ips ch.19

Unit 5: The AtomUnit 5: The Atom

Chapter 19: RadioactivityChapter 19: Radioactivity

19.1 Alpha, Beta, and Gamma 19.1 Alpha, Beta, and Gamma RadiationRadiation

RadioactivityRadioactivity – The – The process by which certain process by which certain elements emit particular elements emit particular forms of radiation.forms of radiation.

Three Forms: Alpha, Beta, Three Forms: Alpha, Beta, and Gammaand Gamma

Alpha ParticlesAlpha Particles – Fast- – Fast-flying, positively charged flying, positively charged subatomic particles make subatomic particles make up radiation.up radiation.

Alpha ParticleAlpha Particle – Made of – Made of two protons and two two protons and two neutrons. (Nucleus of a neutrons. (Nucleus of a helium atom.helium atom.


Beta ParticlesBeta Particles – Fast- – Fast-flying, negatively flying, negatively charged subatomic charged subatomic particles that make up particles that make up beta radiation.beta radiation.

Beta ParticleBeta Particle – An – An electron that is ejected electron that is ejected by an atomic nucleus.by an atomic nucleus.


Gamma RadiationGamma Radiation – – An extremely energetic An extremely energetic form of form of electromagnetic electromagnetic radiation. Has more radiation. Has more energy than visible energy than visible light. Carries no light. Carries no electric charge and electric charge and has no mass.has no mass.

19.2 Radioactivity Is a Natural 19.2 Radioactivity Is a Natural PhenomenonPhenomenon

Radioactivity has been around forever.Radioactivity has been around forever. Cells can repair most kinds of molecular damage Cells can repair most kinds of molecular damage

caused by radiation.caused by radiation. MutationMutation – When radiation alters the genetic – When radiation alters the genetic

information of a cell by damaging its DNA information of a cell by damaging its DNA molecules. (Can lead to cancer)molecules. (Can lead to cancer)

Radon-222Radon-222 – An inert gas arising from uranium – An inert gas arising from uranium deposits.deposits.

EPA suggest that anywhere from 7,000 to 30,000 EPA suggest that anywhere from 7,000 to 30,000 cases of lung cancer each year are attributed to cases of lung cancer each year are attributed to radon exposure.radon exposure.


1/5 of our annual exposure to radiation 1/5 of our annual exposure to radiation comes from non-natural sources.comes from non-natural sources.

Nuclear power industries generate about Nuclear power industries generate about 10,000 tons of radioactive waste a year that 10,000 tons of radioactive waste a year that is is containedcontained!!


Origins of radiation exposure for average Origins of radiation exposure for average American:American:

81% Natural background81% Natural background

15% Medicine and diagnostics15% Medicine and diagnostics

4% Consumer products4% Consumer products

19.3 Radioactivity Results from an 19.3 Radioactivity Results from an Imbalance of Forces in the NucleusImbalance of Forces in the Nucleus

Electrical charges with the same sign repel one Electrical charges with the same sign repel one another.another.

Strong nuclear forceStrong nuclear force – Acts between nucleons, is – Acts between nucleons, is very strong, but acts only over small distancesvery strong, but acts only over small distances

Repulsive electrical interactions are long-ranged.Repulsive electrical interactions are long-ranged. Large nucleus are not as stable as small ones.Large nucleus are not as stable as small ones. Radioactive decayRadioactive decay - Large, unstable nucleus - Large, unstable nucleus

breaks down releasing gamma radiation.breaks down releasing gamma radiation.

19.3 Radioactivity Results from an 19.3 Radioactivity Results from an Imbalance of Forces in the NucleusImbalance of Forces in the Nucleus

Neutrons act as a “nuclear cement” that hold the Neutrons act as a “nuclear cement” that hold the atomic nucleus together.atomic nucleus together.

A neutron has two drawbacks:A neutron has two drawbacks:1. Neutrons are not stable when they are by 1. Neutrons are not stable when they are by

themselves. (It decays to a proton by themselves. (It decays to a proton by emitting an electron and sheds alpha emitting an electron and sheds alpha particles.)particles.)2. Neutrons are not stable when protons are 2. Neutrons are not stable when protons are attracted by strong protons in the nucleus. attracted by strong protons in the nucleus. (Elements with more than 83 protons are (Elements with more than 83 protons are radioactive.)radioactive.)

19.4 A Radioactive Element Can 19.4 A Radioactive Element Can Transmute to a Different ElementTransmute to a Different Element

TransmutationTransmutation - The changing of one element to - The changing of one element to another.another.

