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Nuclear Chemistry PowerPoint PPT Presentation


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Nuclear Chemistry. Chapter 23. Radioactivity. One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934). She discovered radioactivity , the spontaneous disintegration of some elements into smaller pieces.

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Nuclear chemistry l.jpg

Nuclear Chemistry

Chapter 23


Radioactivity l.jpg

Radioactivity

  • One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work ofMarie Curie(1876-1934).

  • She discoveredradioactivity, the spontaneous disintegration of some elements into smaller pieces.


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Nuclear Reactions vs. Normal Chemical Changes

  • Nuclear reactions involve the nucleus

  • The nucleus opens, and protons and neutrons are rearranged

  • The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy

  • “Normal” Chemical Reactions involve electrons, not protons and neutrons


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Types of Radiation

  • Alpha (ά) – a positively charged (+2) helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons

  • Beta (β) – an electron

  • Gamma (γ) – pure energy; called a ray rather than a particle


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Other Nuclear Particles

  • Neutron

  • Positron – a positive electron

  • Proton – usually referred to as hydrogen-1

  • Any other elemental isotope


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Penetrating Ability


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Geiger-Müller Counter

23.7


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Geiger Counter

  • Used to detect radioactive substances


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A

X

Mass Number

Element Symbol

Z

Atomic Number

a particle

proton

neutron

electron

positron

or

or

1H

1p

0b

0e

or

1

1

+1

1n

0e

+1

0b

0

-1

-1

4a

4He

or

2

2

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

A

1

1

0

0

4

1

0

-1

+1

2

Z

23.1


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+

+

+

+

+ 2

+ 2

1n

1n

1n

1n

96

96

0

0

0

0

Rb

Rb

37

37

138

235

138

235

Cs

Cs

U

U

92

55

92

55

Balancing Nuclear Equations

  • Conserve mass number (A).

The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants.

235 + 1 = 138 + 96 + 2x1

  • Conserve atomic number (Z) or nuclear charge.

The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants.

92 + 0 = 55 + 37 + 2x0

23.1


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or

alpha particle -

212Po 4He + AX

2

Z

84

212Po 4He + 208Pb

2

82

84

4a

4He

2

2

212Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212Po.

212 = 4 + A

A = 208

84 = 2 + Z

Z = 82

23.1


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14C 14N + 0b + n

6

7

-1

40K 40Ca + 0b + n

19

20

-1

1n 1p + 0b + n

0

1

-1

11C 11B + 0b + n

6

5

+1

38K 38Ar + 0b + n

19

18

+1

1p 1n + 0b + n

1

0

+1

n and n have A = 0 and Z = 0

Nuclear Stability and Radioactive Decay

Beta decay

Decrease # of neutrons by 1

Increase # of protons by 1

Positron decay

Increase # of neutrons by 1

Decrease # of protons by 1

23.2


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37Ar + 0e 37Cl + n

18

17

-1

55Fe + 0e 55Mn + n

26

25

-1

1p + 0e 1n + n

1

0

-1

212Po 4He + 208Pb

2

82

84

252Cf 2125In + 21n

98

49

0

Nuclear Stability and Radioactive Decay

Electron capture decay

Increase # of neutrons by 1

Decrease # of protons by 1

Alpha decay

Decrease # of neutrons by 2

Decrease # of protons by 2

Spontaneous fission

23.2


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Learning Check

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He ? + 1n

5 2 0


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Learning Check

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He 13N + 1n

5 2 7 0


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Write Nuclear Equations!

Write the nuclear equation for the beta emitter Co-60.

60Co0e+60Ni27 -128


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Artificial Nuclear Reactions

New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4He and 11B.

Reactions using neutrons are called g reactions because a g ray is usually emitted.

Radioisotopes used in medicine are often made by g reactions.


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Artificial Nuclear Reactions

Example of a g reaction is production of radioactive 31P for use in studies of P uptake in the body.

3115P + 10n ---> 3215P + g


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Transuranium Elements

Elements beyond 92 (transuranium) made starting with an g reaction

23892U + 10n ---> 23992U + g

23992U ---> 23993Np + 0-1b

23993Np ---> 23994Pu + 0-1b


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Nuclear Stability

  • Certain numbers of neutrons and protons are extra stable

    • n or p = 2, 8, 20, 50, 82 and 126

    • Like extra stable numbers of electrons in noble gases (e- = 2, 10, 18, 36, 54 and 86)

  • Nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of neutron and protons

  • All isotopes of the elements with atomic numbers higher than 83 are radioactive

  • All isotopes of Tc and Pm are radioactive

23.2


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Band of Stability and Radioactive Decay


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Half-Life

  • HALF-LIFE is the time that it takes for 1/2 a sample to decompose.

