ChemCh25.2.ppt

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25.2 Nuclear Transformations >

Chapter 25

Nuclear Chemistry

25.1 Nuclear Radiation

25.2 Nuclear Transformations

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What is the source of radon in homes?

CHEMISTRY & YOU

CHEMISTRY & YOU

Radon may

accumulate in a

basement that is

not well ventilated.

Test kits are

available to

measure the

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25.2 Nuclear Transformations > Nuclear Stability and _DecayNuclear Stability and _Decay

Nuclear Stability and Decay

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The

nuclear force

is an attractive force

that acts between

all

nuclear particles that

are extremely close together, such as

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The

nuclear force

is an attractive force

that acts between

all

nuclear particles that

are extremely close together, such as

protons and neutrons in a nucleus.

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The stability of a nucleus depends on the

ratio of neutrons to protons.

Interpret Data

• This graph shows the number of

neutrons vs. the number of

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The stability of a nucleus depends on the

ratio of neutrons to protons.

Interpret Data

• The region of the graph in which these points are located is called the band of

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The stability of a nucleus depends on the

ratio of neutrons to protons.

Interpret Data

• For elements of low atomic number

(below about 20), this ratio is about 1. • Above atomic

number 20, stable nuclei have more neutrons than

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A nucleus may be unstable and undergo

spontaneous decay for different reasons.

The neutron-to-proton ratio in a

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Some nuclei are unstable because they

have too many neutrons relative to the

number of protons.

• When one of these nuclei decays, a neutron

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Some nuclei are unstable because they

have too many neutrons relative to the

number of protons.

emits a beta particle (fast-moving electron) from the nucleus.

– A neutron that emits an electron becomes a proton.

n

1

0 p +

1

1 e

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Some nuclei are unstable because they

have too many neutrons relative to the

number of protons.

emits a beta particle (fast-moving electron) from the nucleus.

– A neutron that emits an electron becomes a proton.

n

1

0 p +

1

1 e

0 –1

– This process is known as beta emission. – It increases the number of protons while

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Radioisotopes that undergo beta emission

include the following.

Cu

66

29 Zn +

66

30 e

0 –1

C

14

6 N +

14

7 e

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Other nuclei are unstable because they

have too few neutrons relative to the

number of protons.

• These nuclei increase their stability by converting a proton to a neutron.

– An electron is captured by the nucleus during this process, which is called electron capture.

Co

59 27

Ni + e

59 28 0 –1 Cl 37 17

Ar + e

37 18

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A

positron

is a particle with the mass of an

electron but a positive charge.

• Its symbol is e.

• During positron emission, a proton changes to a neutron, just as in electron capture.

0 +1

B

8

5 Be +

8 4 e 0 +1 O 15

8 N +

15

7 e

0 +1

– When a proton is converted to a neutron, the

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Nuclei that have an atomic number greater

than 83 are radioactive.

• These nuclei have both too many neutrons and too many protons to be stable.

– Therefore, they undergo radioactive decay.

– Alpha emission increases the neutron-to-proton ratio, which tends to increase the stability of the nucleus.

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In alpha emission, the mass number

decreases by four and the atomic number

decreases by two.

Ra

226

88 Rn + He

222 86

4 2

Th

232

90 Ra + He

228 88

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Recall that conservation of mass is an

important property of chemical reactions.

• In contrast, mass is not conserved during nuclear reactions.

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During nuclear decay, if the atomic

number decreases by one but the mass

number is unchanged, the radiation

emitted is

A. a positron.

B. an alpha particle.

C. a beta particle.

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During nuclear decay, if the atomic

number decreases by one but the mass

number is unchanged, the radiation

emitted is

A. a positron.

B. an alpha particle.

C. a beta particle.

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25.2 Nuclear Transformations > Half-LifeHalf-Life

Half-Life

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1 2

A

half-life

(t ) is the time required for

one-half of the nuclei in a radioisotope sample

to decay to products.

Interpret Graphs

After each half-life, half of the original

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Half-lives can be as short as a second or as long as billions of years.

