1

Chapter 37

Nuclear Chemistry

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

2

37.1 RadioactivityRadioactive 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)

EAZ

3

B115

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

4

• 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.

5

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.

6

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 42

42

2 42

2p, 2n

7

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 01-

01 pure electron

8

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

00or

9

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

10

Nuclear Equations

– The radioactive decay of by alpha-particle emission (loss of a nucleus) is written

U23892

He42

HeThU 42

23490

23892

– Reactant and product nuclei are represented in nuclear equations by their nuclide symbol.

lost 2p & 2n

11

Nuclear Equations

– Other particles are given the following symbols.

n10Neutron

00Gamma photon

H11 p1

1Proton or

01 e0

1Electron or

01 e0

1Positron or

12

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).

13

Alpha Decay

He Th U 42

23490

23892

238 = 234 + 4

92 = 90 + 2

mass number

atomic numberEx. Plutonium 239 emits an alpha particle when it decays, write the balanced nuclear equation.

He E? Pu 42

AZ

23994

mass A:

239 = 4 + A

A = 239 – 4 = 235

Z:

94 = 2 + Z

Z = 94 – 2 = 92He U Pu 4

223592

23994

U

14

Beta Decay

e ON 01-

168

167

Ex. Protactinium 234 undergoes beta decay. Write the balanced nuclear equation.

e E? Pa 01-

AZ

23491 mass A:

234 = 0 + A

A = 234 – 0 = 234

Z:

91 = -1 + Z

Z = 91 + 1 = 92

n p

e U Pa 01-

23492

23491

U

15

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

16

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

Z:

43 = -1 + Z

Z = 43 + 1 = 44 Ru

17

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

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

Ru9944

18

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).

19

• 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.

20

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.

21

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.

22

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

23

Predicting the Type of Radioactive Decay

• Nuclides outside the band of stability are generally radioactive.

– 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

24

Predicting the Type of Radioactive Decay

• Nuclides outside the band of stability are generally radioactive.

– 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

25

Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, 2005.

26

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.

He42

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

HeRnRa 42

22286

22688

Lost 2p & 2n

27

Types of Radioactive Decay

• There are six common types of radioactive 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.

epn 01

11

10

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

01

147

146 NC n p

28

Types of Radioactive Decay• There are six common types of radioactive

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

a positron from an unstable nucleus.

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

enp 01

10

11

– The radioactive decay of technetium-95 is an example of positron emission.

eMoTc 01

9542

9543

p n

29

Types of Radioactive Decay• 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.

nep 10

01

11

– An example is the radioactive decay of potassium-40.

AreK 4018

01

4019

p n

30

Types of Radioactive Decay• 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

31

Types of Radioactive Decay

• There are six common types of radioactive 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

32

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.

33

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

k693.0

t2

1

34

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

ktNN

lno

t

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

21t

t 693.0NN

lno

t

35

A Problem To Consider• 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) (14.35.5d)( 693.0

NN

lno

t

21t

t 693.0NN

lno

t

remainingor

eN

N

o

t

%77

77.0remaining nucleiFraction 267.0

HW 51

2/1

2

1remaining nucleiFraction

or

t

tn

n

code: second

36

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

n4MoBaCf 10

10642

14256

25298

37

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 10

14156

9236

23692

23592

10

– 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.

38

• Chain reaction - the reaction sustains itself by producing more neutrons

Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed., Houghton Mifflin, New York, NY, 2005.

39

Nuclear Fission and Nuclear Fusion

• 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.

40

Nuclear Fission and Nuclear Fusion

• 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 10

42

31

21