1 © 2006 brooks/cole - thomson chemistry and chemical reactivity 6th edition john c. kotz paul m....

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© 2006 Brooks/Cole - Thomson

Chemistry and Chemical Reactivity 6th Edition

John C. Kotz Paul M. Treichel

Gabriela C. Weaver

CHAPTER 23

Nuclear Chemistry

© 2006 Brooks/Cole Thomson

Lectures written by John Kotz

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Nuclear Chemistry

Pictures of human heart before and after

stress using gamma rays from radioactive

Tc-99m

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Why do you care?• PET scans• Nuclear Power• Space travel• Smoke Detectors (Am-241)• Ionizing Radiation and X-rays• Neutron Activation• Exposure (pilots, nuclear accidents, Radon)• Carbon Dating• Nuclear Weapons

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Nuclear Radiation

• The Process of emitting energy in the form of waves or particles.

• Comes from the Nucleus of the Atom– The Neutrons– Instability – Binding Energy– E=mc2

– Non-conservation of Mass

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ATOMIC COMPOSITION• Protons– positive electrical charge– mass = 1.672623 x 10-24 g– relative mass = 1.007 atomic mass units

(amu)• Electrons

– negative electrical charge– relative mass = 0.0005 amu

• Neutrons– no electrical charge– mass = 1.675523 x 10-24 g– relative mass = 1.009 amu

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Isotopes

• Atoms of the same element (same Z) but different mass number (A).

• Boron-10 (10B) has 5 p and 5 n: 105B

• Boron-11 (11B) has 5 p and 6 n: 115B

10B

11B

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Radioactivity

• One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of

Marie Curie (1876-1934). • She discovered

radioactivity, the spontaneous disintegration of some elements into smaller pieces.

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Types of Radiation

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Penetrating Ability

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Nuclear Reactions• Alpha emission

Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.

Nucleons are rearranged but conserved

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Nuclear Reactions• Beta emission

Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

How does this happen?

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Other Types of Nuclear Reactions

Positron (0+1b): a positive electron

K-capture: the capture of an electron from the first or K shell

An electron and proton combine to form a neutron.0

-1e + 11p --> 1

0n

207 207

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Radioactive Decay Series

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Stability of Nuclei• Heaviest naturally

occurring non-

radioactive isotope is

209Bi with 83 protons

and 126 neutrons

• There are 83 x 126 =

10,458 possible

isotopes. Why so few

actually exist?

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Stability of Nuclei

• Up to Z = 20 (Ca), n = p (except for 73Li, 11

5B, 199F)

• Beyond Ca, n > p (A > 2 Z)

• 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 isotopes

and most stable isotope has EVEN N.

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Stability of Nuclei

• Suggests some PAIRING of NUCLEONS• Something inside the nucleus gives each atom a

probability of radioactive decay

Even Odd

Odd

Even

Z N

157 52

50 5

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Band of Stability and Radioactive Decay 243

95Am --> 42a + 239

93Np

a emission reduces Z

b emission increases Z

6027Co --> 0

-1b + 6028Ni

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

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Binding Energy, Eb

Eb is the energy required to separate the

nucleus of an atom into protons and

neutrons.

Use E=mc2

Find the mass of the isotope.

Sum the masses of the nucleons.

For m, use the DIFFERENCE between those

masses.

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Calculate Binding Energy

For deuterium, 21H: 2

1H ---> 11p + 1

0n

Mass of 21H = 2.01410 g/mol

Mass of proton = 1.007825 g/mol

Mass of neutron = 1.008665 g/mol

∆m = 0.00239 g/mol = 2.39x10-6 kg/mol

c = 3x108 m/sec

From Einstein’s equation:

Eb = (∆m)c2 = 2.15 x 1011 J/mol

How much binding energy is there per nuclear particle?

Eb per nucleon = Eb/2 nucleons

= 1.08 x 108 kJ/mol nucleons

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Half-Life

• HALF-LIFE is the time it takes for 1/2 a sample to disappear.

• The rate of a nuclear transformation depends only on the “reactant” concentration. It does not depend on any factors outside the nucleus.

• Half-life is a property that can be used to identify an element.

• Half-life cannot predict the likelihood a single atom will decay

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Half-Life

Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?

