chapter 37 nuclear chemistry - sfactl

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Chapter 37

Nuclear Chemistry

Copyright (c) 2011 by Michael A. Janusa, PhD. All rights reserved.

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37.1 Radioactivity Radioactive decay is the process in which a

nucleus spontaneously disintegrates, giving off

radiation.

Radiation are the particles or rays emitted.

Radiation comes from the nucleus as a result of

an alteration in nuclear composition or structure.

This occurs in a nucleus that is unstable and

hence radioactive.

Nuclear symbols are used to designate the nucleus and

consists of

- Atomic symbol (element symbol)

- Atomic number (Z : #protons)

- Mass number (A : #protons + #neutrons)

EA

Z

3

B11

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atomic symbol

atomic number Z number of

protons

mass number A number of

protons and neutrons

• This symbol is the same as writing boron-11 and

defines an isotope of boron.

• In nuclear chemistry this is often called a nuclide.

• This is not the only isotope (nuclide) of boron.

5 protons , 6 neutrons

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• Some isotopes are stable

• The unstable isotopes are the ones that

produce radioactivity (emit particles); the

actual process of radioactive decay.

• Radioactive decay is the process in which

nucleus spontaneously disintegrates,

giving off radiation.

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Radioactivity

• Radioactivity was discovered by Antoine

Henri Becquerel in 1896.

– The work involved uranium salts which lead to the

conclusion that the minerals gave off some sort of

radiation.

– This radiation was later shown to be separable by

electric (and magnetic) fields into three types;

alpha ( ), beta ( ), and gamma ( ) rays.

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Radioactivity – Alpha rays ( ) bend away from a positive plate

indicating they are positively charged.

– They are known to consist of helium-4 nuclei

(nuclei with two protons and two neutrons).

– Slow moving (relatively large mass compared to

other nuclear particles; therefore, moves slow -

10% speed of light)

– Stopped by small barriers as thin as few pages of

paper.

– Symbolized in the following ways:

α α He He 4

2

4

2

2 4

2

2p, 2n

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Radioactivity – Beta rays ( ) bend in the opposite direction

indicating they have a negative charge.

• They are known to consist of high speed electrons

(90% speed of light).

• Emitted from the nucleus by conversion of neutron

into a proton.

• Higher speed particles; therefore, more penetrating

than alpha particles (stopped by only more dense

materials such as wood, metal, or several layers of

clothing).

• The symbol is basically equivalent to electron

β β e 0

1-

0

1pure electron

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Radioactivity

– Gamma rays ( ) are unaffected by electric and

magnetic fields.

– They have been shown to be a form of

electromagnetic radiation (pure energy) similar

to x rays, but higher in energy and shorter in

wavelength.

– Alpha and beta radiation are matter; contains p, n,

or e while gamma is pure energy (no p, n, e).

– Highly energetic, the most penetrating form of

radiation (barriers of lead, concrete, or more often,

a combination is required for protection).

– Symbol is

0

0or

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37. 2 Nuclear Equations

• A nuclear equation is a symbolic

representation of a nuclear reaction using

nuclide symbols.

– For example, the nuclide symbol for

uranium-238 is

U23892 92 p, 146 n

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Nuclear Equations

– The radioactive decay of by alpha-particle

emission (loss of a nucleus) is written U

23892

He42

HeThU42

23490

23892

– Reactant and product nuclei are represented

in nuclear equations by their nuclide symbol.

lost 2p & 2n

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Nuclear Equations

– Other particles are given the following symbols.

n10Neutron

00Gamma photon

H11 p

11Proton or

01 e

01Electron or

01

e01Positron or

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Nuclear Equations

• In a nuclear equation, you do not balance the

elements, instead...

– the total mass on each side of the reaction arrow

must be identical (this means that the sum of the

superscripts for the products must equal the

sum of the superscripts for the reactants).

– the sum of the atomic numbers on each side of the

reaction arrow must be identical (this means that the

sum of the subscripts for the products must

equal the sum of the subscripts for the

reactants).

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Alpha Decay

He Th U 4

2

234

90

238

92

238 = 234 + 4

92 = 90 + 2

mass number

atomic number Ex. Plutonium 239 emits an alpha particle when it decays, write the

balanced nuclear equation.

