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Chapter 20. Nuclear Chemistry. HISTORY. Radioactivity was discovered by Henry Bequerel in 1896 by observing uranium salts emit energy. Madame Curie and her husband extended Bequerel’s work on radioactivity

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Chapter 20

Chapter 20

Nuclear Chemistry


History

HISTORY

Radioactivity was discovered by Henry Bequerel in 1896 by observing uranium salts emit energy.

Madame Curie and her husband extended Bequerel’s work on radioactivity

Curie was the first to use Radioactivity to describe the spontaneous emission of alpha, beta, and gamma particles from an unstable nucleus.

Both Curies suffered the effects of radiation poisoning.

Rutherford took over and bombarded gold with alpha particles


Identification of radiation

Identification of Radiation


Penetrating effects

Penetrating Effects


Nuclear chemistry

Nuclear Chemistry

  • Nuclear chemistry is the study of reactions that involve changes in the nuclei of atoms.

  • Radioactive decay is the spontaneous disintegration of alpha, beta, and gamma particles.

  • Radioactive decay follows first-order kinetics.


Nuclear stability

NUCLEAR STABILITY

Kinetic stability describes the probability that a nucleus will undergo decomposition to form a different nucleus (radioactive decay)

Thermodynamic Stability - the potential energy of a particular nucleus as compared with the sum of the potential energies of its component protons and neutrons. (Binding energy)


Stability radioactive decay

Stability radioactive decay

Light elements are stable if the neutrons and protons are equal, i.e. 1:1 ration, heavier nuclides require a ratio of 1:1.5

Magic numbers of protons and neutrons seem to exist, much like 8 electrons to make elements and ions Nobel.


Magic numbers

Magic Numbers

Even number protons and neutrons are stable compared to odd ones

Magic numbers (protons or neutrons) 2,8,20,28,50,82 and 126

For example tin has 10 stable isotopes, even number, but on either side of elemental tin indium and antimony have only two stable isotopes

Nuclei with magic numbers of both protons and neutrons are said to be “doubly magic” and even more stable i.e. Helium-4 two protons and two neutrons and Pb-208 with 82 protons and 126 neutrons

Could be shells for nucleons, like electrons


Number of stable nuclides related to numbers of protons and neurons

Stability radioactive decay

Number of stable nuclides related to numbers of protons and neurons


Nuclear stability1

Nuclear Stability


Stability radioactive decay1

Stability radioactive decay

Nucleons are protons and neutrons

The strong nuclear force keeps the nucleus together by overcoming the repulsive force of the protons.

Neutrons are present to help dissipate the repulsive forces between the protons

As the atomic number (number of protons) increases, so does the number of neutrons to shield the repulsion of the protons

All nuclides with 84 or more protons are unstable and radioactive. This means the strong force is only strong enough neutralize the force of 84 protons.


Decay series

Decay Series


The half lives of nuclides in the 238 92 u decay series

The Half-Lives of Nuclides in the 23892U Decay Series


Types of radioactive decay

Types of Radioactive Decay

Decay processes

Neutron-rich nuclei, converts a neutron to a proton, thus lowering the neutron/proton ration

Neutron-poor nuclei, net effect of converting a proton to a neutron thus causing an increase neutron/proton ratio

Heavy nuclei, Z>200 just unstable regardless of the neutron/proton ratio, just too many positive protons


Decay types

Decay Types

Alpha particle emitters (Mass number changes)

Nuclei with atomic mass number>200

The daughter nuclei contains two fewer protons and two fewer neutrons than the parent

U-238, Th-230


Types of radioactive decay1

Types of Radioactive Decay

Decay processes

Neutron-rich nuclei, converts a neutron to a proton, thus lowering the neutron/proton ration

Neutron-poor nuclei, net effect of converting a proton to a neutron thus causing an increase neutron/proton ratio

Heavy nuclei, Z>200 just unstable regardless of the neutron/proton ratio, just too many positive protons


Decay types1

Decay Types

Beta particle decay

Too many neutrons

Atomic number increases, thus more protons

Neutron splits into a proton and electron called transmutation.

