1

Chapter 19 Nuclear Chemistry I. Introduction

a. ordinary chemical reactions, as discussed in the previous chapters, involve changes in the outer electron structures of atoms or molecules.

b. Nuclear reactions result in changes taking place within the atomic nuclei.

c. Atomic Symbols A Review i. Examples

This atom has 6 protons, 6 electrons, and since the

atomic number is 12, then it has 6 neutrons.

This atom has 6 protons, 6 electrons, however, since the atomic number is 14, then it has 8 neutrons.

ii. Nuclei with the same number of protons, but a different

number of neutrons are called isotopes. II. Radioactivity

a. An atom with a radioactive nucleus will spontaneously decompose, or decay.

b. Energy is given off as the atoms nucleus decays. c. Natural radioactivity some (very few) atoms are naturally

radioactive; the nuclei decomposes on their own, without outside influences.

d. Artificial radioactivity - many atoms can be made to be radioactive in the laboratory by bombarding stable nuclei with high-energy particles.

!

6

12C

!

6

14C

2

e. Modes of decay naturally occurring radioactive nuclei commonly decompose by the following modes.

i. Alpha particle emission an ordinary helium nucleus,

!

2

4He

is given off. 1. Example: Uranium-238

!

92

238U "

2

4He +

90

234Th

2. When a nucleus decays by alpha emission, the mass

number decreases by 4, whereas the atomic number decrease by 2; consistent with the loss of a helium particle.

ii. Beta particle emission produces an electron, given the

symbol

!

"1

0e .

1. Example

!

90

234Th "

-1

0e +

91

234Pa

2. Notice that the product nucleus,

!

91

234Pa , has the same

mass number as the reactant,

!

90

234Th , but its atomic

number is one unit larger.

iii. Gamma radiation emission which consists of high energy photons. Because gamma emission changes neither the mass number nor the atomic number.

3

f. Modes of decay - artificial radioactive decay produces -emissions, -emissions, and -emissions when bombarded with high-energy particles.

i. Positron emission a positron is identical to an electron

except that it has a charge of +1 rather than -1. The symbol for a positron is

!

1

0e .

!

19

40K "

+1

0e +

18

40Ar

ii. K-electron capture an electron in the innermost energy

level (n=1) falls into the nucleus.

!

37

82Rb "

-1

0e +

36

82Kr

1. Electron capture is more common with heavy nuclei,

presumably because the n=1 level is closer to the nucleus.

2. Notice that the result of K-electron capture is the same as positron emission; the mass number remains unchanged whereas atomic number decreases by one unit.

g. Example: Promethium exists as two isotopes,

!

61

142Pm and

!

61

147Pm, and is essentially nonexisitent in nature. All isotopes

are radioactive. Write the balanced equations for the decomposition of a.

!

61

142Pm by positron emission; by K-electron capture.

!

61

142Pm "

1

0e +

60

142Nd |

!

61

142Pm "

-1

0e +

60

142Nd

b.

!

61

147Pm by beta emission.

!

61

147Pm "

1

0e +

62

142Sm

4

h. Bombardment Reactions i. Bombardment reactions are utilized to prepare radioactive

isotopes.

ii. Bombarding the nucleus of an atom with a stable nucleus will cause a radioactive nucleus to form.

iii. The newly formed radioactive nucleus will in turn decay

to form stable products.

iv. There are more than 1500 radioactive isotopes have been prepared in the lab.

v. The number of isotopes per element range from 1

(hydrogen and boron) to 34 (indium).

vi. Types of bombarding particles

a) A neutron (produced from a nuclear fission reaction, to be discussed later in this chapter).

1. Example A stable aluminum atom is bombarded by neutrons.

!

13

27Al +

0

1n"

13

28Al

the product nucleus, Al-28, is radioactive, which decays by beta emission.

!

13

28Al "

14

28Si +

-1

0e

5

b) A charged particle (for example, electron, positron, alpha particle) which can be accelerated to high velocities in electric and/or magnetic fields.

1. This added velocity allows the charged particle to acquire enough energy to cause a nuclear reaction.

2. Without the added acceleration, the charge particle would be repelled by the charged components of the atom.

3. The first laboratory-prepared radioactive isotopes were made in 1934 by Irene Curie and her husband, Frederic Joliot. They bombarded stable isotopes with high-energy alpha particles.

One example of their reactions

!

13

27Al +

2

4He "

15

30P +

0

1n

The product, Phosphorous-30, is radioactive, decaying by positron emission.

!

15

30P"

14

30Si +

1

0e

Transuranium elements those elements with atomic numbers greater than Uranium (92) have been synthesized in the lab.

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i. Applications 1. Medicine

a. Radioactive isotopes are used in cancer therapy to eliminate malignant cells left after surgery.

b. Cobalt-60, a gamma-ray emitter

(energy only) is focused on a small area where cancer is suspected.

c. Thyroid cancer can be treated

with radioisotopes of iodine

!

131I or

123I , since iodine moves

toward the thyroid gland when taken into the body.

d. Positron Emission Tomography

(PET) is a technique used to study brain disorders. Glucose prepared with carbon-11, a positron emitter is given to the patient. The brain is then scanned for positrons from the labeled glucose. This technique determined that the brain of a schizophrenic metabolizes only about 20% as much glucose as that of most people.

Diagnostic Uses of Radioactive Isotopes

Isotope Use

!

6

11C

PET brain scans

!

11

24Na

circulatory disorders

!

15

32P

detection of eye tumors

!

26

59Fe

anemia

!

31

67Ga

scan for lung tumors

!

34

75Se

pancreas scan

!

43

99Tc

imaging of the brain, liver, kidneys and

bone marrow.

!