He E? Pu 4

2

A

Z

239

94

mass A:

239 = 4 + A

A = 239 – 4 = 235

Z:

94 = 2 + Z

Z = 94 – 2 = 92 He U Pu 4

2

235

92

239

94 U

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Beta Decay

e ON 0

1-

16

8

16

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Ex. Protactinium 234 undergoes beta decay. Write the balanced

nuclear equation.

e E? Pa 0

1-

A

Z

234

91

mass A:

234 = 0 + A

A = 234 – 0 = 234

Z:

91 = -1 + Z

Z = 91 + 1 = 92

n p

e U Pa 0

1-

234

92

234

91 U

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A Problem To Consider

• Technetium-99 is a long-lived radioactive

isotope of technetium. Each nucleus decays

by emitting one beta particle. What is the

product nucleus? – The nuclear equation is

01

AZ

9943 XTc

mass A:

99 = 0 + A

A = 99 – 0 = 99

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product nucleus?

– The nuclear equation is

01

AZ

9943 XTc

Z:

43 = -1 + Z

Z = 43 + 1 = 44 Ru

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product nucleus?

– The nuclear equation is

01

AZ

9943 XTc

– Hence A = 99 and Z = 44 (Ruthenium), so the

product is

Ru9944

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37.4 Nuclear Structure and Stability

• Binding Energy - the energy that holds the

protons, neutrons, and other particles

together in the nucleus.

• Binding energy is very large for unstable

isotopes.

• When isotopes decay (forming more stable

isotopes,) binding energy is released (go to

lower E state; more stable arrangement).

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• Important factors for stable isotopes- nuclear stability correlates with: – Ratio of neutrons to protons in the isotope.

– Nuclei with large number of protons (84 or more) tend to be unstable.

– The “magic numbers” of 2, 8, 20, 50, 82, or 126 help determine stability. These numbers of protons or neutrons are stable. These numbers, called magic numbers, are the numbers of nuclear particles in a completed shell of protons or neutrons.

– Even numbers of protons or neutrons are generally more stable than those with odd numbers.

– All isotopes (except 1H) with more protons than neutrons are unstable.

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Nuclear Stability

• Several factors appear to contribute the

stability of a nucleus.

– when you plot each stable nuclide on a graph of

neutrons vs. protons, these stable nuclei fall in a

certain region, or band.

– The band of stability is the region in which stable

nuclides lie in a plot of number of neutrons against

number of protons.

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Figure :

Band of stability.

p n

more protons than

needed for stability

n p

more neutrons than

needed for stability

Ebbing, D. D.; Gammon, S. D. General Chemistry,

8th ed., Houghton Mifflin, New York, NY, 2005.

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Predicting the Type of Radioactive Decay

• Nuclides outside the band of stability are

generally radioactive.

– Nuclides to the left of the band have more

neutrons than that needed for a stable nucleus.

– These nuclides tend to decay by beta emission

because it reduces the neutron-to-proton ratio.

n p emit electron in process

more neutrons than needed for stability

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– In contrast, nuclides to the right of the band of

stability have a neutron-to-proton ratio smaller

than that needed for a stable nucleus.

– These nuclides tend to decay by positron

emission or electron capture because it

increases the neutron to proton ratio.

p n

more protons than needed for stability

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– In the very heavy elements, especially those with

Z greater than 83, radioactive decay is often by

alpha emission.

Lose 2p and 2n - emit alpha particle

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37.3 Types of Radioactive Decay

• There are six common types of radioactive

decay.

– Alpha emission (abbreviated ): emission of

a nucleus, or alpha particle, from an

unstable nucleus. He4

2

– An example is the radioactive decay of radium-226.

HeRnRa42

22286

22688

Lost 2p & 2n

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Types of Radioactive Decay


decay.

– Beta emission (abbreviated or -): emission

of a high speed electron from a unstable

nucleus. – This is equivalent to the conversion of a neutron to a

proton.

epn01

11

10

– An example is the radioactive decay of carbon-14.

01

147

146 NC n p

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decay. – Positron emission (abbreviated +): emission of

a positron from an unstable nucleus.