n → P + β

Examples Th-234, I-131

1

o

1

0

-1

1

234

0

234

e

Th

Pa

+

-1

91

90


Decay types2

Decay Types

Electron Capture

Neutron-poor nuclides

Electron in an inner shell reacts with a proton 11P+ 0-1β → 10n

AZX + 0-1β → AZ-1X’ + x-ray

No change in mass number

Example iron-55


Decay types3

Decay Types

Positron Emission

Neutron-poor nuclides

Same mass as an electron, but opposite charge, the positron emission is opposite beta decay

11P → 10n + 0+1β

AZX → AZ-1X’ + 0+1β

Example C-11


Decay types4

Decay Types

Electron Capture

Neutron-poor nuclides

Electron in an inner shell reacts with a proton 11P+ 0-1β → 10n

AZX + 0-1β → AZ-1X’ + x-ray

No change in mass number

Example iron-55


Decay types5

Decay Types

Gamma Emission 00γ

Many nuclear decay daughters are in an elevated, or excited, energy state

These meta stable isotopes emit gamma rays to lower their potential energy

This emission can be instantaneous, or delayed for sever hours

Te-99m has a half life of about 6 hours

9843Tc* → 9843Tc + 00γ


Decay types6

Decay Types

Spontaneous Fission

Very massive nuclei Z > 103

Usually large amounts of energy are released

Usually neutrons are released

Example:

25498Cf → 11846Pd + 13252Te + 4 10n


Various types of radioactive processes showing the changes that take place in the nuclides

Decay Types

Various Types of Radioactive Processes Showing the Changes That Take Place in the Nuclides


Radioactive decay

Radioactive Decay


Radiochemical dating

Radiochemical Dating

n = t/t1/2

t - time, t1/2 - time for a half-life, and n - the number of half-lives

At/Ao = 0.5n

Ao - amount initially present, At - amount at time t, and n - the number of half-lives

If we know what fraction of sample is left (At/Ao) and its half-life (t1/2), we can calculate how much time has elapsed.


Radiocarbon dating of artifacts

Radiocarbon Dating of Artifacts


Calibration curves

Calibration Curves


Kinetics of radioactive decay

Kinetics of Radioactive Decay

Radioactive decay is a first order process, but using atoms instead of concentration

Radioactive decay rates

Activity is defined as the number of nuclei that decay per unit time

A = -ΔN/Δt, the units are usually disintegrations per second or minute (dps), dpm

The activity is directly proportional to the number of atoms, thus A(Rate)=kN

From Che162 we know the first order rate law is lnN/N0 = -kt

Also t1/2 ln1/2N0/N0 = -kt1/2 → t1/2 = 0.693/k


Example problem

Example problem

Fort Rock Cave in Oregon is the site where archaeologists discovered several Indian sandals, the oldest ever found in Oregon. Analysis of the 14C/12C ratio of the sandals gave an average decay rate of 5.1 dpm per gram of carbon. Carbon found in living organisms has a C-14/C-12 ratio of 1.3 X 10-12, with a decay rate of 15 dpm/g C. How long ago was the sage brush in the sandals cut? The half life of carbon-14 is 5730 years. Note dpm is disintegrations per minute


Sample problem solution

Sample Problem Solution

First calculate the rate constant k from the half-life: k=0.693/5730 = 0.000121 yr-1

Substitute into the first order rate equation.

ln(N/N0) = kt

t = ln(N/N0)/k = ln(15/5.1)/0.000121

t = 8910 years oldsandals


Practice

Practice

A mammoth tusk containing grooves made by a sharp stone edge (indicating the presence of humans or Neanderthals) was uncovered at an ancient camp site in the Ural Mountains in 2001. The 14C/12C ratio in the tusk was only 1.19% of that in modern elephant tusks. How old is the mammoth tusk?


Practice1

Practice

Radioactive radon-222 decays with a loss of one  particle. The half-life is 3.82 days. What percentage of the radon in a sealed vial would remain after 7.0 days?


