chapter 2 nuclear chemistry - copy

8/13/2019 Chapter 2 Nuclear Chemistry - Copy

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GENERAL CHEMISTRY

CHAPTER 2: NUCLEAR CHEMISTRY

Nuclear chemistry is the study of reactions involving changes in atomic nuclei.

This branch of chemistry began with the discovery of natural radioactivity by

Antoine Becquerel and grew as a result of subsequent investigations by Pierre

and Marie Curie and many others.

Nuclear chemistry is very much in the news today. In addition to applications in

the manufacture of atomic bombs, hydrogen bombs, and neutron bombs, even

the peaceful use of nuclear energy has become controversial, in part because of

safety concerns about nuclear power plants and also because of problems with

radioactive waste disposal.

In this chapter, we will study nuclear reactions, the stability of the atomic nucleus,

radioactivity, and the effects of radiation on biological systems.


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GENERAL CHEMISTRY

CHAPTER 2: NUCLEAR CHEMISTRY

2.1RadioactivityNuclear reaction Chemical reaction

Elements may be converted fromone to another

No new elements can be produced

Particles within the nucleus are

involved

Only the electrons participate

Nuclide Is an atom characterized by a definite atomic

number and mass number

Atomic number (Z) The number of protons in a nuclide

Mass number (A) The total number protons and neutrons

1. Isotopes: Atoms of the same element can have different numbers of neutrons

For example, the most common isotope of hydrogen has no neutrons at all.

There is also a hydrogen isotope called deuterium with one neutron and

another tritium with two neutrons.

Lithium-6 Lithium-7 Lithium-8

No. of electron 3 3 3

No. of proton 3 3 3

No. of neutron 3 4 5

2. Two types of nuclear reactions:

Radioactive Decay

In this process the nucleus spontaneously disintegrates (decay), giving off

radiation.

Nuclear bombardment reactions

A nucleus is bombardedby another nucleus or a nuclear particle.

3. Nuclear decay reactions : For example, if we had a sample of an element whose half

life had been determined to be ten days, all we could be sure of is that in ten days

only half the original specimen would remain -- the other half having beentransformed into some other collection of nuclides and/or decay particles.


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GENERAL CHEMISTRY

4. In a nuclear bombardment reaction, a nucleus is struck by another nucleus or by a

nuclear particle of sufficient energy to cause the original nucleus to be converted

into a product nucleus.

5. Nuclear equations:

Decay of uranium -238

You can represent the uranium-238 nucleus by the nuclide symbol

The products are thorium-234 and helium-4

The superscript equals the mass number

The subscript equals the atomic number

Potassium-40 is a naturally occurring radioactive isotope. It decays to

calsium-40 by beta emission.

6. Nuclear Stability:

How to use simple rules to predict the stability of nuclei?

Ask the following questions:

i. Is the atomic number Z > 83?

ii. Is atomic number or mass number a magic number?

iii. Are the nucleons paired?

iv. Does the nucleus lie in the band of stability?

(i) Is atomic number > 83?

o The band of stability terminate at Z = 82. Nuclides of Z > 83 are not

stable. Nuclides of Z > 83 often decay by alpha emission.

o It appears that when the number of protons becomes very large, the

proton-proton repulsions become so great that stable nuclides are

impossible.

o No stable nuclides are known with atomic numbers greater than 83 .

(ii)Magic Number

o The shell model of the nucleussays that protons and neutrons exist in

shells.


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GENERAL CHEMISTRY

o This explains why nuclei with certain number of protons and neutrons

appear to be stable.

o These numbers are called magic numbers.

o A magic number is the number of nuclear particles in a completed shell of

protons or neutrons.

o Helium contains 2 protons and 2 neutrons.

o That is contains a magic number of protons (2) and a magic number of

neutrons (2)

o Calcium contains 82 protons ( magic number) , 126 neutrons (magic

number)

Proton Neutron

2

8

20

28

50

82

114

2

8

20

28

50

82

126

(iii)Are the Nucleons Paired?

o Nuclides with even numbers of nucleons enjoy special stability.

o In the odd-even rule, when the numbers of neutrons and protons in the

nucleus are both even numbers, the isotope tends to be far more stablethan when they are both odd.

o Out of all the 264 stable isotopes, only 5 have both odd numbers of both,

whereas 157 have even numbers of both and the rest have a mixed

number.

