Nuclear Chemistry

Atomic Nucleus Very small Very dense----1.6 x 1014 g/cm3

----Ping-Pong ball of nuclear matter = 2.5 billion tons of mass Large magnitude of energy in nucleus Composed of neutrons and protons Neutrons & protons made of quarks

Nuclear Vocabulary

• Nucleons: particles in the nucleus:–p+: proton–n0: neutron.

• Mass number (A): the number of p+ + n0.

• Atomic number (Z): the number of p+.

• Isotopes: same number of p+; different numbers of n0.

• Nuclide: unique atom: AZ X

Why Are Some Nuclei Unstable?

• Proton has high mass and high charge.• Proton-proton repulsion is large.• In nucleus protons are very close to

each other.• Strong nuclear forces: cohesive forces

in nucleus. Neutrons involved with strong nuclear force.

• As more protons are added the proton-proton repulsion gets larger.

Neutron-to-Proton Ratio

•The heavier the nucleus, the more neutrons are required for stability.•The belt of stability deviates from a 1:1 neutron to proton ratio for high atomic mass.

Radioactive Decay

Many nuclei are unstable (out of 2000 nuclides, 279 are stable).

Light nuclides stable when n: p+ = 1 More stability with even numbers of n and p+ Super stable: magic numbers 2, 8, 20, 28, 50, 82,

126 Nuclides with 84 or more protons are unstable.• Radioactivity: decomposition of a nucleus to form

another nucleus plus particle(s).• In nuclear equations, number of nucleons is

conserved: 238

92U 23490Th + 4

2He

Packet Example Page 7

Which of the following nuclei are especially stable: helium-4, calcium-40, or technetium-98?

helium-4 and calcium-40, because 1:1 proton:neutron ratio and even numbers

technetium-98, NO! Few nuclei are stable with odd numbers of protons and neutrons, plus above Z = 84

Three Types of Radioactive Decay

Three types:– -Radiation: loss of 4

2He from nucleus

– -Radiation: loss of an e- from nucleus

– -Radiation: loss of high-energy photon from nucleus

• Write all particles with their atomic and mass numbers: 4

2He and 42a

represent -radiation

Alpha-particle Production

particle is a helium nucleus: 42He

Very common decay for heavy radioactive nuclides

Decaying nucleus changes sheds particle, changes mass

23892U 234

90Th + 42He

Practice Problems p 3

1. What product is formed when radium-226 undergoes alpha decay?

2. What element undergoes alpha decay to form lead-208?

Beta-radiation Neutron splits into p+ and e-

(Electron is created by release of energy)

High-speed electron is particle -radiation is loss of electron from

nucleus and change of a neutron to a proton

Mass of decaying nucleus remains constant

10n 1

1p+ + 0-1e-

Gamma Ray Production

-ray is high energy photon

Not a particle! Part of EM spectrum Accompanies some

nuclear decays Release of -rays allows

relaxation of excited nucleus to its ground state

Spontaneous Fission

Another decay process Spontaneous fission: splitting of

heavy nuclide into 2 lighter nuclides with similar mass numbers

Mass number changes as with -decay

Occurs at very slow rate for most nuclides.

Positron Emmision Occurs for nuclides whose

neutron/proton ratio if too small Positron: particle with mass of

electron but opposite charge Positron emission changes a proton

to a neutron, causing a higher neutron/proton ratio than original atom.

10p+ 1

0n + 01e+

Positron Annihilation

Positron is the antiparticle of the electron.

Collision of electron and positron changes matter to EM gamma radiation.

0-1e- + 0

1e+ 200g

Electron Capture Inner-orbital electron captured by

nucleus Proton captures electron and forms

neutron Occurs very slowly Gamma rays are produced 1

1p+ + 0-1e- 1

0n

Nuclear Stability•At Bi (the belt of stability ends and all nuclei are unstable.•Nuclei above belt have -emission. •Nuclei below belt have +-emission or electron capture. •Nuclei with atomic numbers greater than 83 usually have -emission.

Packet Examples Pages 4-5

Write nuclear equations for the following processes:

a. Mercury-201 undergoes electron capture

b. Thorium-231 decays to form protactinium-231

Packet Examples Page 6

Predict the mode of decay of carbon-14

xenon-118

Short-Hand Notation

• Nuclear transmutations can occur using high velocity -particles:

14N + 4 17O + 1p.• The above reaction is written

in short-hand notation: 14N(,p)17O.