Nuclear transformations

Nuclear Transformations

Rutherford (1919) was the first to carry out a bombardment reaction, when he combined an alpha particle with nitrogen-14, creating oxygen-17 and a proton

The next successful bombardment reaction was done 14 years later when Aluminum-27 to make phosphorus-30 and a neutron

If the bombarding particle has a positive charge then repulsion by the nucleus hinders the process, thus particle accelerators are required.

Cyclotron and linear accelerator pg850

Neutrons, do not suffer from the repulsive effect

Synthetic elements have been made, called transuranium elements


Cyclotron

Cyclotron

Nuclear reactions can be induced by accelerating a particle and colliding it with the nuclide.


An aerial view of fermilab a high energy particle accelerator cyclotron

Cyclotron

An Aerial View of Fermilab, a High Energy Particle Accelerator Cyclotron.


The accelerator tunnel at fermilab

The Accelerator Tunnel at Fermilab


Linear accelerator

Linear Accelerator


Linear accelerator cyclotron

Linear AcceleratorCyclotron


Detection and uses of radioactivity

Detection and Uses of Radioactivity

Geiger counter, high energy from radioactive substances ionizes the Ar, thus allowing a current to flow. The more ions the more current, thus more radioactive

Scintillation counter, measures the amount of light given off by a phosphor such as ZnS, which is measured by a photometer

Badges


Geiger counter

Geiger Counter

One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.

The ionizing radiation creates ions, which conduct a current that is detected by the instrument


Geiger counter1

Geiger Counter


Thermodynamic stability

Thermodynamic Stability

  • This is done by comparing the mass of the individual protons and neutrons to the mass of the nucleus itself. The difference in mass is called the mass defect (Δm), which when plugged into E = mC2, or ΔE = ΔmC2 for change in energy

  • The mass of an atom is always less than the mass of the subatomic particles

    • Protium is the only exception, since there is no defect

    • The other isotopes of hydrogen deuterium and tritium have defects

    • Mass of neutron = 1.008665 amu

    • Mass of proton = 1.007276 amu

    • Mass of electron = 0.0005446623amu, note mass of electron is not really necessary in calculations since it subtracts out when finding the difference


Subatomic particles

Subatomic Particles

ParticleMass(g)Charge

Electron(e)9.11 x 10-28 -1

Proton(p)1.67 x 10-24 +1

Neutron(n)1.67 x 10-24 0

Particle6.64 x 10-24 +2

Positron9.11 x 10-28 +1


Thermodynamic stability1

Thermodynamic Stability

  • Just like a molecule is more stable that its atoms, an nucleus in more stable than its individual atoms.

  • Energy changes for nuclear process are extremely large when compared to normal chemical and physical changes, thus very valuable energy source.

  • Normal units are expressed per nucleon, in MeV (million electron volts)

  • MeV = 1.60 X 10-13 J OR amu = 931 MeV

  • All nuclei have different relative stabilities, see figure 18.9


Sample problem

Sample problem:

  • Calcualte the changes in mass (in amu) and energy (in J/mol and eV/atom) that accompany the radioactive decay of 238U to 234Th and an alpha particle. The alpha particle absorbs two electrons from the surrounding matter to form a helium atom.

    Solution (Note: AMU = g/mole)

    Δm = mass prod. – mass react.

    Δm = (mass 234Th + mass 42He) - mass238U

    Δm = (234.43601 + 4.002603) - 238.050788 =

    -0.004584 amu or -4.584X10-6kg

    ΔE = mC2↔ ΔE =( -4.584X10-6kg)(2.998X108m/s)2

    =-4.120X1011j/mole

    ΔE = -0.004584 amu X 931 MeV/amu

    Divide by the mass number to get energy per nucleon, called binding energy


Practice2

Practice

What is the binding energy of 60Ni? The mass of a 60Ni atom is 59.9308 amu. The mass of an electron is 9.10939 x 10-31 kg and 1 amu is 1.66054 x 10-27 kg.


Thermodynamic stability2

Thermodynamic Stability

Revisiting the graph on page 988

  • Notice that Iron is the most stable nuclide

  • ∆E is negative when a process goes from a less stable to a more stable state

  • In nuclear reactions more stable nuclei can be achieved by combining nuclei (fusion) or splitting a nucleus (fission)

  • Lighter elements typically undergo fusion, while elements heavier than iron undergo fission.


