© 2015 pearson education, inc. chapter 21 lecture presentation radioactivity and nuclear chemistry...

© 2015 Pearson Education, Inc.

Chapter 21

Lecture presentation

Radioactivity

and Nuclear Chemistry

Catherine E. MacGowan

Armstrong Atlantic State University


What Is Radioactivity?

• Radioactivity is the release of tiny, high-energy particles or high-energy electromagnetic radiation from the nucleus of an atom.

• Atoms that eject particles and/or energy from their nucleus are referred to as being radioactive.


Radioactivity

• Radioactive nuclei spontaneously decompose into smaller nuclei. This process is referred to as radioactive decay.– Radioactive nuclei are unstable.– Decomposing involves the nuclide emitting a particle

and/or energy.

• The parent nuclide is the nucleus that is undergoing radioactive decay.

• The daughter nuclide is the new nucleus that is made.

• All nuclides with 84 or more protons are radioactive.


• Antoine-Henri Becquerel designed an experiment to determine whether phosphorescent minerals gave off X-rays.

– Phosphorescence is the long-lived emission of light by atoms or molecules that sometimes occurs after they absorb light.

– X-rays are detected by their ability to penetrate matter and expose a photographic plate.

The Discovery of Radioactivity

The greenish light emitted from glow-in-the-dark toys is phosphorescence.


• Becquerel discovered that certain minerals were constantly producing energy rays that could penetrate matter.

• Becquerel determined that 1. all the minerals that produced these rays contained

uranium; and 2. the rays were produced even though the mineral was

not exposed to outside energy.

• He called them uranic rays because they were emitted from minerals that contained uranium.– Like X-rays– Not related to phosphorescence

Discovery of Radioactivity: Becquerel


• Marie Curie determined that the rays were emitted from specific elements.

• She also discovered new elements by detecting their rays.– Radium is named for its green

phosphorescence.– Polonium is named for her homeland.

• Because these rays were no longer just a property of uranium, she renamed them radioactivity.

Discovery of Radioactivity: Marie Curie


• Radioactive rays can ionize matter.– They cause uncharged matter to become

charged.• This is the basis of how a Geiger counter and

electroscope work.

• Radioactive rays– have high energy;– can penetrate matter; and– cause phosphorescent chemicals to glow.

Properties of Radioactivity


• An atom’s nucleus has the following characteristics:– Very small volume compared to the volume of the atom– Constitutes essentially the entire mass of the atom– Very dense– Composed of protons and neutrons that are tightly held together

• The particles that make up the nucleus are called nucleons.

• Every atom of an element has the same number of protons.– Atomic number (Z)

• Atoms of the same elements can have different numbers of neutrons.– Isotopes

• Different atomic masses

Facts about the Nucleus


• Isotopes are identified by their mass number (A).– Mass number = number of protons + number of neutrons

• The number of neutrons is calculated by subtracting the atomic number from the mass number.

– Isotopic symbol:

• The nucleus of an isotope is called a nuclide.• Less than 10% of the known nuclides are nonradioactive;

most are radionuclides.• Each nuclide is identified by a symbol.

– Element – mass number = (X – A)

Facts about Isotopes


Types of Radioactive Decay• Natural radioactivity can be categorized by the type of decay (particles

or energy rays).

• Rutherford discovered three types of rays:– Alpha (α) rays

• Have a charge of +2 and a mass of 4 amu• What we now know to be helium nucleus

– Beta (β) rays• Have a charge of −1 and negligible mass• Electron-like

– Gamma (γ) rays• Form of light energy (not a particle like α and β)

• In addition, some unstable nuclei emit positrons.– Like a positively charged electron

• Some unstable nuclei will undergo electron capture.– A low-energy electron is pulled into the nucleus.


Alpha (α) Decay

• An alpha particle is a 42He nucleus.

• Alpha decay occurs when an unstable nucleus emits a particle composed of two protons and two neutrons.

• It is the most ionizing but least penetrating of the types of radioactivity.– Protection from alpha decay: paper or light cloth

• Loss of an alpha particle means that– the atomic number increases by 2; and– the mass number decreases by 4.