Example Packet Page 7

Write the balanced equation for the process summarized as 27Al(n, )24Na

Patterns of Nuclear Stability• A nucleus usually undergoes more

than one transition on its path to stability.

• Radioactive series: series of nuclear reactions that accompany path to stability.

• Daughter nuclei: nuclei resulting from radioactive decay.

Radioactive SeriesFor 238U, first decay is to 234Th (-decay). 234Th undergoes -emission to 234Pa and 234U. 234U undergoes -decay (several times) to 230Th, 226Ra, 222Rn, 218Po, and 214Pb. 214Pb undergoes -emission (twice) via 214Bi to 214Po which undergoes -decay to 210Pb. The 210Pb undergoes -emission to 210Bi and 210Po which decays () to the stable 206Pb.

Nuclear Transmutations

• Nuclear transmutations: change of one element into another by collisions between nuclei.

• First observed by Rutherford in 1919 (N to O)• Irene & Frederic Joliot-Curie in 1933 (Al to P)• Can occur using high velocity -particles:

14N + 4 17O + 1p• To overcome electrostatic forces, charged

particles need to be accelerated in particle accelerators before they react.

Kinetics of Radioactive Decay Rate law for radioactive decay is first-order.

ln(N/N0) = -kt

Half-life (t½): time required for the number of nuclides to reach half the original value.

Remember: t½ = 0.693/k k = decay constant

Half-lives

• Each isotope has a characteristic half-life.

• Half-lives not affected by temperature, pressure or chemical composition.

• Half-lives of natural radioisotopes longer than half-lives of synthetic radioisotopes.

Half-lives range from fractions of a second to millions of years.

Units of Radioactivity

Activity: rate of decay (disintegrations per unit of time in seconds: dps)

SI unit of radioactivity = Becquerel (Bq)

1 Bq = 1 dps Older Curie (Ci) still widely

used 1 Ci = 3.7 x 1010 dps

Radioactive Dating• Naturally occurring

radioisotopes can be used to determine how old a sample is.

• Carbon-14 is used to determine ages of organic compounds.

• We assume the ratio of 12C to 14C has been constant over time.

• Object must be less than 50,000 years old.

Shroud of Turin dated 1260-1390 A.D.

Examples Packet Top Page 9 The half-life of cobalt-60 is 5.3 years. How much of a 1.000 mg sample of cobalt-60 is left after a 15.9 year period?

Half-life (t½): time required for the number of nuclides to reach half the original value.

Packet Example Middle Page 9 Carbon-11, used in medical imaging, has a half-life of 20.4 min. The carbon-11 nuclides are formed, then incorporated into a desired compound. The resulting sample is injected into a patient and the medical image is obtained. The entire process takes five half-lives. What percentage of the original carbon-11 remains at this time? Assume you start with 1.00 g carbon-11.

Packet Example 1 Page 11

A rock contains 0.257 mg of lead-206 for every milligram of uranium-238. The half-life for the decay of uranium-238 to lead-206 is 4.5 x 109 yr. How old is the rock?

Remember: t½ = 0.693/k

Remember: t = -1/k ln N/N0

Packet Example 2 Page 11 A wooden object from an archeological

site is subjected to carbon dating. The activity of the sample due to carbon-14 is measured to be 11.6 dps. The activity of a carbon sample of equal mass from fresh wood is 15.2 dps. The half-life of carbon-14 is 5715 yr. What is the age of the archeological sample?

Remember: t½ = 0.693/k

Packet Example 3 Page 11

A sample to be used for medical imaging is labeled with fluorine-18, which has a half-life of 110 minutes. What percentage of the original sample remains after 300 minutes?

Detection of Radiation

• Matter is ionized by radiation.• Geiger-Muller counter (Geiger

counter) determines the amount of ionization of Ar by radioactive particles.

• Scintillation counter: measures flashes of light given off as substance is struck by radiation.

Geiger-Muller Counter

Medical Applications

Radiotracers: radioactive nuclides that can be put into organisms and whose pathways can be traced.

Sensitive and noninvasive Iodine-131 diagnoses and

treats illnesses of thyroid Thallium-201 and technetium-

99m assess damage to heart

Energy Changes in Nuclear Reactions

• Einstein showed that mass and energy are proportional:

• If a system loses mass it loses energy; if a system gains mass it gains energy.

