George M. McKelvy, Ph.D.1 Chapter 4 The Structure of the Atom.

George M. McKelvy, Ph.D. 1

ChemistryChemistry: Matter and : Matter and ChangeChange

Chapter 4Chapter 4

The Structure of The Structure of the Atomthe Atom



Section 4.4Section 4.4

Unstable Nuclei and Unstable Nuclei and Radioactive DecayRadioactive Decay



ObjectivesObjectives

Explain Explain the relationship between the relationship between unstable nuclei and radioactive decay.unstable nuclei and radioactive decay.

Characterize Characterize alpha, beta, and alpha, beta, and gamma radiation in terms of mass and gamma radiation in terms of mass and charge.charge.



Review VocabularyReview Vocabulary

element:element: a pure substance that a pure substance that cannot be broken down into simpler cannot be broken down into simpler substances by physical or chemical substances by physical or chemical meansmeans



New VocabularyNew Vocabulary

radioactivityradioactivity alpha alpha particleparticle

radiationradiation nuclear nuclear equationequation

nuclear reactionnuclear reaction beta beta radiationradiation

radioactive decayradioactive decay beta particlebeta particle

alpha radiationalpha radiation gamma raysgamma rays



Unstable atomic nuclei emit Unstable atomic nuclei emit radiation to gain stability.radiation to gain stability.

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ChemistryChemistry: Matter and : Matter and ChangeChangeRadioactivityRadioactivity

Nuclear reactions or decay can change one Nuclear reactions or decay can change one element into another element.element into another element.

In the late 1890s, scientists noticed some In the late 1890s, scientists noticed some substances spontaneously emitted radiation, substances spontaneously emitted radiation, a process they called a process they called radioactivityradioactivity..

The rays and particles emitted are called The rays and particles emitted are called radiationradiation..

A process that involves a change in an atom’s A process that involves a change in an atom’s nucleus is called nucleus is called nuclear reaction or decaynuclear reaction or decay..


ChemistryChemistry: Matter and : Matter and ChangeChangeRadioactive DecayRadioactive Decay

Unstable nuclei lose energy by emitting Unstable nuclei lose energy by emitting radiation in a spontaneous process called radiation in a spontaneous process called radioactive decayradioactive decay..

Unstable radioactive elements undergo Unstable radioactive elements undergo radioactive decay thus forming stable radioactive decay thus forming stable nonradioactive elements.nonradioactive elements.

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Alpha radiationAlpha radiation is made up of positively is made up of positively charged particles called charged particles called alpha particlesalpha particles..

Each alpha particle contains two protons and Each alpha particle contains two protons and two neutrons and has a 2+ charge.two neutrons and has a 2+ charge.

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The figure shown below is aThe figure shown below is a nuclear nuclear equationequation showing the radioactive decay of showing the radioactive decay of plutonium-240 to uranium-236.plutonium-240 to uranium-236.

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plutonium-240 uranium-236 alpha particleplutonium-240 uranium-236 alpha particle

Pu24094 U236

92 42+

The mass is conserved in nuclear equations.The mass is conserved in nuclear equations.

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Beta radiationBeta radiation is radiation that has a negative is radiation that has a negative charge and emits beta particles.charge and emits beta particles.

Each Each beta particlebeta particle is an electron with a 1- charge. is an electron with a 1- charge.

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potassium-40 calcium-40 beta particlepotassium-40 calcium-40 beta particle

Ca4020

K4019 +

1

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Alpha, beta, and gamma radiation can be Alpha, beta, and gamma radiation can be determined by using a magnetic field. Alpha determined by using a magnetic field. Alpha and beta have opposite charges and undergo and beta have opposite charges and undergo deflection in opposite directions.deflection in opposite directions.

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Gamma raysGamma rays are high-energy radiation with are high-energy radiation with no mass and no charge (neutral).no mass and no charge (neutral).

Gamma rays account for most of the energy Gamma rays account for most of the energy lost during radioactive decay.lost during radioactive decay.

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Table 4.5Table 4.5 Characteristics of RadiationCharacteristics of Radiation

Alpha Beta Gamma

SymbolSymbol or e- or

Mass (amu)Mass (amu) 4 1/1840 0

Mass (kg)Mass (kg) 6.65 x 10-27 9.11 x 10-31 0

ChargeCharge 2+ 1- 0

He42

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Atoms that contain too many or too Atoms that contain too many or too few neutrons are unstable and lose few neutrons are unstable and lose energy through radioactive decay to energy through radioactive decay to form a stable nucleus.form a stable nucleus.

Few exist in nature—most have already Few exist in nature—most have already decayed to stable forms.decayed to stable forms.



AssessmentAssessmentA reaction that changes one element A reaction that changes one element into another is called what?into another is called what?

A. chemical reaction

B. beta radiation

C. nuclear reaction

D. physical reaction



AssessmentAssessmentWhy are radioactive elements rare in Why are radioactive elements rare in nature?nature?

A. They do not occur on Earth.

B. Most have already decayed to a stable form.

C. They take a long time to form.

D. They afre too hard to detect.

George M. McKelvy, Ph.D.1 Chapter 4 The Structure of the Atom.

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