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Chapters 18

NUCLEAR

CHEMISTRY

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CHAPTER OUTLINE

Atomic Structure Radioactivity Alpha Decay Beta & Gamma Decays Radionuclide Half Life Nuclear Fission & Fusion

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ATOMICSTRUCTURE

# of e + = # of p (neutral)

The general designation for an atom is shown below:

No. of protons

0# of n = A - Z

No. of protons & neutrons

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Example 1:

Determine the number of protons, electrons and neutrons in a fluorine atom .

199 F

A = 19

Z = 9

# of protons = atomic no. (Z)= 9

# of electrons = # of protons = 9

# of neutrons = A-Z = 19-9 = 10

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Example 2:

Argon (Ar) has 18 protons, 18 electrons and 22 neutrons. Write a formula designation for an argon atom.

Atomic no. = # of protons = 18

Mass no. = p+ + n0 = 18 + 22 = 401840 Ar

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HISTORY OFRADIOACTIVITY

The discovery of radioactivity can be attributed to several scientists.

Wilhem Roentgen discovered X-rays in 1895. Shortly after that, Henri Becquerel observed

radioactive behavior while experimenting with salts of uranium.

However, the term radioactivity was first coined by Marie Curie in 1898.

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RADIOACTIVITY

Radioactivity is the spontaneous emission of particles and/or rays from the nucleus of an atom.

Nuclides are said to be stable (non-radioactive) or unstable (radioactive).

All elements having an atomic number greater than 83 (bismuth) have unstable nuclei, which undergo spontaneous decay (disintegration), and are therefore radioactive.

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RADIOACTIVITY

These elements can undergo one of 3 different kinds of decay called (alpha), (beta) and (gamma).

When decaying, the original nucleus (A) changes into another nucleus (B) and gives off radiation (b).

+ A B b ¾¾®Parent Daughter Radiation

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TYPES OFRADIATION

The three different kinds of radiations can be distinguished by their interactions with an electric field.

-Radiations carry a positive charge and are attracted to the negative plate.

β-Radiations carry a negative charge and are attracted to the positive plate.

γ-Radiations carry no charge and are attracted to neither plate.

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TYPES OFRADIATION

The three different kinds of radiations have different energies and penetration abilities.

α-Particles have low energy and can be blocked by a thin sheet of paper.

β-Particles have moderate energy and can be blocked by a thin sheet of metal.

γ-Rays have high energy and can only be blocked by a thick sheet of lead.

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ALPHA DECAY

Alpha decay occurs when a nucleus changes into another nucleus and gives off an -particle, which is a helium nucleus 4

2( He).

90 88232 228 4

2Th Ra + He¾¾®

Note that the sum of the atomic numbers and the mass numbers on the left and the right side of the equation are equal.

90 = 88 + 2232 = 228 + 4

Loss of alpha particle from the nucleus results in loss of 4 in mass number (A) and 2 in atomic number (Z).

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Examples:23892 U1. undergoes alpha decay. Write the equation for this process.

92

8 42

23 U ??? + He¾¾®

92 – 2 = 90

90

238 – 4 = 234

234

238 2342

49 90 2U Th + He¾¾®

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Examples:

226 88 Ra .

2. Write the equation for alpha decay of radium isotope

88

22 42

6 Ra ??? + He¾¾®

88 – 2 = 86

86

226 – 4 = 222

222

88 86226 222 4

2Ra Rn + He¾¾®

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BETA DECAY

Beta decay occurs when a nucleus changes into another nucleus and gives off a ß-particle, which is an electron 0

-1( e).

In beta decay, a neutron is transformed into a proton and an electron. The proton remains in the nucleus, while the electron is emitted as a beta particle.

6 = 7 -114 = 14 + 0

14 146

07 -1C N + e¾¾®

1 10 1

0-1n p + e¾¾®

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GAMMA DECAY

Gamma decay occurs when a nucleus gives off -rays , and becomes a less energetic form of the same nucleus.