• Since c2 is a large number (8.99 1016 m2/s2) small changes in mass cause large changes in energy.

• Mass and energy changed in nuclear reactions are much greater than chemical reactions.

2mcE

Energy as Matter Energy is a form of matter!

Consider 23892U 234

90Th + 42He

For 1 mol the masses are 238.0003 g 233.9942 g + 4.0015

g. Change in mass during reaction is

233.9942g+ 4.0015 g - 238.0003 g = -0.0046 g

The process is exothermic because the system has lost mass.

Mass Defect

• Mass of a nucleus is less than the mass of its nucleons!

• Mass defect: difference in mass between nucleus and component nucleons.

• Binding energy: energy needed to separate a nucleus into its nucleons.

Energy change = E = ∆mc2

Example Packet Top Page 14 How much energy is lost or gained when a mole of cobalt-60 undergoes beta decay to form nickel-60? The mass of cobalt-60 is 59.933819 amu and that of nickel-60 is 59.930788 amu.

Use E = ∆mc2

Example Packet Bottom Page 14

Positron emission from carbon-11 to form boron-11 occurs with the release of 2.87 x 1011 joule per mol of carbon-11. What is the mass change per mole of carbon-11 in this nuclear reaction?

Use E = ∆mc2

Binding Energy

• Larger binding energy = more stable nucleus.

• Average binding energy per nucleon increases to a maximum at mass number 50 - 60, and decreases afterwards.

Most stable nucleus is iron-56.

Fission and Fusion

Energy is released when a process goes from less stable to more stable state.

Fusion: combining two light nuclei to form a heavier, more stable one.

Fission: splitting a heavy nucleus into two nuclei with smaller mass numbers.

Both processes energy changes millions of times larger than those from chemical reactions!

Nuclear Fission Discovered in late 1930’s. Fission of uranium-235 gives 26

million times more energy than combustion of CH4.

Nuclear Chain Reactions

• For every 235U fission 2.4 neutrons are produced.

• Each neutron produced can cause the fission of another 235U nucleus.

• Number of fissions and energy increase rapidly.

• Eventually, a chain reaction forms.• Without controls, an explosion results.

Critical Mass• Each neutron can cause another fission.• Minimum mass of fissionable material is

required for a chain reaction (or neutrons escape before they cause another fission).

• When enough material is present for a chain reaction, we have critical mass.

• Subcritical mass: neutrons escape and no chain reaction occurs.

• Supercritical mass: any mass over critical

Nuclear Bomb

• Two subcritical wedges of 235U separated by a gun barrel.

• Explosives used to bring two subcritical masses together to form one supercritical mass = nuclear explosion.

• Manhattan Project during WWII: Hiroshima and Nagasaki in 1945.

Nuclear Reactors Controlled fission used to produce

electricity.• Use a subcritical mass of 235U (enrich 238U

with about 3% 235U).• Enriched 235UO2 pellets are encased in Zr or

stainless steel rods to absorb neutrons.• Heat produced in the reactor core is

removed cooling fluid to a steam generator.

• Steam drives an electric generator.

Future of Nuclear Reactors?Cons of Nuclear Reactors

Pros of Nuclear Reactors

Storage of used fuel rods

Cost to build and maintain new reactors

Safety: Three-Mile Island and Chernobyl

Produces no greenhouse gases

Lower volume of waste products than fossil fuels

Almost unlimited supply of fuel

Excellent safety record

Nuclear Fusion• Light nuclei can fuse to form heavier nuclei.• Most reactions in the Sun are fusion.• Fusion products are not usually radioactive,

so fusion is a good energy source.• Hydrogen needed for reaction can easily be

supplied by seawater.• However, high energies are needed to

overcome repulsion between nuclei before reaction can occur.

Effects of Radiation

Energy transferred to cells can break chemical bonds and wreak havoc with sell systems.

Radiation damage can be subtle-feel effects years later.

Somatic damage: damage to organism = sickness of death

Genetic damage: produces malfunction in offspring

Biological Effects Depend on… Energy of the radiation: dose measured

in rads (radiation absorbed dose) Penetrating ability of radiation: < <

Ionizing ability of radiation: extraction

of electrons from biomolecules very damaging

Chemical property of radiation source: affects residence time in organism