Half-Lives of Some Naturally Occurring Radioisotopes

Isotope Half-life Radiation emitted

Carbon-14 5.73 × 103_years 

Potassium-40 1.25 × 109_years 

Radon-222 3.8 days  Radium-226 1.6 × 103_years 

Thorium-234 24.1 days 

Uranium-235 7.0 × 108_years 

Uranium-238 4.5 × 109_years 

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• Scientists use half-lives of some long-term

radioisotopes to determine the age of ancient objects.

• Many artificially produced radioisotopes have short half-lives, which makes them useful in nuclear medicine.

– Short-lived isotopes are not a long-term radiation hazard for patients.

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Comparing Half-Lives

• The age of

uranium-containing minerals can be estimated by measuring the ratio of uranium-238 to

lead-206.

• Because the half-life of uranium-238 is 4.5 × 109 years, it is possible to use its half-life to date rocks as old as the solar system.

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Uranium compounds are found in

rocks and in soils that form from these

rocks. How can these uranium

compounds lead to a buildup of radon

in homes and other buildings?

CHEMISTRY & YOU

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Uranium compounds are found in

rocks and in soils that form from these

rocks. How can these uranium

compounds lead to a buildup of radon

in homes and other buildings?

CHEMISTRY & YOU

CHEMISTRY & YOU

Radon gas is a product of the decay of

uranium. As the uranium compounds in

the soil beneath homes and buildings

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Radiocarbon Dating

Plants use carbon dioxide to produce

carbon compounds, such as glucose.

• The ratio of carbon-14 to other carbon isotopes is constant during an organism’s life.

• When an organism dies, it stops exchanging carbon with the environment and its radioactive C atoms decay without being replaced.

• Archaeologists can use this data to estimate when an organism died.

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Exponential Decay Function

• _A_{stands for the amount remaining.}

• A₀ stands for the initial amount.

• _n_{stands for the number of half-lives.}

You can use the following equation to

calculate how much of an isotope will

remain after a given number of half-lives.

A

=

A

₀



1 ₂

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• The exponent n indicates how many times A₀

must be multiplied by to determine 12 A.

A

=

A

₀



1 ₂

n

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Decay of Initial Amount (A₀) of Radioisotope

Half-Life Amount Remaining

0 A₀ × ( )0 = A 0

1 A₀ × ( )1 = A

0 ×

2 A₀ × ( )2 = A

0 × ×

This table shows examples in which

n

= 1 and

n

= 2.

Exponential Decay Function

1 2 1 2 1 2 1 2 1

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Using Half-Lives in Calculations

Sample Problem 25.1

Carbon-14 emits beta radiation and decays with a half-life (t ) of 5730 years. Assume that you start

with a mass of 2.00 × 10–12 g of carbon-14.

1 2

a. How long is three half-lives?

b. How many grams of the

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KNOWNS UNKNOWNS

3 half-lives = ? years

mass remaining = ? g

Analyze List the knowns and the unknowns.

1

• To calculate the length of three half-lives, multiply the half-life by three.

• To find the mass of the radioisotope

remaining, multiply the original mass by for each half-life that has elapsed.

1 2

t = 5730 years

initial mass (A₀) = 2.00 × 10–12 g

number of half-lives (n) = 3

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a. Multiply the half-life of carbon-14 by

the total number of half-lives.

Calculate Solve for the unknowns.

2

t × 1 n = 5730 years × 3 = 17,190 years

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Calculate Solve for the unknowns.

2

b. The initial mass of carbon-14 is

reduced by one-half for each half-life. So, multiply by three times.1

2

Remaining mass = 2.00 × 10–12 g × × ×1

2 12 12

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Calculate Solve for the unknowns.

2

b. You can get the same answer by

using the equation for an exponential decay function.

= (2.00 × 10–12 g)

= 0.250 × 10–12 g

= 2.50 × 10–13 g

A = A₀

( )

1 = (2.00 × 10–12 g)

2

n

( )

1 2

3

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Evaluate

Do the results make sense?

