Nuclear Chemistry

Atoms are composed of

PROTONS

NEUTRONS

ELECTRONS

positively charged mass = 1.6726 x 10–27 kg+

neutral mass = 1.6750 x 10–27 kg

negatively charged–• mass = 9.1096 x 10–31 kg

the Nucleus

+

+

made up of protons and neutrons

Strong Nuclear Force

+

+

the neutrons within the nucleus act as sort of glue countering the electrostatic

repulsion between the protons

Why/how do the protons stay so close to each other?

when an unstable nuclei increases its stability by altering its number of

neutrons and protons

Radioactivity

Types of Nuclear Decay

Alpha emission ( α)42

0 -1Beta emission ( β )

Electron capture ( e) 0 -1

Positron emission ( β ) 0 +1

Gamma emission ( γ)00

Alpha emission ( α)42

the nucleus emits a helium nuclei ( 2 protons and 2 neutrons

α42 = 4

2 He

Po Pb +210 20684 82

42 He

Beta emission ( β )the nucleus changes a neutron into a proton by

emitting an electron

0 -1

β 0 -1

10

n 1 1p+

C +146

147 Nβ 0

-1

Positron emission ( β )the nucleus changes a proton into a neutron by

emitting a positron

0 +1

β 0 +1

11

p 1 0n+

B +85

84 Beβ 0

+1

Electron capture ( e )the nucleus captures an electron and changes a

proton into a neutron

0

β 0 -1

11

p 1 0n+

-1

C116

115 Bβ 0

-1 +

electromagnetic radiation emitted during nuclear decay

Gamma emission ( γ )00

Nuclear Stability

the principle factor in determining whether a nucleus is stable is the neutron-to-proton ratio

as the mass number increases, the neutron-to-proton ratios become greater than one

for elements of low atomic number the value is close to one

nuclei that contain 2, 8, 20, 50, 82, and 126 protons are generally more stable (magic

numbers)

nuclei with even numbers of both protons and neutrons are generally more stable than odd

numbers

Nuclear Stability

If an isotope’s mass number is greater than its atomic weight, beta emission is expected

If an isotope’s mass number is less than its atomic weight, positron emission or electron

capture is expected

All elements having an atomic number greater than 83 are radioactive. Alpha particles are

emitted by most of these isotopes.

Nuclear Stability

Po Pb +210 20684 82 X

Cs Ba +137 13755 56

Na Ba +20 2011 10

X

X

balance the following nuclear equations

Po Pb +210 20684 82

42 He

Cs Ba +137 13755 56

Na Ba +20 2011 10

X

X

balance the following nuclear equations

Po Pb +210 20684 82

42 He

Cs Ba +137 13755 56

Na Ne +20 2011 10 X

0-1 β

balance the following nuclear equations

Po Pb +210 20684 82

42 He

Cs Ba +137 13755 56

Na Ne +20 2011 10

0-1 β

0+1β

balance the following nuclear equations

a decay series

when a radioactive nucleus disintegrates, the products formed may also be unstable and under go further disintegration’s until a stable product

is formed

U Pb238 20692 82

involves 14 steps

U238

92 Pb206

82

+U23892 Th234

9042 He

+Th23490 Pa234

910

-1β

+Pa23491 U234

920

-1β+U

23492 Th230

9042 He

+Th23090 Ra226

8842 He

+Ra22688 Rn222

8642 He

+Rn22286 Po218

8442 He

+Rn22286 Po218

8442 He

+Po21884 Pb214

8242 He

+Pb21482 Bi214

830

-1β

+Bi21483 Po214

840

-1β

+Po21484 Pb210

8242 He

+Pb21082 Bi210

830

-1β

+Bi21083 Po210

840

-1β

+Po21084 Pb206

8242 He

all radioactive decays obey first-order kinetics

Kinetics of Radioactive Decay

Rate of decay at time t = Nk

k = rate constant

the number of radioactive nuclei present at time tN =

Time ( s )The plot shows the decay of uranium-238 to thorium-234

First-order rate plot

U23892 U Th +

238 23492 90

42 He

First-Order rate law Integrated

the integrated form of the rate law is:

t1/2 = k

0.693= kt

N0

Nt

ln

Integrated rate law

is an equation for a straight line

Plot ln Nt versus t

ln Nt = -kt + ln N0

y = mx + b

Slope = -k

y intercept is ln N0

Half-life

the time for the concentration of a reactant to decrease to one-half of its

initial concentration

Time ( s )0

U23892

t 1/2 = 4.51 x 107yr

U Th +238 23492 90

42 He

Half-life

Radiocarbon Dating

Willard Libby (Nobel Prize, 1960)