Occurs when a radioactive nucleus emits an alpha Occurs when a radioactive nucleus emits an alpha or beta particle and the atomic number and or beta particle and the atomic number and identity of the element change.identity of the element change.

When transmutation occurs, energy is released.When transmutation occurs, energy is released. When an element ejects an alpha particle from it When an element ejects an alpha particle from it

nucleus, the mass number of the atom is nucleus, the mass number of the atom is decreased by 4 and its atomic number is decreased by 4 and its atomic number is decreased by 2. (Falls back 2)decreased by 2. (Falls back 2)

19.4 A Radioactive Element Can 19.4 A Radioactive Element Can Transmute to a Different Element Transmute to a Different Element

When an beta particle is ejected the mass of When an beta particle is ejected the mass of the atom is not changed, but its atomic the atom is not changed, but its atomic number increases by 1.(Steps up one)number increases by 1.(Steps up one)

19.5 The Shorter the Half-life, the 19.5 The Shorter the Half-life, the Greater the RadioactivityGreater the Radioactivity

Radioactive materials by their very Radioactive materials by their very nature, decay via various methods to nature, decay via various methods to form other elements, ultimately to form form other elements, ultimately to form stable non-radioactive isotopes. stable non-radioactive isotopes.

The radioactive decay is measured in a The radioactive decay is measured in a characteristic of time called Half-life.characteristic of time called Half-life.

The half-life is the time needed for half of The half-life is the time needed for half of the radioactive atoms to decay. the radioactive atoms to decay.

19.5 The Shorter the Half-life, the 19.5 The Shorter the Half-life, the Greater the RadioactivityGreater the Radioactivity

After 20 half-lives, the initial quantity is After 20 half-lives, the initial quantity is diminished to one-millionth of the original.diminished to one-millionth of the original.

Remarkably constant and not affected by Remarkably constant and not affected by external conditions.external conditions.

Measured by a Geiger counter.Measured by a Geiger counter.

Geiger CounterGeiger Counter

If you monitor a radioactive substance If you monitor a radioactive substance with a Geiger counter, you will hear with a Geiger counter, you will hear "clicks" every time the device detects a "clicks" every time the device detects a particle of radiation.particle of radiation. Over time, if you Over time, if you graphed the number of clicks per second graphed the number of clicks per second you would see a graph resembling you would see a graph resembling something like you see below: something like you see below:


Depending on how quickly an element decays, the Depending on how quickly an element decays, the time needed to make the above graph could take time needed to make the above graph could take anywhere from a fraction of a second to billions of anywhere from a fraction of a second to billions of years. years.

One way to describe how fast a radioactive One way to describe how fast a radioactive substance decays is to determine how long it will substance decays is to determine how long it will take half of the currently present atoms to decay. take half of the currently present atoms to decay. Let's look at a particular example - Iodine-131 Let's look at a particular example - Iodine-131 which is used to diagnose problems with the which is used to diagnose problems with the thyroid gland. thyroid gland.


By looking at the graph you can see that half By looking at the graph you can see that half of the Iodine-131 has decayed every 8 days. of the Iodine-131 has decayed every 8 days. We then say that Iodine-131 has a "half-life" We then say that Iodine-131 has a "half-life" of 8 days. of 8 days.

19.6 Isotopic Dating Measures the 19.6 Isotopic Dating Measures the Age of MaterialAge of Material

All plants and animals contain a tiny quantity All plants and animals contain a tiny quantity of radioactive carbon-14.of radioactive carbon-14.

Carbon-14 emits a beta particle, and decays Carbon-14 emits a beta particle, and decays back to nitrogen.back to nitrogen.

The longer a plant or animal is dead the less The longer a plant or animal is dead the less carbon-14 it contains.carbon-14 it contains.

The half-life of carbon-14 is about 5,730 The half-life of carbon-14 is about 5,730 years.years.

19.6 Isotopic Dating Measures the 19.6 Isotopic Dating Measures the Age of MaterialAge of Material

Carbon-14 dating – The way scientists Carbon-14 dating – The way scientists calculate the age of artifacts.calculate the age of artifacts.

Carbon-14 dating is only useful to 50,000 Carbon-14 dating is only useful to 50,000 years into the past. Then they guess!years into the past. Then they guess!

Fluctuations in the sun’s magnetic field, Fluctuations in the sun’s magnetic field, Earth’s climate, and water temperature Earth’s climate, and water temperature means that carbon-14 has fluctuated means that carbon-14 has fluctuated throwing off this dating method.throwing off this dating method.

ips ch.19

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