3

• The mass of carbon-14 after three half-lives should be one-eighth of the original mass.

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The half-life of phosphorus-32 is

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The half-life of phosphorus-32 is

14.3 days. How many milligrams of

phosphorus-32 remain after 100.1

days if you begin with 2.5 mg of the

radioisotope?

n = 100.1 days × = 7 half-lives 1 half-life_{14.3 days}

= (2.5 mg) = 2.0 × 10–2 mg

A = A₀

( )

1₂ = (2.5 mg) n

( )

1₂ 7

( )

1

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25.2 Nuclear Transformations > Transmutation ReactionsTransmutation Reactions

Transmutation Reactions

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For thousands of years, alchemists tried to

change lead into gold.

• What they wanted to achieve is

transmutation, or the conversion of an atom of one element into an atom of another

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Transmutation can occur by radioactive

decay, or when particles bombard the

nucleus of an atom.

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Ernst Rutherford performed the earliest

artificial transmutation in 1919.

• He bombarded nitrogen gas with alpha particles.

• The unstable fluorine atoms quickly decay to form a stable isotope of oxygen and a proton.

O +

17 8 p 1 1 F 18 9 Proton Oxygen-17 Fluorine-18

N +

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Rutherford’s experiment eventually led to

the discovery of the proton.

• He and other scientists noticed a pattern as they did different transmutation experiments. Hydrogen nuclei were emitted.

• Scientists realized that these hydrogen nuclei (protons) must have a fundamental role in atomic structure.

He Alpha particle N Nitrogen atom F Unstable fluorine atom O Oxygen P Proton 4

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James Chadwick’s discovery of the neutron

in 1932 also involved a transmutation

experiment.

• Neutrons were produced when beryllium-9 was bombarded with alpha particles.

C +

12 6 n 1 0 Neutron Carbon-12

Be +

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Elements with atomic numbers above 92,

the atomic number of uranium, are called

transuranium elements

.

• None of these elements occurs in nature. • All of them are radioactive.

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Transuranium elements are synthesized in nuclear reactors and nuclear accelerators.

• Reactors produce beams of low-energy particles.

• Accelerators are used to increase the speed of bombarding

particles to very high speeds.

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• When uranium-238 is bombarded with the relatively slow neutrons from a nuclear reactor, some uranium nuclei capture these neutrons. The product is uranium-239.

• Uranium-239 is radioactive and emits a beta particle. The other product is an isotope of the artificial

radioactive element neptunium (atomic number 93).

• Neptunium is unstable and decays, emitting a beta particle and a second artificial element, plutonium (atomic number 94).

Np +

239 93 e 0 –1 U 239 92

U +

238 92 n 1 0 U 239 92

Pu +

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Scientists in Berkeley, California,

synthesized the first two artificial elements

in 1940.

• Since that time, more than 20 additional

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Which of the following always changes

when transmutation occurs?

A. The number of electrons

B. The mass number

C. The atomic number

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Which of the following always changes

when transmutation occurs?

A. The number of electrons

B. The mass number

C. The atomic number

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25.2 Nuclear Transformations > Key ConceptsKey Concepts

The neutron-to-proton ratio in a

radioisotope determines the type of decay that occurs.

After each half-life, half of the original

radioactive atoms have decayed into atoms of a new element.

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25.2 Nuclear Transformations > Key EquationKey Equation

A

=

A

₀



1 ₂

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25.2 Nuclear Transformations > Glossary TermsGlossary Terms

• nuclear force: an attractive force that acts

between all nuclear particles that are

extremely close together, like protons and

neutrons in a nucleus

• band of stability: the location of stable nuclei on a neutron-vs.-proton plot

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25.2 Nuclear Transformations > Glossary TermsGlossary Terms

• half-life: the time required for one-half of the nuclei of a radioisotope sample to decay to products

• _{transmutation:}_{the conversion of an atom of}

one element to an atom of another element

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Electrons and the Structure of Atoms

• Unstable atomic nuclei decay by emitting alpha or beta particles.

• Often gamma rays are emitted.

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