(iv)Band of Stability

o A plot of the number of protons (Z) versus the number of neutrons (N)

reveals the band of stability, the region in which stable nuclides lie in.

o For nuclides up to Z=20, the ratio of neutrons to protons is about 1.0 to

1.1.

o AsZincreases, the neutron-to-proton ratio increases to about 1.5.

o This is due to increasing repulsions of protons from their electric charges.


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GENERAL CHEMISTRY

o More neutrons are required to give attractive nuclear forces to balance

these repulsions.

7. Types of nuclear radioactive decay:

i. Alpha emissions

o Emission of a nucleus or alpha particle

o All nuclides with Z > 83 are radioactive

o Example: radioactive decay of radium-226

o The product nucleus has an atomic number that is two less and

mass number that is four less than original nucleus

ii. Beta emission

o Emission of a high-speed electron from an unstable nucleuso Neutron-to-proton ratio too large


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GENERAL CHEMISTRY

o beta emission is equivalent to the conversion of a neutron to a proton

iii. Positron emission

o Positron is a particle identical to an electron in mass but having a

positive charge instead of a negative chargeo Positron emission is equivalent to the conversion of a proton to a

neutron.

o Neutron-to-proton ratio too small

o It occurs most often the lighter elements

iv. Electron capture

o Is about the decay of an unstable nucleus by capturing an electron

from an inner orbital of an atom.

o Neutron-to-proton ratio too small

o It occurs most often with heavier elements

v. Gamma emission

o Emission from an excited nucleus, corresponding to radiation with

a short wavelength of about 10-12m

o The excited state is an unstable and goes to a lower energy state

with the emission of electromagnetic radiation

o A metastable nucleus is a nucleus in an excited state with a lifetime

ofat least one nanosecond

o The product nucleus is simply a lower-energy state of the original

nucleus, so there is no change of atomic number or mass number

vi. Spontaneous fission

o Is the spontaneousdecay of an unstable nucleus in which a heavy

nucleus of mass number greater than 89 splits into lighter nuclei

and energy is released.

o Uranium-236 can spontaneously undergo the following nuclear

reaction


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GENERAL CHEMISTRY

8. Radioactive Decay Series:

o Nuclides withZ > 83are

radioactive.

o Many of these nuclides

decay by alpha

emission by which a

nucleus reduces its

atomic number so that

it becomes more stable.

o If the nucleus has a verylarge atomic number,

such as uranium-238,

the product nucleus is

also radioactive.

o This gives rise to a

radioactive decay

series.

o Radioactive decay series, a sequence in which one radioactive nucleus decays to

a second, which then decays to a third, and so forth. Eventually, a stable nucleus,

which is an isotope of lead, is reached.


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GENERAL CHEMISTRY

Questions and Answers: 2.1 Radioactivity

a) 20.33 Rubidium-87, which forms about 28% of natural rubidium, is radioactive,

decaying by the emission of a single beta particle to strontium-87. Write the nuclear

equation for this decay of rubidium-87.

b) 20.34 Write the nuclear equation for the decay of phosphorus- 32 to sulfur-32 by

beta emission. A phosphorus-32 nucleus emits a beta particle and gives a sulfur-32

nucleus.

c) 20.35 Thorium is a naturally occurring radioactive element. Thorium-232 decays by

emitting a single alpha particle to produce radium-228. Write the nuclear equation

for this decay of thorium-232.

d) 20.36 Radon is a radioactive noble gas formed in soil containing radium. Radium-

226 decays by emitting a single alpha particle to produce radon-222. Write the

nuclear equation for this decay of radium-226.

e) 20.37 Fluorine-18 is an artificially produced radioactive isotope. It decays by

emitting a single positron. Write the nuclear equation for this decay.

f) 20.38 Scandium-41 is an artificially produced radioactive isotope. It decays by

emitting a single positron. Write the nuclear equation for this decay.

g) 20.39 Polonium was discovered in uranium ores by Marie and Pierre Curie.

Polonium-210 decays by emitting a single alpha particle. Write the nuclear equation

for this decay.


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GENERAL CHEMISTRY

h) 20.40 Actinium was discovered in uranium ore residues by Andr-Louis Debierne.

Actinium-227 decays by emitting a single alpha particle. Write the nuclear equation

for this decay.

i) 20.41 From each of the following pairs choose the nuclide that is radioactive. (One is

known to be radioactive, the other stable.) Explain your choice.

j) 20.43 Predict the type of radioactive decay process that is likely for each of the

following nuclides.

k) 20.44 Predict the type of radioactive decay process that is likely for each of thefollowing nuclides.