Alpha (α) Decay Illustration


Beta (β) Decay

• A beta particle is an electron-like particle that is emitted from the nucleus when a neutron in the nucleus transmutes into a proton (remains in the nucleus) and a beta particle (emitted from the nucleus).

• Beta decay occurs when an unstable nucleus emits an electron-like particle.

• A beta particle is about 10 times more penetrating than an alpha particle but has only about half the ionizing ability.– Protection: heavy cloth

• When an atom loses a beta particle,– its atomic number increases by 1; and– its mass number remains the same.

10 neutron → –

01

β + 11 proton


Beta (β) Decay Illustration


Gamma (γ) Emission

• Gamma (γ) rays are high-energy photons.

• With gamma emission there is no loss of particles from the nucleus.

• There is no change in the composition of the nucleus.– Same atomic number and mass number

• Symbol:0

0 γ

• Gamma rays are the least ionizing but have the most penetration.– Protection: lead plates and thick cement walls

• Gamma emission generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange.


• A positron has a charge of +1 and a negligible mass.– It is the antielectron.

• Has the mass of an electron but opposite charge

• Positrons are similar to beta particles in their ionizing and penetrating abilities.

• A positron is formed and ejected from the nucleus when a proton transmutes to a neutron.

• When an atom loses a positron from the nucleus,– its mass number remains the same; and– its atomic number decreases by 1.

Positron Emission (+0

1 e)

11 proton → +

01

e + 10 neutron


Positron Emission Illustration


• It occurs when an electron from an inner orbital is pulled into the nucleus.

• There is no particle emission, but the atom changes because the inner electron combines with a proton in the nucleus to form a neutron.

• When a proton combines with the electron to make a neutron,– its mass number stays the same; and– its atomic number decreases by 1.

Electron Capture (–0

1 e)

11 proton + –

01

e → 10 neutron


Table of Radioactive Particles and Rays


Nuclear Equations• Nuclear processes are described with nuclear equations.

• Atomic numbers and mass numbers are conserved in a nuclear equation.– The sum of the atomic numbers on both sides must be equal.– The sum of the mass numbers on both sides must be equal.

• This conservation can be used to determine the identity of a daughter nuclide if the parent nuclide and mode of decay are known.


Practice Problem: Writing a Nuclear Equation for Alpha Decay


Practice Problem: Writing a Nuclear Equation


• The particles in the nucleus are held together by a very strong attractive force found only in the nucleus called the strong force.– Strong forces act only over very short distances.

• The neutrons play an important role in stabilizing the nucleus as they add to the strong force but don’t repel each other like the protons do.

What Causes Nuclei to Decompose?


N/Z (Neutrons/Protons) Ratio

• The ratio of neutrons to protons (N/Z) is an important measure of the stability of the nucleus.

• If the N/Z ratio is too high, neutrons are converted to protons via beta decay.

• If the N/Z ratio is too low, protons are converted to neutrons via positron emission or electron capture.– Or via alpha decay, though not as efficiently


• For Z = 1 → 20, stable N/Z ≈ 1.

• For Z = 20 → 40, stable N/Z approaches 1.25.

• For Z = 40 → 80, stable N/Z approaches 1.5.

• For Z > 83, there are no stable nuclei.

Valley of Stability


Practice Problem: Predicting Radioactive Decay


• Besides the N/Z ratio, the actual numbers of protons and neutrons affect stability.

• Most stable nuclei have even numbers of protons and neutrons.– Only a few have odd numbers of protons and neutrons.

• If the total number of nucleons adds to a magic number, the nucleus is more stable.– Same principle as stability of the

noble gas electron configuration– Most stable when N or Z = 2, 8,

20, 28, 50, 82; or N = 126

Magic Numbers


Decay Series• In nature, often one radioactive

nuclide changes into another radioactive nuclide.– That is, the daughter nuclide

is also radioactive.

• All atoms with Z > 83 are radioactive.

• All of the radioactive nuclides that are produced one after the other until a stable nuclide is made constitute a decay series.


Natural Radioactivity

• There are small amounts of radioactive minerals in the air, ground, and water.

• They are even in the food you eat!

• The radiation you are exposed to from natural sources is called background radiation.