54

133Xe

lung imaging

!

thalium

81

201Tl

heart disorders

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2. Chemistry

a. Chemical analysis i. Neutron activation analysis

1. Bombarding a sample with neutrons, which create a radioactive element that will emit gamma rays.

!

38

84Sr +

0

1 n"

38

85Sr

2. The magnitude of the energy change

and therefore the wavelength of the gamma ray will vary from one element to another, allowing for the identification of the element (Qualitative Analysis)

3. The intensity of the radiation depends

upon the amount of the element in the sample, allowing for quantitative analysis of samples as small as one picogram (10-12 g).

4. Neutron activation analysis is used in

archeology.

Plants have a high strontium content, so a high Sr content in bones would suggest a vegetarian diet. Measuring the amount of strontium in bones, it is possible to determine the diet of prehistoric humans.

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3. Commercial Applications a. Smoke detectors use small amounts of

radioactive elements (typically Americium (Am-241)). The decay of Am-241 will ionize air molecules within a small chamber. An electric field (created by a battery) causing the ions formed from the air to move across the chamber, creating an electric current.

If smoke gets into the chamber, the electric current is impeded, causing a circuit to detect the voltage drop, and to sound the alarm. As a fail-safe, the alarm will also go off if voltage drops due to a dead battery.

b. Gamma rays are often used to sterilize food

products, since they are known to kill insects, larvae, and parasites such as trichina that can cause trichnosis in pork. Since chemical preservatives in food can cause health issues, the use of gamma irradiation to sterilize foods is an attractive alternative.

Gamma-irradiation is also a means to sterilize plastic surgical implements and IV bags, and alternative to standard autoclaving procedures for stainless steel instruments, which can stand up to the temperatures (121 oC) and pressures found in an autoclave. Although there are autoclavable plastics available, gamma-irradiation as a method for sterilization is a suitable alternative.

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III. Rate of Radioactive Decay a. Radioactive decay is a first-order process, therefore the

following equations apply

!

rate = kX

lnX

0

X= kt

k =ln 2

t 12

=0.693

t 12

b. Because of the way in which rate of decay is measured, it is

often described bt the activity (A) of a sample, which expresses the number of atoms decaying in unit time. A new equation can be expressed as follows.

!

A = kN

Activity can be expressed in terms of the number of atoms decaying per second, or becquerels (Bq)

!

Bq = 1 atom

second Alternatively, activity may be cited in disintergrations per minute, or more commonly, in curies (Ci)

!

1 Ci = 3.700 " 10

10atom

second

Where k is the first order rate constant

!

t 12

is the half-life X is the present amount of the radioactive species X0 is the initial amount of the radioactive species at time 0.

Where A is the activity k is the first order rate constant N is the number of radioactive

nuclei present.

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c. Example: The half-life of radium-226 is 1.60 x 103 y, or 5.05 x 1010 s. Calculate k in s-1.

!

k = 0.693

5.05 "1010

s = 1.37 "10

#11/s

What is the activity in curies of a 1.00 g sample of Ra-226?

!

N = 1.00g " 1 mol Ra - 226

226g "

6.022 "1023atoms

1 mol"

1 nucleus

1 atom= 2.66 "1021 radioactive atomic nuclei

A = kN = 1.37 "10-11 /s( ) 2.66 "1021atoms( )1 Ci

3.700 "10 atomss

#

$ %

&

' ( = 0.985 Ci

What is the mass in grams of a sample of Ra-226 that has an activity of 1.00 x 109 atom/s?

!

N =A

k=

1.00 "109 atomss1.37 "10#11 s

= 7.30 "1019 atoms

mass Ra - 226 = 7.30 "1019 atoms( )1 mol

6.022 "1023 atoms

$

% &

'

( )

226 g

1 mol

$

% &

'

( ) = 0.0274 g

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d. Age of Organic Material i. Carbon-14 dating

1. Professor W.F. Libby in the 1950s is credited for creating this method of determining the age of organic (carbon containing) material.

2. Carbon-14 dating is based upon the known decay rate

of carbon-14. 3. Carbon-14 is produced in the atmosphere by the

interaction of neutrons from cosmic radiation with ordinary nitrogen atoms.

!

7

14N +

0

1n"

6

14C +

1

1H

4. The carbon-14 formed by this nuclear reaction is

eventually incorporated into the carbon dioxide of the air. A steady-state concentration of one

!

6

14C

atoms for every 1012

!

6

12C atoms, established in

atmospheric CO2. 5. The concentration of

!

6

14C is such that a sample

containing one gram of carbon has an activity of 13.6 atoms/min.

6. A living plant or a plant-eating human or animal is

assumed to have this same activity. When a plant or animal dies, the intake of radioactive carbon stops, and the process of radioactive decay takes over.

!

6

14 C " 714N + -1

0e t 12

= 5730 years

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7. The following equation can be used to determine the age of an organic sample.

!

ln A

0

A= kt

8. Example: A tiny piece of paper taken from the dead sea scrolls, believed to date back to the first century, A.D., was found to have an activity per gram of carbon of 10.8 atoms/min. Taking A0 to be 13.6 atoms/min, estimate the age of the scrolls.

!

k =0.693

t 12

=0.693

5730 years=1.21"10#4 per year

lnA0

A= kt

ln13.6 atoms/min

10.8 atoms/min= 1.21"10#4 /year( ) t

t =0.231

1.21"10#4 /year=1.91"103 years

9. Considerations. It is not easy to determine the activities of atoms that decay at the rate of 10 atoms per minute. Large samples must be used to increase the counting rate. Background radiation must be carefully excluded.

Where A0 is the original activity, assumed to be 13.6 atoms/min A is the measured activity today k is the first order rate constant t is the age of the sample

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IV. Mass-Energy Relations V. Nuclear Fission VI. Nuclear Fusion