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GENERAL CHEMISTRY

2.2Nuclear Bombardment Reactions1. In 1919, Rutherford discovered that it is possible to change the nucleus of one

element into the nucleus of another element by processes that can be controlled in

the laboratory.

2. Transmutationis the change of one element to another by bombarding the nucleus of

the element with nuclear particles or nuclei.

3. Rutherford used a radioactive element as a source of alpha particles and allowed

these particles to collide with nitrogen nuclei.

4. Nuclear bombardment reaction is often referred to by an

abbreviated notation.

5. Transuranium elements:

o Transuranium elements are the chemical elements with atomic numbers

greater than 92.

o None of these elements are stable.

o They decay radioactively into other elements.

o [a] Produce neptunium

o Neptunium (Np) is produced by bombarding uranium-238 with neutron

o

This nucleus decayed in a few days by beta emission


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GENERAL CHEMISTRY

o [b] Produce plutonium

Questions and Answers: 2.2 Nuclear Bombardment Reactions

a) 20.47 Write the abbreviated notations for the following bombardment reactions.

b) 20.49 Write the nuclear equations for the following bombardment reactions.

c) 20.51 A proton is accelerated to 12.6 MeV per particle. What is this energy in kJ/mol?

d) 20.53 Fill in the missing parts of the following reactions.


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GENERAL CHEMISTRY

2.3Radiations and Matter: Detection and Biological Effects1. Radiation counters:

o Two types of devicesionization counters and scintillation countersare

used to count particles emitted from radioactive nuclei and other nuclear

processes.

o Ionization counters detect the production of ions in matter. Scintillation

counters detect the production of scintillations, or flashes of light.

2. A Geiger counter, a kind of ionization counter used to count particles emitted by

radioactive nuclei, consists of a metal tube filled with gas, such as argon.

o It is used to count particles emitted from radioactive nuclei

o Alpha and beta particles can be directly detected

o To detect neutrons, boron trifluoride is added. The alpha particles are

produced, then can be detected

3. A scintillation counter is a device that detects nuclear radiation from flashes of light

generated in a material by the radiation. Aphosphor is a substance that emits flashes

of light when struck by radiation


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GENERAL CHEMISTRY

4. Biological Effects and Radiation Dosage

o Although the quantity of energy dissipated in a biological organism from a

radiation dosage might be small, the effects can be quite damaging because

important chemical bonds may be broken.

o DNA in the chromosomes of the cell is especially affected, which interferes

with cell divisionn. Cells that divide the fastest, such as those in the blood-

forming tissue in bone marrow, are most affected by nuclear radiations.

2.4Rate of Radioactive Decay1. Rate of radioactive decay and half-life:

o The rate of radioactive decay is proportional to the

number of radioactive nuclei in the sample.

o Nt is the number of radioactive nuclei at time t

o kis the radioactive decay constant

o Half-life is defined as the time is takes for one-half of the

nuclei in a sample to decay

o The shape of exponential decay curve can be seen in the decay of a 1-g

sample of iodine-131:

o Note the inverse relationship between the decay constant and the halflife;

those compounds with very small decay constants have very long half-lives.


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GENERAL CHEMISTRY

o Example:Phosphorus-32 is a radioactive isotope with a half-life of 14.3 d. A

biochemist has a vial containing a compound of phosphorus-32. If the

compound is used in an experiment 5.5 d after the compound was prepared

what fraction f the radioactive isotope originally present remains? Suppose

the sample in the vial originally contained 0.28g of phosphorus-32. How

many grams remain after 5.5d?


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GENERAL CHEMISTRY

o Carbon-14 dating

o The age of organic object can be estimated by radiocarbon dating (14C).

o The decay reaction as below

o The half-life is 5730 yearso The 14C in the environment gives a specific activity of 15.3 disintegrations

per minute per gram of total carbon, that is 0.255 Bq/g

o Example: A piece of charcoal from a tree killed by the volcanic eruption gave

7 disintegrations of carbon-14 nuclei per minute per gram of total carbon.

Present-day carbon (in living matter) gives 15.3 disintegrations per minute

per gram of total carbon. Determine the date of the volcanic eruption. Recall

that the half-life of carbon-14 is 5730 years. Answer: 6500 years ago.

Questions and Answers: 2.4 Rate of Radioactive Decay

a) 20.57 Tritium, or hydrogen-3, is prepared by bombarding lithium-6 with neutrons.

