Nuclear Chemistry

Chapter 19Nuclear Chemistry

General Chemistry: An Integrated Approach

Hill, Petrucci, 4th Edition

Mark P. HeitzState University of New York at Brockport

© 2005, Prentice Hall, Inc.

Chapter 19: Nuclear Chemistry 2

Introduction

Nuclear properties can be used to distinguish among the various isotopes of an element

Examples: carbon-13, carbon-14 C,C 146

136

EOS

Radioactivity, or radioactive decay, is the spontaneous change of the nuclei accompanied by the emission of subatomic particles and/or high-frequency electromagnetic radiation


Radioactivity andNuclear Equations

A nucleus with a specified number of protons and neutrons is a nuclide

The two sides of a nuclear equation must have the same totals of atomic numbers and mass numbers EOS

Together, protons and neutrons are called nucleons

C136

Mass number

Atomic number


Radioactive Decay Products

Effect of an electric field on particle emission:

EOS


Modes of Radioactive Decay

HeThU 42

23490

23892

Sum of mass numbers: 238 = 234 + 4

EOS

Sum of atomic numbers: 92 = 90 + 2


Types of Radioactive Decay

ePaTh 01

23491

23490 Beta Decay

00

23490

m23490 ThTh Gamma Decay

eMgAl 01

2612

2313 Positron Decay

EOS

TeeI 12552

01

12553 Electron Capture


Radioactive Elements

Most of the naturally occurring nuclides of the lighter elements have stable nuclei; they are not radioactive

Even though they are radioactive, many nuclides of high atomic number are found in natural sources

EOS

The half-life (t1/2) of a radioactive nuclide is the time required for one-half the nuclei in a sample of the nuclide to decay


Radioactive Decay Series

A series of radioactive decays beginning with a long-lived radioactive nuclide and ending with a nonradioactive one is called a radioactive decay series

EOS


Radioactive Decay Rates

The radioactive decay law states that the rate of disintegration of a radioactive nuclide, called the decay rate or activity, A, is directly proportional to the number of atoms present

Rate of Radioactive Decay = A = N

EOS

Radioactive decay is a first-order process. The decay constant, , is analogous to k in the rate of reaction


Example of First Order Decay

EOS


Selected Nuclide Half-lives

EOS


Radiocarbon Dating

Carbon-14 is formed at a nearly constant rate in the upper atmosphere by the bombardment of nitrogen-14 with neutrons from cosmic radiation

EOS

Carbon-14 in living matter decays by – emissions at a rate of about 15 disintegrations per minute per gram of carbon

The half-life for carbon-14 is 5730 years. This dating method works well if an object is between 5000 and 50,000 years old


Synthetic Nuclides

Rutherford, in 1919, was able to convert nitrogen-14 into oxygen-17 plus some extra protons by bombarding the nitrogen atoms with particles. This a naturally occurring form of oxygen and is not radioactive

EOS

Phosphorus-30 was the first synthetic radioactive nuclide


Transuranium Elements

In 1940, the first of the transuranium elements—elements with a Z > 92—was synthesized by bombarding uranium-238 nuclei with neutrons

EOS

PueNp

NpeU

UnU

23994

01

23993

23993

01

23992

23893

01

23892


Transuranium ElementsConsiderable energy must be imparted to a positive ion in order for it to overcome repulsion by a positively charged nucleus. A machine, called a charged-particle accelerator, or cyclotron, is capable of this process

EOS


Nuclear Stability

EOS

Stable, nonradioactive nuclei are found within the “belt of stability”


Energetics of Nuclear ReactionsWhile working out the details of the theory of special relativity, Einstein derived the equation for the equivalence of mass and energy: E = mc2

Nuclear energies are normally expressed in the unit MeV (megaelectronvolt): 1 u = 931.5 MeV

EOS

m = –0.0061 u or –5.7 MeV


Nuclear Binding EnergyThe energy released in forming a nucleus from its protons and neutrons is called the nuclear binding energy and is expressed as a positive quantity

EOS

This explains why there is a mass loss of 0.0304 u in the formation of a helium nucleus from the two protons and two neutrons which comprise it. This quantity is called the mass defect of the nucleus


Nuclear Binding Energyfor Helium

EOS


Average Binding Energies

EOS


Nuclear FissionThe breakup of a heavy nucleus into two lighter fragments is called nuclear fission

EOS


Nuclear Fission Reactions

A nuclear reactor is designed to tame the nuclear fission process so that energy is released in a controlled manner

EOS

When a nucleus undergoes fission, some mass is converted into energy; about 3.2 × 10–11 J or approximately 8 × 107 kJ/g


A Model Reactor

EOS


Nuclear Fusion

The process of combining light nuclei into a heavier one is called nuclear fusion

EOS

Fusion is much more difficult to accomplish than fission because, with fusion, the nuclei must be forced extremely close together

– accomplished in the uncontrolled fusion reactions of hydrogen bombs


A Fusion ReactorThis close approach requires that the nuclei have enormously high thermal energies (over 40,000,000 K)

EOS


Effect of Radiation on Matterparticles only dislodge electrons from atoms and are termed ionizing radiation

Electrons freed by ionizing radiation are called primary electrons

Some primary electrons are energetic enough to ionize other atoms and molecules, producing secondary electrons

EOS


Effects of Ionizing Radiation

EOS

Ionizing radiation can excite electrons to higher energy levels with the emission of electromagnetic radiation such as X rays and ultraviolet light


Radiation DetectorsOne of the simplest and oldest ways to detect ionizing radiation is to observe the clouding it produces on photographic film

EOS

The most familiar radiation detection device is the Geiger–Müller counter


Radiation Units

EOS


Radiation Dosage1000 rem: Almost certain to cause death

450 rem: A short-term dose would kill 50% of a population within 30 days1 rem: A short-term dose would likely cause about 100 cases of cancer within 20 to 30 years for every 1 million people exposed130 mrem/y: The normal average background radiation dosage20 mrem: The typical dose in a chest X ray examination

EOS5 mrem/y: Result of nuclear power production


Approximate Stopping Distances

Materials vary in ability to block radiation

EOS


Medical Diagnosis and Therapy

Radioisotopes are widely used to diagnose various disorders: most have very short half-lives

Although ionizing radiation can induce cancer, it can also be used to treat cancer cells, which are destroyed more easily by radiation than are healthy, normal cells

EOS

In some instances, radioactive chemicals (called radiopharmaceuticals) can be ingested and allowed to find their own way to the targeted tissue


Radioactive TracersRadioactive nuclides can be used as radioactive tracers, and their atoms can be attached to other substances, which are then said to be “tagged”

EOS

These tracers can be used to:

– Detect leaks in underground piping systems

– Determine frictional wear in piston rings

– Determine the uptake of phosphorus and its distribution in plants


Summary of Concepts

• The five types of radioactive nuclides involve emission of alpha () particles, beta () particles, gamma () rays, positrons, and electron capture

• All known nuclides with Z > 83 are radioactive, and many of them occur naturally as members of four radioactive decay series

EOS

• In the formation of an atomic nucleus from its protons and neutrons, a quantity of mass is converted into energy


Summary of Concepts

• Radiation from radioactive materials interacts with matter, principally by forming ions and breaking chemical bonds

EOS

Nuclear Chemistry

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