Chapter 23 Nuclear Chemistry

Jeffrey MackCalifornia State University,

Sacramento

Chapter 23

Nuclear Chemistry

Images of a human heart before and after stress detecting gamma rays from radioactive Tc-99m

Nuclear Chemistry

• Protons– (+1) electrical charge– mass = 1.672623 1024 g– mass = 1.007 atomic mass units (amu)

• Electrons– negative electrical charge– relative mass = 0.0005 amu

• Neutrons– no electrical charge– mass = 1.009 amu

Atomic Composition

• Isotopes are atoms of the same element (same Z) but different mass numbers (A).

• Boron-10:5 protons and 5 neutrons: • Boron-11: 5 protons and 6 neutrons:

10B

11B

Isotopes

• The isolation and characterization of radium & polonium by Marie Curie was one of milestones of chemistry.

• It is a credit to her skills as a chemist that she was able to isolate only a single gram of radium from 7 tons of uranium ore.

Marie and Pierre Curie

Radioactivity

• -particles can be stopped by paper.• -particles require at least a cm of lead (Pb).• -particles require at least 10 cm of lead (Pb).

Energy: > >

Natural Radioactivity

Penetrating Ability

• Ernest Rutherford isolated Radium forms Radon gas while studying alpha particle emission.

• 1902 Rutherford and Soddy proposed radioactivity was the result of the natural change of the isotope of one element into an isotope of a different element.

Nuclear Reactions

Alpha emission

• Nucleons must be conserved in any nuclear reaction.

• In emission, the mass number (A) decreases by 4 and the atomic number (Z) decreases by 2.

Nuclear Reactions

Beta emission

In emission, the mass number (A) remains unchanged and the atomic number (Z) decreases by 1.

Nuclear Reactions

Radioactive Decay Series

Positron (positive electron) emission

207 207

• Positrons have the mass of an electron, but positive charge. They are the antimatter analog of an electron.

• Positron emission arises from “electron capture”.• An inner shell electron is absorbed by the nucleolus

converting a proton into a neutron along with an emitted positron.

Other Types of Nuclear Reactions

• H is most abundant element in the universe.• H represents 88.6% of all atoms• He represents 11.3% of all atoms• Together 99.9% of all atom & 99% of mass of

the universe.

Stability of Nuclei

• Hydrogen: – 1

1H, protium– 2

1H, deuterium– 3

1H, tritium (radioactive)

• Helium, 42He

• Lithium, 63Li and 7

3Li• Boron, 10

5B and 115B

• Iron– 54

26Fe, 5.82% abundant

– 5626Fe, 91.66%

abundant– 57

26Fe, 2.19% abundant

– 5826Fe, 0.33%

abundant

Isotopes

• 209Bi with 83 protons and 126 neutrons is the heaviest naturally occurring non-radioactive isotope.

• There are 83 x 126 = 10,458 possible isotopes.

• Why do so few exist in nature?

Stability of Nuclei

• Up to Z = 20 (Ca) stable isotopes often have the same # of neutrons and protons. Only H and He-3 have more protons than neutrons.

• Beyond Ca, the ratio of neutrons to protons is >1.• As Z increases, the n:p ratio deviates further from 1:1• Above Bi all isotopes are radioactive. Fission leads to

smaller particles, the heavier the nucleus the greater the rate.

• Above Ca: elements of EVEN Z have more stable isotopes than ODD Z elements.

• The more stable isotopes have an EVEN number of neutrons.

Stability of Nuclei

Out of > 300 stable isotopes:

Even Odd

Odd

Even

ZN

157 52

50 5

3115P

199F

21H, 6

3Li, 105B, 14

7N, 18073Ta

Stability of Nuclei

• The trend suggests some PAIRING of NUCLEONS

• There are “nuclear magic numbers”2 He 28 Ni8 O 50 Sn20 Ca 82 Pb

Even Odd

Odd

Even

ZN

157 52

50 5

Stability of Nuclei

Band of Stability and Radioactive Decay

Isotopes with low n/p ratio, below band of stability decay, decay by positron emission or electron capture

• The energy required to separate the nucleus of an atom into protons and neutrons.