• Particles emitted by radioactive nuclei have a lot of energy and therefore can be readily detected.

• Radioactive rays can expose light-protected photographic film.– Photographic film can be used to detect the presence

of radioactive rays.• Film-badge dosimeters

Detecting Radioactivity


• Radioactive rays cause air to become ionized.• An electroscope detects radiation by its ability to

penetrate the flask and ionize the air inside.• A Geiger-Müller counter works by counting electrons

generated when argon gas atoms are ionized by radioactive rays.



• When radioactive rays strike certain chemicals, they cause those chemicals to give off a flash of light.

• A scintillation counter is able to count the number of flashes per minute.



• The rate of change in the amount of radioactivity is constant and is different for each radioactive isotope.

• A particular length of time—a constant half-life—is required for each radionuclide to lose half its radioactivity.

• The shorter the half-life, the more nuclei decay every second; therefore, the “hotter” the sample, the more radioactive it is.

• The rate of radioactive change is not affected by temperature.– In other words, radioactivity is

not a chemical reaction!

Rate of Radioactive Decay Is First-Order Kinetics


Rate of Radioactive Decay Is First-Order Kinetics

• Radioactive decay follows first-order kinetics.

Rate law: Rate = kN

Integrated rate law:

Where

Nt = number of radioactive nuclei at time t

N0 = initial number of radioactive nuclei

Half life: t1/2 = 0.693/k or ln(2)/k

lnN0

Nt = –kt


Half-Lives of Various Nuclides


Practice Problem: Radioactive Decay Kinetics


• The change in the amount of radioactivity of a particular radionuclide is predictable and not affected by environmental factors.

• By measuring and comparing the amount of a parent radioactive isotope and its stable daughter, we can determine the age of the object.– Using the half-life and the previous equations

• Mineral (geological) dating: U to Pb– Compares the amount of U-238 to the amount of

Pb-206 in volcanic rocks and meteorites• Dates Earth to between 4.0 and 4.5 billion years old

– Compares amount of K-40 to amount of Ar-40

Radiometric Dating


• All things that are alive or were once alive contain carbon.

• Three isotopes of carbon exist in nature, one of which, C-14, is radioactive.– C-14 radioactive with

half-life = 5730 years

• Atmospheric chemistry keeps producing C-14 at nearly the same rate it decays.

Radiocarbon Dating


• While an organism is still living, C-14/C-12 is constant because the organism replenishes its supply of carbon.– CO2 in air is the ultimate source of

all C in an organism.

• Once the organism dies the C-14/C-12 ratio decreases.

• By measuring the C-14/C-12 ratio in a once-living artifact and comparing it to the C-14/C-12 ratio in a living organism, we can tell how long ago the organism was alive.

• The limit for this technique is 50,000 years old.– About 9 half-lives, after which

radioactivity from C-14 will be below the background radiation

Radiocarbon Dating


Practice Problem: Radiocarbon Dating


Practice Problem: Radioactive Decay—U/Pb Dating


Practice Problem continued


Nonradioactive Nuclear Changes• Fission

– The large nucleus splits into two smaller nuclei.

• Fusion – Small nuclei can be accelerated to smash together to make a

larger nucleus.

• Both fission and fusion release enormous amounts of energy.– Fusion releases more energy per gram than fission does.


Fission Chain Reaction

• A chain reaction occurs when a reactant in the process is also a product of the process.– In the fission process it is the neutrons.– So, you need only a small amount of neutrons to start

the chain.

• Many of the neutrons produced in fission are either ejected from the uranium before they hit another U-235 or absorbed by the surrounding U-238.

• The minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass.


• Fissionable isotopes include U-235, Pu-239, and Pu-240.

• Natural uranium is less than 1% U-235.– The rest is mostly U-238.– There is not enough U-235 to sustain chain reaction.

• To produce fissionable uranium, the natural uranium must be enriched in U-235.– To about 7% for “weapons grade”– To about 3% for reactor grade

Fissionable Material


Fission Chain Reaction Illustration


Nuclear Power

• Nuclear reactors use fission to generate electricity.– About 20% of U.S. electricity

is generated this way.