Carbon-14

Natural abundance: 1 part in 1012

β − emitter

Half-life = 5730 yearsused to date archeological artifacts younger

than 30,000 years

- +

14 C614 N7

0 e-1 +

Example

The C-14 decay rate of wood obtained from a live tree is 0.260 disintegration per second per gram of sample A sample of wood from an archaeological site has C-14 decay rate

of 0.186 disintegration per second per gram. How old is the sample?

The C-14 decay rate of wood obtained from a live tree is 0.260 disintegration per second per gram of sample A sample of wood

from an archaeological site has C-14 decay rate of 0.186 disintegration per second per gram. How old is the sample?

t 1/2 for 14C is known to be 5730 years

t1/2 = k

0.693

Therefore,k = (0.693/5730 yr)= 1.21 x 10-4 yr-1

ln[A]0

[A]= kt ln

260

186= (1.21 x 10-4 yr-1 ) t

t =2770 years

Some representative half-lives

Tc-99 6 hours

C-14 5730 years

Sr-90 28.8 years

Mo-99 67 hours

K-40 1,300,000 years

U-238 45, 000,000 years

By bombarding a sample of nitrogen with α particles an oxygen-17 isotope was produced

with the emission of a proton.

An experiment performed by Rutherford in 1919 produced artificial radioactivity

He +42

178 Op1

1147 N +

Converting one element into another element

Nuclear Transmutation

He +42

178 Op1

1147 N +

balance the following nuclear equation

(d,a )5626Fe 54

25Mn , where d represents the

deuterium nucleus H )21(

+H21+56

26Fe 5425MnHe4

2

Transuranium Elements

Elements with atomic numbers greater than 92

made in particle accelerators

a device used to accelerate nuclear particles near the speed of light

Nuclear Fission

Nuclear Fission

process in which a heavy nucleus (mass number> 200) divides to form smaller nuclei of

intermediate mass and one or more neutrons

Nuclear Fission

Although many heavy nuclei can be made to undergo fission only uranium-235 and

plutonium-239 have any practical importance

Enriched Uranium

Isotope Natural % Abundance

238U 99.2745

235U 0.72

234U 0.0055

http://www.epa.gov/radiation/radionuclides/uranium.htm

http://www.washingtonpost.com/wp-dyn/content/article/2007/04/24/AR2007042401055.html

http://www.cbsnews.com/stories/2007/04/26/eveningnews/main2732837.shtml?source=search_story

Nuclear Fission

n +10 n1

023592U +

14354Xe

9038Sr + 3

For one mole of uranium-235, the energy released is 2.0 x 1013 J

For one ton of coal, the energy released is only 8.0 x 107 J

n +10 n1

023592U +

14354Xe

9038Sr + 3

the fact that more neutrons are produced then captured during uranium-235 fission makes a

possible chain reaction possible

a self-sustaining sequence of nuclear fission reactions

Chain Reaction

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Critical Mass

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•The minimum mass of fissionable material required to

generate a self-sustaining nuclear chain reaction

the first application of nuclear fissionAtomic Bomb

critical mass is formed by using TNT to force the fissionable sections together

Lowell H.S.

yehwoooo

yeh

excellent•

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Nuclear Fusion

Nuclear Fusion

combining small nuclei into larger ones

HHHe

Nuclear Fusion

Because fusion reactions take place at very high temperatures, they are often

called thermonuclear reactions.

H1 1

32

He2 1H+

H1 1

21

H1 1H+ 0

+1β+

He3 2

42

He3 2He+ + H1

12

H1 1

16 O1 0n+8 8

8

Mass of = 2.65535 x 10-2316 O8 g

Mass of H1 1

1 0n+8 8 = 2.67804 x 10-23 g

the difference in mass for the formation of one mole16 O8

= -0.1366 g /molof

Mass and Energy

difference in mass = 2.269 x 10-25

Mass Defect

- equivalence of mass and energy (derived from Einstein’s theory of special

relativity)

E = MC2

energymass

speed of light3.00 x 108 m/s

when a system gains or loses energy, it also gains or loses a quantity of mass.