*82 82

204 204Pb Pb + γ¾¾®

More energetic Less energeticgamma

radiation

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Examples:

21 2146 46*Sc Sc + γ¾¾®

1. Complete the following equation for nuclear decay:

γ-decay2146 *Sc ??? + ???¾¾®

*246

1Sc ??? + γ¾¾®

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Examples:

47 4721 22 -1

0 Sc Ti + e¾¾®

2. Complete the following equation for nuclear decay:

β-decay

2147Sc ??? + ???¾¾®

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472 -1

0Sc ??? + e¾¾®

47 = 47 + 0

47

21 = 22 -1

22

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POSITRONDECAY

Certain nuclear processes can also lead to a fourth form of radiation, called a positron

17 = 17 + 09 = 8 + 1positron

0+1( e).

17 179

08 +1F O + e¾¾®

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REVIEW: TYPES OF RADIATION

Alpha () particle is two protons and two neutrons

Beta () particle is a high-energy electron

Positron (+) is a positive electron

Gamma ( ) rayis high-energy released from a nucleus

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SUMMARY OF TYPES OF RADIATION

General, Organic, and Biological Chemistry

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RADIONUCLIDES

A nuclide will be radioactive if it meets any of the following criteria:

1. Its atomic number is greater than 83.

2. It has fewer n0 than p+ in the nucleus.

3. It has odd # of n0 and odd # of p+.

1 31 2Exceptions: H ; He

2 6 10 141 3 5 7Exceptions: H ; Li ; B ; N

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Examples:

Which is the radionuclide in each of the following pairs:

208 22282 861) Pb and Rn

19 2010 102) Ne and Ne

63 6429 293) Cu and Cu

Z > 83

n0 < p+

p+ and n0 are odd

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SMOKE DETECTORS

A practical use of radionuclides is their use in some smoke detectors.

In these detectors, a radionuclide decays to form α–particles.

The α–particles ionize (charge) the air particles and keep a current running through the circuit.

When smoke particles interfere with the ions, current is reduced in the circuit, and a alarm goes off.

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HALF-LIFE

The half-life of a radioisotope is the time for the radiation level to decrease (decay) to one-half of the original value.

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DECAY CURVE

A decay curve shows the decay of radioactive atoms and the remaining radioactive sample.

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Each half-life reduces the amount of radio-nuclide to half of the previous amount.

DECAY CURVE

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Example 1:

Iodine-131 has a half-life of 8 days. What mass of a 40 g sample of iodine-131 will remain after 24 days.

24 days1 half-life

x 8 days

= 3 half-lives

40 g 1st ½ -life 20 g 2nd ½ -life 10 g 3rd ½ -life 5 g

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Example 2:

How long would it take for a sample of C-14 to decay from 20.0 g to 0.625 g? (½- life of C=14 = 5730 y)

5730 y5 half-lives x = 28650 y

1 half-life

½ life 0 1 2 3 4 5

g 20.0 10.0 5.00 2.50 1.25 0.625

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NUCLEARFISSION

Fission is the process by which a large nucleus is “split” into smaller nuclei, with a large amount of energy released.

1 235 20 92

3926n+ U U (unstable)¾¾®

236 140 94 192 54 38 0U Xe+ Sr+2 n¾¾®

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NUCLEAR FISSION DIAGRAM AND EQUATION

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1 235 236 91 142 10 92 92 36 56 0 + U U Kr + Ba + 3 + energyn n

A nuclear bomb is an example of an uncontrolled fission, while a nuclear reactor is an example of a controlled fission.

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CHAIN REACTION

A chain reaction occurs

when a critical mass of uranium undergoes fission

releasing a large amount of heat and energy that produces an atomic explosion

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NUCLEAR POWER PLANTS

In nuclear power plants, fission is used to produce energy control rods in the reactor absorb

neutrons to slow and control the chain reactions of fission

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

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NUCLEAR FUSION

Nuclear fusion occurs at extremely high temperatures

(100 000 000 °C) smaller nuclei combine to form larger nuclei large amounts of energy are released this occurs continuously in the sun and stars

General, Organic, and Biological Chemistry

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NUCLEARFUSION

1 2 +1 4 0

14 H He + 2 e + energy¾¾®

The sun uses fusion to produce a helium nucleus from 4 hydrogen nuclei, giving off 2 positrons and energy.

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THE END