• Nuclear reactors use the fission of U-235 to produce heat.– The heat boils water, turning

it to steam.– The steam turns a turbine,

generating electricity.


Nuclear Power Plants versus Coal-Burning Power Plants

Nuclear Power Plants

• Use about 50 kg of fuel to generate enough electricity for 1 million people

• No air pollution

Coal-Burning Power Plants

• Use about 2 million kg of fuel to generate enough electricity for 1 million people

• Produce NO2 and SOx that add to acid rain

• Produce CO2 that adds to the greenhouse effect


• The fissionable material is stored in long tubes called fuel rods, which are arranged in a matrix.– Subcritical

• Between the fuel rods are control rods made of neutron-absorbing material.– Boron and/or cadmium– Neutrons needed to sustain the chain reaction

• The rods are placed in a material called a moderator to slow down the ejected neutrons.– Allows chain reaction to occur below critical mass

Nuclear Power Plants—Core


Nuclear Power Plant Reactor Core


• Core meltdown– Water loss from core; heat melts core– Chernobyl and Fukushima Daiichi

• Waste disposal– Waste highly radioactive– Reprocessing; underground storage?

• Federal high-level radioactive waste storage facility at Yucca Mountain, Nevada

• Transporting waste

• Dealing with old, no longer safe nuclear power plants – Yankee Rowe in Massachusetts

Concerns about Nuclear Power


Where Does the Energy from Fission Come From?

• During nuclear fission, some of the mass of the nucleus is converted into energy.– E = mc2

• Each mole of U-235 that fissions produces about 1.7 × 1013 J of energy.– A very exothermic chemical reaction produces

106 J per mole.


• When a nucleus forms, some of the mass of the separate nucleons is converted into energy.

• The difference in mass between the separate nucleons and the combined nucleus is called the mass defect.

• The energy that is released when the nucleus forms is called the binding energy.– 1 MeV = 1.602 × 10−13 J – 1 amu of mass defect = 931.5 MeV– The greater the binding energy per nucleon, the more

stable the nucleus.

Mass Defect and Binding Energy


Binding Energy Curve


Mass Defect and Binding Energy: Conversion of Mass to Energy

Mass lost (m) = 236.05258 amu – 235.86769 amu = 0.18489 amu

0.18489 amu × (1.66054 × 10–27 kg/1 amu) = 3.0702 × 10–28 kg

Energy produced: E = mc2

E = 3.0702 × 10–28 kg (2.9979 × 108 m/s)2

E = 2.7593 × 10–11 J


Practice Problem: Mass Defect and Nuclear Binding Energy


• Fusion is the combining of light nuclei to make a heavier, more stable nuclide.

• The sun uses the fusion of hydrogen isotopes to make helium as a power source.

• It requires high input of energy to initiate the process.– Because it needs to overcome

repulsion of positive nuclei

• It produces 10 times the energy per gram as fission.

• It produces no radioactive by-products.

• Unfortunately, the only currently working application is the H bomb.

Nuclear Fusion


• Rutherford discovered that during the radioactive process, atoms of one element are changed into atoms of a different element.– This process is referred to as transmutation.

• Showing that statement 3 of Dalton’s atomic theory is not valid all the time—only for chemical reactions

• For one element to change into another, the number of protons in the nucleus must change!

Transmutation


Making New Elements: Artificial Transmutation

• High-energy particles can be smashed into target nuclei, resulting in the production of new nuclei.

• The particles may be radiation from another radionuclide or charged particles that are accelerated.– Rutherford made O-17 by bombarding N-14 with alpha

rays from radium.– Cf-244 is made by bombarding U-238 with C-12 in a

particle accelerator.


Artificial Transmutation• It involves the bombardment of one nucleus with another,

causing new atoms to be made.– Can also bombard with neutrons

• The reaction is done in a particle accelerator.– Linear– Cyclotron


Cyclotron


Effects of Radiation on Life

• Radiation has high energy—enough energy to knock electrons from molecules and break bonds.– Ionizing radiation

• Energy transferred to cells can damage biological molecules and cause malfunction of the cells.