Thermodynamic stability3

Thermodynamic Stability

  • For lighter elements, fusion processes lead to nuclei with greater binding energy, whereas heavy elements are formed through other processes.


Artificial elements

Artificial Elements

Scientists have been transmuting elements since 1919 when oxygen-17 and hydrogen-1 were produced from nitrogen-14 and  particles.

147N + 42He 178O + 11H

Artificial transmutation requires bombardment with high velocity particles.

Alpha particles are positivly charged so how do they strike the nucleus, since the nucleus is positivly charged?


Energy in nuclear reactions

Energy in Nuclear Reactions

  • In the types of chemical reactions we have encountered previously, the amount of mass converted to energy has been minimal.

  • However, these energies are many thousands of times greater in nuclear reactions.


Energy in nuclear reactions1

Energy in Nuclear Reactions

For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g.

The change in energy, E, is then

E = (m) c2

E= (−4.6  10−6 kg)(3.00  108 m/s)2

E= −4.1  1011 J


Fission process

Fission Process

  • Discovered in the 1930’s when U-235 was bombarded with neutrons

  • Neutrons, due to their neutral charge do not require accelerators

  • 11n + 23592U → 14156Ba + 9236Kr + 3 11n

    • This process delivers 2.1X1013J/mole, compared to 8.0X105j of energy for the combustion of methane

    • About 26 million times more energy

    • Another splitting process produces the elements Te-137 and Zr-97, with two neutrons

    • There are 200 different isotopes of 35 different element produced, thus the nucleus fragments in many different ways


Fission process1

Fission Process

  • Since neutrons are produced, then it is possible to have a self-sustaining reaction

    • If the average production of neutrons is less than one, the reaction is called subcritical

    • If the neutron production is equal to one then it is called critical

    • If the neutron production is greater than one then the reaction is called super-critical

    • To achieve the a critical state, then a critical mass is required

    • If the mass is too small then the neutrons escape before splitting other nuclei


Nuclear fission

Nuclear Fission

  • How does one tap all that energy?

  • Nuclear fission is the type of reaction carried out in nuclear reactors.


Nuclear fission1

Nuclear Fission

Bombardment of the radioactive nuclide with a neutron starts the process.

Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.


Nuclear fission2

Nuclear Fission

If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.


Nuclear fission3

Nuclear Fission

Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: critical mass.


Nuclear reactors

Nuclear Reactors

  • In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator


Nuclear reactors1

Nuclear Reactors

The reaction is kept in check by the use of control rods.

These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.


Fusion process

Fusion Process

  • Combining of nuclei, such as the reaction the occurs on the sun

  • Problem is that the nuclei are positive in charge, thus high temperatures (4X 107K) necessary to give the nuclei the correct amount of kinetic energy to overcome the repulsion

    • Electric current heat

    • Laser heat

  • Because to the high temperature then what about containment?


Nuclear fusion

Nuclear Fusion

Fusion would be a superior method of generating power.

The good news is that the products of the reaction are not radioactive.

The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins


Hydrogen fusion

Hydrogen Fusion

  • Heavier elements formed through the process of fusion.


Effects of radiation

Effects of Radiation

  • What happens when one is exposed to radiation?

  • Somatic damage is damage to the organism itself

  • Genetic damage is damage to the genetic machinery, RNA DNA for example

  • Damage depends on the following factors


Quantities of radiation

Quantities of Radiation


Damage factors

Damage Factors

  • The energy of the radiation, measured in rads ( radiation absorbed dose), where one rad = 10-2 J of energy deposited per kg of tissue

    • Since different radioactive particles do different kinds of damage the rad is not the best way to consider the effects

  • Penetrating ability of the radiation

    • Gamma highly penetrating, since electromagnetic energy consisting of photons

    • Beta particles penetrate up to one cm

    • Alpha particles are stopped by the skin


Damage factors1

Damage Factors

  • Ionizing ability of the radiation

    • Gamma radiation only occasionally ionize

    • Alpha particles, highly ionizing and leave a trail of damage, since it is an ion itself, it will strip electrons from other substances

  • Chemical properties of the radiation source.

