Section 24-2

Section 24.2 Radioactive Decay

• Explain why certain nuclei are radioactive.

radioactivity: the process by which some substances spontaneously emit radiation

• Apply your knowledge of radioactive decay to write balanced nuclear equations.

• Solve problems involving radioactive decay rates.

Section 24-2

Nuclear Stability

• Except for gamma radiation, radioactive decay involves transmutation, or the conversion of an element into another element.

• Protons and neutrons are referred to as nucleons.

• All nucleons remain in the dense nucleus because of the strong nuclear force.

Section 24-2

Nuclear Stability (cont.)

• The strong nuclear force acts on subatomic particles that are extremely close together and overcomes the electrostatic repulsion among protons.

Section 24-2

Nuclear Stability (cont.)

• As atomic number(number of protons) increases, more and more neutrons are needed to produce a strong nuclear force that is sufficient to balance the electrostatic repulsion between protons.

• Neutron to proton ratio increases gradually to

about 1.5Neutron : 1proton

Section 24-2

Nuclear Stability (cont.)

• The area on the graph within which all stable nuclei are found is known as the band of stability.

• All radioactive nuclei are found outside the band.

• The band ends at Pb-208; all elements with atomic numbers greater than 82 are radioactive.

Section 24-2

Types of Radioactive Decay

• Atoms can undergo different types of decay—beta decay, alpha decay, positron emission, or electron captures—to gain stability.

Section 24-2

Types of Radioactive Decay (cont.)

• In beta decay, radioisotopes above the band of stability have too many neutrons to be stable.

• Beta decay decreases the number of neutrons in the nucleus by converting one to a proton and emitting a beta particle.

Section 24-2

Types of Radioactive Decay (cont.)

• In alpha decay, nuclei with more than 82 protons are radioactive and decay spontaneously.

• Both neutrons and protons must be reduced.

• Emitting alpha particles reduces both neutrons and protons.

Section 24-2

Types of Radioactive Decay (cont.)

Section 24-2

Types of Radioactive Decay (cont.)

• Nuclei with low neutron to proton ratios have two common decay processes.

• 1) Positron emission is a radioactive decay process that involves the emission of a positron from the nucleus.

• A positron is a particle with the same mass as an electron but opposite charge.

Section 24-2

Types of Radioactive Decay (cont.)

• During positron emission, a proton in the nucleus is converted to a neutron and a positron, and the positron is then emitted.

p → n + ß

• 2)Electron capture occurs when the nucleus of an atom draws in a surrounding electron and combines with a proton to form a neutron.

Section 24-2

Types of Radioactive Decay (cont.)

Section 24-2

Types of Radioactive Decay (cont.)

Section 24-2

Writing and Balancing Nuclear Equations

• Nuclear reactions are expressed by balanced nuclear equations.

• In balanced nuclear equations, mass numbers and charges are conserved.

Section 24-2

Radioactive Series

• A series of nuclear reactions that begins with an unstable nucleus and results in the formation of a stable nucleus is called a radioactive decay series.

Section 24-2

Radioactive Decay Rates

• Radioactive decay rates are measured in half-lives.

• A half-life is the time required for one-half of a radioisotope to decay into its products.

N is the remaining amount.

N0 is the initial amount.

n is the number of half-lives that have passed.

t is the elapsed time and T is the duration of the half-life.

Section 24-2

Radioactive Decay Rates (cont.)

Section 24-2

Radioactive Decay Rates (cont.)

Section 24-2

Radioactive Decay Rates (cont.)

• The process of determining the age of an object by measuring the amount of certain isotopes is called radiochemical dating.

• Carbon-dating is used to measure the age of artifacts that were once part of a living organism.

A. A

B. B

C. C

D. D

Section 24-2

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Section 24.2 Assessment

The process of converting one element into another by radioactive decay is called ____.

A. half-life

B. nuclear conversion

C. transmutation

D. trans-decay

A. A

B. B

C. C

D. D

Section 24-2

Section 24.2 Assessment

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An unknown element has a half-life of 40 years. How much of a 20.0g sample will be left after 120 years?

A. 0.00g

B. 2.50g

C. 5.00g

D. 7.50g

• Q1:Describe what happens to unstable nuclei.

• Unstable nuclei decay by emitting alpha or beta radiation until they form a stable element

• Q2: Explain how you can predict whether or not an isotope is likely to be stable if you know its number of neutrons and protons.

• Isotopes are likely to be stable if their n/p ratio is within the range of 1 : 1 to 1.5 : 1. The lightest isotopes have n/p ratios close to 1 : 1, while the heaviest isotopes have ratios close to 1.5 : 1.

• Q3: Describe the forces acting on the particles within a nucleus and explain why neutrons are the glue holding the nucleus together.

• Protons are positively charged and repel each other. Neutrons do not repel each other or protons. The strong nuclear force is an attractive force that acts on both protons and neutrons in the nucleus. The greater the number of neutrons in the nucleus, the greater the strong nuclear force.

A. A

B. B

C. C

D. D

Chapter Assessment 1

The half-life of a radioisotope is:

A. one-half its total life

B. 2500 years

C. the amount of time it takes to completely decay

D. the amount of time it takes for one-half to decay

A. A

B. B

C. C

D. D

Chapter Assessment 2

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What is a positron?

A. a nucleon with the same mass as a neutron and a positive charge

B. a nucleon with the same mass as a proton and a negative charge

C. a nucleon with the same mass as an electron and a positive charge

D. a type of radioactive emission with a negative charge

A. A

B. B

C. C

D. D

Chapter Assessment 3

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What is the force that holds the protons and neutrons together in the nucleus of an atom?

A. nuclear magnetic force

B. strong nuclear force

C. ionic bonding

D. nuclear bond

A. A

B. B

C. C

D. D

Chapter Assessment 4

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During positron emission, a proton is converted to:

A. a neutron and electron

B. an electron and positron

C. a proton and neutron

D. a neutron and positron

A. A

B. B

C. C

D. D

STP 4

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The half-life of K-40 is 1.26 × 109 years. How much of a 10.0g sample will be left after 200 million years?

A. 8.96g

B. 8.03g

C. 7.75g

D. 4.99g

• End of section 24.2