• Acute radiation damage:– High levels of radiation over a short period of time kill large numbers

of cells.• From a nuclear blast or exposed reactor core

– It causes weakened immune system and lower ability to absorb nutrients from food.• May result in death, usually from infection


• Low doses of radiation over a period of time show an increased risk for the development of cancer.– Radiation damages DNA that may not get repaired

properly.

• Low doses over time may damage reproductive organs, which may lead to sterilization.

• Damage to reproductive cells may lead to genetic defects in offspring.

Chronic Effects: Genetic Defects and Cancer Risks


• The curie (Ci) is an exposure of 3.7 × 1010 events per second.– No matter the kind of radiation

• The gray (Gy) measures the amount of energy absorbed by body tissue from radiation.– 1 Gy = 1 J/kg body tissue

• The rad also measures the amount of energy absorbed by body tissue from radiation.– 1 rad = 0.01 Gy

• A correction factor is used to account for a number of factors that affect the result of the exposure. – This biological effectiveness factor is the RBE, and the result is the

dose in rems.– Rads × RBE = rems– Rem = roentgen equivalent man

Measuring Radiation Exposure


Table of Radiation Exposure by Source in the United States


1. More energetic radiation has a larger effect.

2. More ionizing radiation penetrates human tissue more deeply.– Gamma >> Beta > Alpha

3. More ionizing radiation has a larger effect.– Alpha > Beta > Gamma

4. Characteristics of the radionuclide are as follows:The radioactive half-life of the radionuclideThe biological half-life of the elementThe physical state of the radioactive material

Factors that Determine the Biological Effects of Radiation


Biological Effects of Radiation

• The amount of danger of radiation to humans is measured in rems.


Nuclear Medicine

• Changes in the structure of the nucleus are used in many ways in medicine.

• Nuclear radiation can be used to visualize or test structures in your body to see if they are operating properly.– For example, labeling atoms

so their intake and output can be monitored

• Nuclear radiation can also be used to treat diseases because the radiation is ionizing, allowing it to attack unhealthy tissue.


• Certain organs absorb most or all of a particular element.

• Radiotracers can measure the amount of an element absorbed by using tagged isotopes of the element and a Geiger counter.

– Tagged = radioisotope that can then be detected and measured– They use a radioisotope with a short half-life.– They use a radioisotope that is low ionizing.

• Beta or gamma

Medical Uses of Radioisotopes, Diagnosis


• PET scan– Positron emission tomography

• It employs glucose tagged with F-18.• F-18 is a positron emitter.

– Brain scan and function

Medical Uses of Radioisotopes, Diagnosis


• Cancer treatment– Cancer cells are more sensitive to

radiation than healthy cells; radiation can be used to kill cancer cells without doing significant damage.

• Brachytherapy– Places radioisotope

directly at site of cancer• Teletherapy

– Uses gamma radiation from Co-60 outside to penetrate inside

– IMRT• Radiopharmaceutical therapy

– Uses radioisotopes that concentrate in one area of the body

Medical Uses of Radioisotopes, Treatment—Radiotherapy


Nonmedical Uses of Radioactive Isotopes• Smoke detectors

– Am-241– Smoke blocks ionized air; breaks circuit

• Insect control– Sterilize males

• Food preservation

• Radioactive tracers– Follow progress of a “tagged”

atom in a reaction

• Chemical analysis– Neutron activation analysis


• Authenticating art objects – Many older pigments and

ceramics were made from minerals with small amounts of radioisotopes.

• Crime scene investigation• Measuring thickness or

condition of industrial materials– Corrosion– Track flow through process– Gauge in high-temperature

processes – Weld defects in pipelines– Road thickness

• Agribusiness– Develop disease-resistant

crops– Trace fertilizer use

• Enhancing data integrity in computer disks

• Nonstick pan coatings– Initiate polymerization

• Keeping paper from jamming in photocopiers

• Sterilizing cosmetics, hair products, contact lens solutions, and other personal hygiene products

Nonmedical Uses of Radioactive Isotopes

© 2015 pearson education, inc. chapter 21 lecture presentation radioactivity and nuclear chemistry...

Documents

pearson education

mass number

energy rays

atoms nucleus

new nucleus

radioactive decay

uranic rays

atomic number z atoms