  • The rate of a nuclear transformation depends only on the “reactant” concentration.


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Half-Life

Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?


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Kinetics of Radioactive Decay

For each duration (half-life), one half of the substance decomposes.

For example: Ra-234 has a half-life of 3.6 daysIf you start with 50 grams of Ra-234

After 3.6 days > 25 grams

After 7.2 days > 12.5 grams

After 10.8 days > 6.25 grams


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A daughter

DA

rate = -

Dt

Ln 2

=

k

Kinetics of Radioactive Decay

A = A0e(-kt)

lnA = lnA0 - kt

A = the amount of atoms at time t

A0 = the amount of atoms at time t = 0

k is the decay constant (sometimes called l)

0.693

=

k

23.3


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14N + 1n 14C + 1H

0

7

6

1

14C 14N + 0b + n

6

7

-1

238U 206Pb + 8 4a + 6 0b

92

82

2

-1

Radiocarbon Dating

t½ = 5730 years

Uranium-238 Dating

t½ = 4.51 x 109 years

23.3


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Learning Check!

The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 31 hours?


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Biological Effects of Radiation

Radiation absorbed dose (rad)

1 rad = 1 x 10-5 J/g of material

Roentgen equivalent for man (rem)

1 rem = 1 rad x Q

Quality Factor

g-ray = 1

b = 1

a = 20

23.8


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Effects of Radiation


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Nuclear Fission

Fission is the splitting of atoms

These are usually very large, so that they are not as stable

Fission chain has three general steps:

1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron)

2. Propagation. 236U fission releases neutrons that initiate other fissions

3. Termination.


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Nuclear Fission


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235U + 1n 90Sr + 143Xe + 31n + Energy

0

38

0

54

92

Nuclear Fission

Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2

Energy = 3.3 x 10-11J per 235U

= 2.0 x 1013 J per mole 235U

Combustion of 1 ton of coal = 5 x 107 J

23.5


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Representation of a fission process.


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Mass Defect

  • Some of the mass can be converted into energy

  • Shown by a very famous equation!

    E=mc2

Energy

Mass

Speed of light


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BE + 19F 91p + 101n

0

9

1

binding energy per nucleon =

binding energy

number of nucleons

2.37 x 10-11 J

19 nucleons

=

Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons.

E = mc2

BE = 9 x (p mass) + 10 x (n mass) – 19F mass

BE (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984

BE = 0.1587 amu

1 amu = 1.49 x 10-10 J

Converts amu to kg and multiplies by c2

BE = 2.37 x 10-11J

= 1.25 x 10-12 J

23.2


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Nuclear binding energy per nucleon vs Mass number

nuclear binding energy

nucleon

nuclear stability

23.2


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Non-critical

Critical

Nuclear Fission

Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions.

The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass.

23.5


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Nuclear Fission & POWER

  • Currently about 103 nuclear power plants in the U.S. and about 435 worldwide.

  • 17% of the world’s energy comes from nuclear.


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Diagram of a nuclear power plant


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35,000 tons SO2

4.5 x 106 tons CO2

70 ft3 vitrified waste

3.5 x 106

ft3 ash

1,000 MW coal-fired

power plant

1,000 MW nuclear

power plant

Nuclear Fission

Annual Waste Production

23.5


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Nuclear Fusion

Fusion

small nuclei combine

2H + 3H 4He + 1n +

1 1 2 0

Occurs in the sun and other stars

Energy


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Nuclear Fusion

Fusion

  • Excessive heat can not be contained

  • Attempts at “cold” fusion have FAILED.

  • “Hot” fusion is difficult to contain


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Radioisotopes in Medicine

  • 1 out of every 3 hospital patients will undergo a nuclear medicine procedure

  • 24Na, t½ = 14.8 hr, b emitter, blood-flow tracer

  • 131I, t½ = 14.8 hr, b emitter, thyroid gland activity

  • 123I, t½ = 13.3 hr, g-ray emitter, brain imaging

  • 18F, t½ = 1.8 hr, b+ emitter, positron emission tomography

  • 99mTc, t½ = 6 hr, g-ray emitter, imaging agent

Brain images with 123I-labeled compound

23.7


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Chemistry In Action: Food Irradiation


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