie (mCi): or millicurie (mCi): measure of the

number of radioactive disintegrations occurring each second in a sample. (1Ci = 3.7 x 1010 dps)

– Roentgen (R):Roentgen (R): amount of radiation delivered by a radiation source

– Radiation absorbed dose (Rad):Radiation absorbed dose (Rad): a unit for measuring the energy absorbed per g of material exposed to a radiation source

– Roentgen-equivalent-man (Rem):Roentgen-equivalent-man (Rem): measures the tissue damage caused by radiation

• Preferred for medical purposes

Radiation DosimetryAverage exposure to radiation from common sources

Naturally Occurring RadiationDose (mrem/y)

Cosmic raysTerrestrial radiation (rocks, buildings)Inside human body (K-40 and Ra-226 in bones)Radon in the airTotal

Artificial RadiationMedical x-raysNuclear medicineConsumer productsNuclear power plantsAll othersTotal

272839

200294

3914100.51.5

65.0

Source

Measuring Devices

• Film Badge

• Geiger-Mueller Counter

• Scintillation counters

Geiger-Müller Counter

Geiger-Müller Counter

Measurement of Radioactivity and Radioactive Exposure

Curie: amount of radioactivity which gives 3.7 x 1010 dps

dps = disintegrations per second

Disintegration = decay of a single atom

Measurement of Radioactive Exposure

Roentgen = amount of radiation that produces ions which have 2.56 x 10-4 coulombs/kg

Radiation absorbed dose (Rad) =Radiation absorbed dose (Rad) = energy absorbed per gram of material exposed to a radiation source

REM = Roentgen Equivalent Man

Rem is measure of the effect of radiation when one Roentgen is absorbed

Medical/Research Uses

• Experimental Tracers– Basic biochemical and medical research

• Diagnostic Uses– Organ scans involving preferential uptake

of isotopes• I-131 concentrates in thyroxine in thyroid gland

– Medical Imaging• PET Scan – positron and electron →2 gamma rays• MRI—imaging of soft tissue, such as brain, spinal

cord

Medical/Research Uses

• Radiation Therapy– Aim high energy radiation at cancer cells

• Radiation affects rapidly growing cells more• Cobalt-60 often used for brain tumors• Actinium- 225 attached to monoclonal antibody

targets prostate cancer (binds to PSA on cell surface)

Fission and Fusion

• FusionCombining smaller nuclei to form a

larger nucleus

• Fission “Splitting” of a larger nucleus to

form smaller nuclei

• Energy is released in both fusion and fission

Nuclear Fission

• “Split” a larger nucleus into smaller nuclei.

• Used in nuclear power plants

• Used in the Atomic bomb

• Energy is released.

Chain Reaction

Chain reaction:

Self-sustaining reaction in which theproducts of one reaction event initiateanother reaction event

Critical Mass:

Minimum amount of an isotope needed to sustain a chain reaction

Chain Reaction

Nuclear Power Plants

• Utilize “controlled” Fission Reaction• Fuel rods contain radioactive material• Moderator slows speed of neutrons (water

or graphite)• Control rods contain neutron-absorbing

material such as Cd or B • Control rods can be raised or lowered to

control the number of neutrons available for the reaction

Nuclear Power Plant

Nuclear Reactor

• Control rods can absorb neutrons

• Control rods can be lowered to absorb more neutrons (slow reaction)

• Control rods can be raised to absorb fewer neutrons (increase reaction)

Use of Nuclear Power

% of total electricity

France 75

Sweden 47

Europe 40-50

US 20

Canada 13

Comparison of Nuclear and Fossil Fuel Power Plants

• Nuclear:

Radioactive material NOT connected to outside world

Does NOT pollute air

• Fossil Fuel: Smoke stack open to air

DOES emit air pollutants

Major Problem

• Fuel rods need to be replaced periodically

• Disposal of “spent” rods--nuclear waste

• Currently dry cask storage on site

• Yucca Mountain proposed as nuclear repository site

Recycling of spent fuel

Possible to reprocess spent fuel to concentrate Pu-239 and U-235

Pu-239 potentially used for nuclear weapons

US does not currently reprocess spent fuel

Problems with Plant Operation

Three Mile Island 1979failure of water pump

partial core meltdown

Chernobyl 1986poor design

only graphite moderator (burns)inadequate reactor containment

Fusion

• Occurs in sun

• Theoretically wonderful source of energy– Lots of water and H sources available

• Have not yet achieved fusion – Requires very high temperatures