    • Inert nuclides such as the noble gases pass through the body

    • A radioactive substance such as iodine, can be concentrated in a specific location of the body. For iodine it is the thyroid.

    • rem= rads X RBE


About rem

About REM

  • rem is the radiation equivalent in man

  • rbe is the relative effectiveness of the radiation in causing biologic damage, which is one for betta and gamma, and 20 for alpha

  • Alpha particles have a higher rbe than beta and gamma, since the helium nuclei is much larger.


Typical radiation exposures for a person living in the united states 1 millirem 10 3 rem

Typical Radiation Exposures for a Person Living in the United States (1 millirem = 10-3 rem)


Sources of radiation

Sources of Radiation


Biological effect of radiation

Biological Effect of Radiation


Radon gas release from rocks

Radon Gas Release from Rocks


Radiation therapy

Radiation Therapy


Medical imaging radionuclides

Medical Imaging Radionuclides


Synthesis of the elements in stars

Synthesis of the elements in stars

  • Stars are formed from the gravitational attraction of interstellar dust, mostly hydrogen

  • The density gradually increases reaching a density of about 100g/mL, with a temperature of about 1.5X107 K

  • At this point hydrogen begins to fuse into He-4, releasing energy, like our sun

  • The overall reaction is 4 protium atoms combining to make helium and 2 beta particles plus 2 photons of gamma radiation

  • The helium then concentrates in the core of the star, thus increasing the density and temperature, thus becoming a red giant star


Synthesis of the elements in stars1

Synthesis of the elements in stars

  • At a temperature of about 2X108 K, the helium nuclei begin to fuse producing Be-8

  • Be-8 is unstable due to low neutron numbers, and absorbs alpha particles creating C, O, Ne, Mg

  • The next stage is formation of a red supergiant star, where Na, Si, S, Ar, and Ca are produced

  • Next in the progressions is the formation of a massive red supergiant star, where Fe and Ni are formed by proton-neutron exchange reactions

    • Finally the supernova is produced where elements with Z>28 being formed by multiple neutron captures


Red giant

Red Giant


Supernova

Supernova


Chemtour half life

ChemTour: Half-Life

Click to launch animation

PC | Mac

Students develop and test their understanding of the concepts of half-life and carbon dating by manipulating interactive graphs and working Practice Exercises.


Chemtour fusion of hydrogen

ChemTour: Fusion of Hydrogen

Click to launch animation

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This ChemTour demonstrates the process by which hydrogen nuclei fuse to form helium nuclei. This nuclear reaction fuels the sun and stars and is the first step in the synthesis of heavier elements.


Chemtour modes of radioactive decay

ChemTour: Modes of Radioactive Decay

Click to launch animation

PC | Mac

This ChemTour presents animated explanations of alpha decay, beta decay, positron emission, and electron capture.


Chemtour balancing nuclear reactions

ChemTour: Balancing Nuclear Reactions

Click to launch animation

PC | Mac

This quantitative exercise teaches nuclear equation balancing through worked examples and Practice Exercises.


Chemtour half life1

ChemTour: Half-Life

Click to launch animation

PC | Mac

Students develop and test their understanding of the concepts of half-life and carbon dating by manipulating interactive graphs and working Practice Exercises.


Chemtour fusion of hydrogen1

ChemTour: Fusion of Hydrogen

Click to launch animation

PC | Mac

This ChemTour demonstrates the process by which hydrogen nuclei fuse to form helium nuclei. This nuclear reaction fuels the sun and stars and is the first step in the synthesis of heavier elements.


Chemtour modes of radioactive decay1

ChemTour: Modes of Radioactive Decay

Click to launch animation

PC | Mac

This ChemTour presents animated explanations of alpha decay, beta decay, positron emission, and electron capture.


Chemtour balancing nuclear reactions1

ChemTour: Balancing Nuclear Reactions

Click to launch animation

PC | Mac

This quantitative exercise teaches nuclear equation balancing through worked examples and Practice Exercises.


End chapter 20 nuclear chemistry

End Chapter #20Nuclear Chemistry


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