A 0.250-mg sample of tritium decays at the rate of 8.94 1010disintegrations per

second. What is the decay constant (in /s) of tritium, whose atomic mass is 3.02 amu?

b) 20.59 Sulfur-35 is a radioactive isotope used in chemical and medical research. A0.48-mg sample of sulfur-35 has an activity of 20.4 Ci. What is the decay constant of

sulfur-35 (in /s)?

c) 20.60 Sodium-24 is used in medicine to study the circulatory system. A sample

weighing 5.2x10-6 g has an activity of 45.3 Ci. What is the decay constant of sodium-

24 (in /s)?


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GENERAL CHEMISTRY

d) 20.61 Tellurium-123 is a radioactive isotope occurring in natural tellurium. The

decay constant is 1.7 10-21/s. What is the half-life in years?

e) 20.62 Neptunium-237 was the first isotope of a transuranium element to be

discovered. The decay constant is 1.03x10-14/s. What is the half-life in years?

f) 20.63 Carbon-14 has been used to study the mechanisms of reactions that involve

organic compounds. The half-life of carbon-14 is 5.73 103 y. What is the decay

constant (in /s)?

g) 20.65 Gold-198 has a half-life of 2.69 d. What is the activity (in curies) of a 0.86-mg

sample?


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GENERAL CHEMISTRY

2.5Applications of Radioactive Isotopes1. Radioactive tracer is a very small amount of radioactive isotope added to a chemical,

biological or physical system to study the system.

2. We expect different isotopes of the same element are having nearly identical chemical

behavior

3. The use of tracers to establish that chemical equilibrium is a dynamic process. The

equilibrium of solid lead(II) iodideand its saturated solution is written as:

4. You prepare PbI2in two separate beakers

5. One beaker contains only natural iodine atoms with nonradioactive isotopes

6. The other beaker contains radioactive iodide ions [131I-]

7. Some of the solution, containing the radioactive iodide ion is now added to the

beaker containing nonradioactive iodide ion8. After a time, the solid lead iodide which was originally nonradioactive becomes

radioactive

9. This is evidence for a dynamic equilibrium , in which radioactive iodide ions in the

solution substitute for nonradioactive iodide ions in the solids

10.Radium-226 and radon-22 were used for cancer therapy

11.Technetium-99m is the radioactive isotope used most often to develop images of

internal body organ

12.Thallium-201 is radioisotope used to determine whether a person has heart disease

13.Iodine-131 used to measure thyroid activity

14.Iron-59 used to measure the rate of formation of red blood cells

2.6Mass Energy Equivalence1. The change of energy is related to changes of mass, according to the mass-energy

equivalence relation

2. Energy and mass are equivalent and are related by equation

3. Atom mass unit:

o The kilogram and gram are inconveniently large units for measuring themasses of single atoms (single nuclides)


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GENERAL CHEMISTRY

o The atomic mass unit (amu) is better suited for measurements on this scale

o The definition of this tiny unit of mass has been arranged so that Avogadros

number of atomic mass units equals 1g of mass

o 6.022X1023. amu =1g

o The mass of a single atom expressed in atomic mass units in numerically

equal to the mass of 1 mol of such atoms expressed in grams


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GENERAL CHEMISTRY

4. Nuclear binding energy:

o The binding energy is the energy needed to break a nucleus into itsindividual protons and neutrons

o The mass of the helium-4 atom is 4.00260 amu (see Table 20.3), so the mass

difference is:


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GENERAL CHEMISTRY

Questions and Answers: 2.6 Energy Mass Equivalence

a) 20.79Find the change of mass (in grams) resulting from the release of heat when 1

mol C reacts with 1 mol O2.

b) 20.81 Calculate the energy change for the following nuclear reaction (in joules per

mole of ). Give the energy change in MeV per

nucleus. See Table 20.3.

c) 20.82 Calculate the change in energy, in joules per mole of , for the following

nuclear reaction. Give the energy change in MeV per

nucleus. See Table 20.3.


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GENERAL CHEMISTRY

d) 20.83 Obtain the mass defect (in amu) and binding energy (in MeV) for the

nucleus. What is the binding energy (in MeV) per nucleon? See Table 20.3.

2.7Nuclear Fission and Nuclear Fusion1. Nuclear fission is a nuclear reaction in which a heavy nucleus splits into lighter nuclei

and energy is released.

2. For example, californium-252 decays both by alpha emission (97%) and by

spontaneous fission (3%).

3. There are many possible ways in which the nucleus can split.

4. One way is represented by the following equation:


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chapter 2 nuclear chemistry - copy

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