– This is equivalent to the conversion of a proton to a

neutron.

enp01

10

11

– The radioactive decay of technetium-95 is an

example of positron emission.

eMoTc01

9542

9543

p n

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• There are six common types of radioactive decay.

– Electron capture (abbreviated EC): the decay of

an unstable nucleus by capturing, or picking up,

an electron from an inner orbital of an atom.

– In effect, a proton is changed to a neutron, as in

positron emission.

nep10

01

11

– An example is the radioactive decay of

potassium-40.

AreK4018

01

4019

p n

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• There are six common types of radioactive decay.

– Gamma emission (abbreviated ): emission from

an excited nucleus of a gamma photon,

corresponding to radiation with a wavelength of

about 10-12 m.

– In many cases, radioactive decay produces a

product nuclide in a metastable excited state. – The excited state is unstable and emits a gamma

photon and goes to a lower energy state (more

stable). The atomic mass and number do not

change.

– An example is metastable technetium-99.

00

9943

9943 TcTcm

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decay.

– Spontaneous fission: the spontaneous decay of

an unstable nucleus in which a heavy nucleus of

mass number greater than 89 splits into lighter

nuclei and energy is released.

– For example, uranium-236 undergoes spontaneous

fission.

n4IYU

10

13653

9639

23692

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37.5 Rate of Radioactive Decay

• The rate of radioactive decay, that is the

number of disintegrations per unit time, is

proportional to the number of radioactive

nuclei in the sample.

– You can express this rate mathematically as

tkN Rate where Nt is the number of radioactive nuclei at time t,

and k is the radioactive decay constant.

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Rate of Radioactive Decay

– All radioactive decay follows first order kinetics

as outlined in kinetics chapter.

– Therefore, the half-life of a radioactive sample is

related only to the radioactive decay constant.

– The half-life, t½ ,of a radioactive nucleus is the time

required for one-half of the nuclei in a sample to decay.

– The first-order relationship between t½ and the

decay constant k is

k

693.0t

21

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Rate of Radioactive Decay

• Once you know the decay constant, you can

calculate the fraction of radioactive nuclei

remaining (Nt/No) after a given period of time.

– Recall the first-order time-concentration equation

is

ktN

Nln

o

t

– Or if we don’t know k we can substitute k = 0.693/t½

and get

21t

t 693.0

N

Nln

o

t

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• Phosphorus-32 has a half-life of 14.3 days. What fraction of a sample of phosphorus-32 would remain after 5.5 days?

267.0d) (14.3

5.5d)( 693.0

N

Nln

o

t

21t

t 693.0

N

Nln

o

t

remainingor

eN

N

o

t

%77

77.0remaining nucleiFraction 267.0

HW 51

2/1

2

1remaining nucleiFraction

or

t

tn

n

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37.4.1 Nuclear Fission and Nuclear Fusion

• Nuclear fission is a nuclear reaction in which

a heavy nucleus splits into lighter nuclei and

energy is released.

– For example, one of the possible mechanisms for

the decay of californium-252 is

n4MoBaCf10

10642

14256

25298

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Nuclear Fission and Nuclear Fusion

– In some cases a nucleus can be induced to

undergo fission by bombardment with neutrons.

energy n 3 Ba Kr U U n 1

0

141

56

92

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236

92

235

92

1

0

– When uranium-235 undergoes fission, more

neutrons are released creating the possibility of

a chain reaction.

– A chain reaction is a self-sustaining series of nuclear

fissions caused by the absorption of neutrons

released from previous nuclear fissions.

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• Chain reaction - the reaction sustains

itself by producing more neutrons



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• Nuclear fusion is a nuclear reaction in which

light nuclei combine to give a stable heavy

nucleus plus possibly several neutrons, and

energy is released.

– Such fusion reactions have been observed in the

laboratory using particle accelerators.

– Sustainable fusion reactions require

temperatures of about 100 million oC.

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• Fusion (to join together) - combination of

two small nuclei to form a larger nucleus.

• Large amounts of energy is released.

• Best example is the sun.

• An Example:

energy n He H H 1

0

4

2

3

1

2

1

chapter 37 nuclear chemistry - sfactl

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