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Kinetics of Radioactive Decay

Activity (A) = Disintegrations/time

N is the number of atoms

Decay is first order, and so

ln (A/Ao) = -kt or

ln (A) – ln (Ao) = -kt

The half-life of

radioactive decay is

t1/2 = 0.693/k

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Radiocarbon DatingRadioactive C-14 is formed in the upper atmosphere

by nuclear reactions initiated by neutrons in cosmic radiation

14N + 1on ---> 14C + 1H

The C-14 is oxidized to CO2, which circulates through the biosphere. There is a constant % of C-14 in the atmosphere. While a plant is alive, it has the same % of C-14 in it as the atmosphere.

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.

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Radiocarbon Dating

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Man-made Eyes to See Small Things

• Humans needed to find a way to extend their senses, to gather knowledge about things beyond our physical constraints.

• Light can be thought of as a piece of information sent between matter.

• The wavelength/frequency/energy of light determines how it interacts with matter and also predicts where it came from.

• Certain materials can “see” light that our eyes cannot.

• Using these materials we learn about the elements in space and on earth.

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Human Limitations

• The molecules in our eyes only work within a very specific range of wavelengths.

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Our Sun- Seen by Ultraviolet Light

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Extending Our Vision

• Common detector materials that interact with light:

• Sodium Iodide crystal:

• Plastic scintillator:

• Germanium Crystal:

• Silicon:

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Cosmic Rays

• Super fast particles from the sun and outer space (protons and ions)---

• Strike the atmosphere and become pions (positively charged fundamental particle), then muons (heavy electrons).

• Built a detector to “see” them using a plastic scintillator.

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• Obtainable info:– Direction of radiation– Shielding effects

• Pyramids example• Depth inside Earth

– Solar activity levels200 muons/m2/second

Protonfrom sun

Pion

Molecule inatmosphere

Muon

Neutrino

Atom ofHydrocarbon

Light

Cosmic Rays

Photomultiplier Tube (PMT)

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Cosmic rays are the source of C-14 used in radiocarbon dating!

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Terrestrial Radiation

• Obtainable info:– Naturally occurring radioactive isotopes can be identified.– Composition of isotopes in rocks is compared to rocks

from around the world. – Background radiation in the air can be measured – Investigation of radiation in the ground.

• Uses gamma ray spectroscopy to “see” light that comes from matter in the ground

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Summary• Certain materials interact with the light that our eyes

don’t detect.• Devices made from these materials have lead to the

field of spectroscopy, meaning “seeing light.”• All modern devices convert a light signal into an

electrical signal.• The electrical signal is arranged in a way that allows

us to ‘see’ what is going on with our eyes.

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Bubble ChambersAlpha, Beta, and Gamma Particles rip through a supercooled gas, ionizing them, and forming bubbles.

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Artificial Nuclear Reactions

New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4He and 11B.

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

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• Applications:– Test for the presence of

heavily shielded dangerous nuclear material.

– Create small amounts of elements (alchemy)

– Find approximate percent compositions of elements in a substance.

Neutron Activation– Shoot neutrons into a substance, stuffing them into a

nucleus to make it unstable. They will then decay in a special way that we can “see” what is in them.

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Transuranium Elements

Elements beyond 92 (transuranium) made

starting with an n,g reaction

23892U + 1

0n ---> 23992U + g

23992U ---> 239

93Np + 0-1b

23993Np ---> 239

94Np + 0-1b

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Transuranium Elements

& Glenn Seaborg

106Sg

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Nuclear Fission

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Nuclear Fission

Fission chain has three general steps:

1. Initiation. Reaction of a single atom

starts the chain (e.g., 235U + neutron)

2. Propagation. 236U fission releases

neutrons that initiate other fissions

3. Termination.

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Nuclear Fission &

Lise Meitner

109Mt

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Nuclear Fission & POWER

• Currently about 104 nuclear

power plants in the U.S. and

about 400 worldwide.

• 17% of the world’s energy

comes from nuclear fission.

• What are would be the benefits

and drawbacks to using nuclear

FUSION instead of nuclear

fission?

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Nuclear Medicine: Imaging

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BNCTBoron Neutron Capture

Therapy• 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

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Food Irradiation

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

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

•USDA has approved irradiation of meats and eggs.

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Effects of Radiation

Rem: Quantifies biological tissue damageUsually use “millirem”

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