• For deuterium, 21H

21H 1

1p + 10n Eb = 2.15 108 kJ/mol

• Eb per nucleon = Eb/2 nucleons= 1.08 108 kJ/mol nucleons

Binding Energy, Eb

For deuterium, 21H: 2

1H 11p + 1

0n

Mass of 21H: = 2.01410 g/mol

Mass of proton: = 1.007825 g/molMass of neutron: = 1.008665 g/mol∆m: = 0.00239 g/molFrom Einstein’s equation: Eb = (∆m)c2

= 2.15 x 108 kJ/molEb per nucleon = Eb/2 nucleons

= 1.08 108 kJ/mol nucleons

Calculate Binding Energy

Binding Energy/Nucleon

• The HALF-LIFE of an isotope is the time it takes for 1/2 a sample to decay from its initial amount.

• The rate of a nuclear transformation depends only on the “reactant” concentration.

• The decay and half-life for a nuclear reaction follows first order kinetics.

Half-Life

After each successive half-life, one half of the original amount remains.

Half-Life

Activity (A) = Disintegrations/time = (k)(N) where N is the number of atomsDecay follows first order kinetics:

The half-life of radioactive decay is t1/2 = 0.693/k

Kinetics of Radioactive Decay

Willard Libby (1908-1980)Libby received the 1960 Nobel Prize in chemistry for developing carbon-14 dating techniques. He is shown here with the apparatus he used. Carbon-14 dating is widely used in fields such as anthropology and archeology.

Radiocarbon Dating

Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation:14N + 1

0n 14C + 1HThe C-14 is oxidized to CO2, which circulates through the biosphere.When a plant dies, the C-14 is not replenished.But the C-14 continues to decay with t1/2 = 5730 years.Activity of a sample can be used to date the sample.

Radiocarbon Dating

• New elements or new isotopes of known elements are produced by bombarding an atom with subatomic particles such as a protons or neutrons, or even a heavier particles such as 4He and 11B.

• Reactions using neutrons are called n, reactions because a -ray is usually emitted.

• Radioisotopes used in medicine are often made by n, reactions.

Artificial Nuclear Reactions

• An Example of a n, reaction is production of radioactive 32P.

• 32P is used in studies of phosphorous uptake in the body.

Artificial Nuclear Reactions

Elements beyond 92 (transuranium) are made via n, reactions.

Transuranium Elements

106Sg

Transuranium Elements & Glenn Seaborg

Nuclear Fission

Fission chain reaction has three general steps:Initiation:

Reaction of a single atom starts the chain (e.g., 235U + neutron)

Propagation:236U fission releases neutrons that initiate other fissions

Termination.Consumption of the fissionable material is completed

Nuclear Fission

109Mt

Nuclear Fission & Lise Meitner

• Currently about 103 nuclear power plants in the U.S. and about 435 worldwide.

• 17% of the world’s energy comes from nuclear.

Nuclear Fission & Power

• Curie: 1 Ci = 3.7 1010 distintegrations/s (dps)

• SI unit is the becquerel: 1 Bq = 1 dps• Rad: measures amount of energy absorbed

1 rad = 0.01 J absorbed/kg tissue• Rem: “roentgen equivalent man” based on

amount and type of radiation. • Quantifies biological tissue damage, usually

represented “millirems”.

Units for Measuring Radiation

Effects of Radiation

Effects of Radiation

Nuclear Medicine: Imaging

Technetium-99m is used in more than 85% of the diagnostic scans done in hospitals each year. Synthesized on-site from Mo-99.

99m43Tc decays to 99

43Tc giving off a -ray.The half-life of the radioisotope is 6.01 hrs.Once ingested, the Tc-99m concentrates in areas of high activity such as the thyroid. -ray imagining detects its presence.

Nuclear Medicine: Imaging

Imaging of a heart using Tc-99m before and after exercise.

Nuclear Medicine: Imaging

• 10B isotope (not 11B) has the ability to capture slow neutrons

• In BNCT, tumor cells preferentially take up a boron compound, and subsequent irradiation by slow neutrons kills the cells via the energetic 10B 7Li neutron capture reaction (that produces a photon and an alpha particle)

• 10B + 1n 7Li + 4He + photon

BNCTBoron Neutron Capture Therapy

• Food can be irradiated with rays from 60Co or 137Cs.

• Irradiation retards the growth of bacteria, molds and yeasts.

• Irradiated milk has a shelf life of 3 mo. without refrigeration.

• USDA has approved irradiation of meats and eggs.

Food Irradiation