Radioactivity Ch16.1 8th Pdf

Radioactivity Chapter 16 Section 1

Objectives:  Describe how radioactivity was discovered  Compare alpha, beta, and gamma decay  Describe the penetrating power of the 3 kinds of nuclear

radiation

 Calculate ages of objects using half-life  Identify uses of radioactive materials

Science Humor…  Why did the unstable nucleus use

toothpaste?

 It wanted to prevent decay!

Discovering Radioactivity  1896, Henri Becquerel, discovered a new area of science  Becquerel hypothesized that fluorescent minerals give off x-

rays

 His test idea:  Put a fluorescent mineral on top of a photographic plate

wrapped in paper  Put it in bright sunlight; developed the plate; saw a strong image of the mineral

Henri Becquerel’s test…

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An unexpected result  Tried the experiment again, but weather was cloudy  He put the plate in a drawer, and found it still created an

image several days later  The mineral gave off energy. It passed through the paper and

made an image on the plate.  Becquerel concluded that the energy came from uranium, an

element in the mineral

Naming the Unexpected  Nuclear radiation: high energy particles and rays that are

emitted by the nuclei of some atoms  Marie Curie, a scientist working with Becquerel, named

the process radioactivity or radioactive decay

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Kinds of Radioactive Decay  Radioactive decay: an unstable nucleus gives off particles and

energy  3 kinds:  Alpha decay

 Beta decay  Gamma decay

Alpha Decay  Release of an alpha particle  Alpha particle is made up of 2 protons and 2 neutrons (same

as the nucleus of a Helium atom)

 Mass number of 4, charge of 2+  Mass number is the sum of the numbers of protons and

neutrons in the nucleus of an atom

Alpha Decay  Many large radioactive nuclei give off alpha particles and

become nuclei of different elements  Example: Radium-226 gives off alpha particles

http://www.sr.bham.ac.uk/xmm/images/atom/alphadecaysymb_370_94.jpg

Conservation in Decay  Mass number is conserved  The sum of the mass numbers of the starting materials is always

equal to the sum of the mass numbers of the products  Also, charge is conserved  The sum of the charges of the starting materials is always equal

to the sum of the charges of the products  Mass number:  Charge:

226 = 222 + 4 (88+) = (86+) + (2+)

Beta Decay  The release of a beta particle from a nucleus  A beta particle can be an electron or a positron

 Electron: charge= 1ˉ  Positron: charge=1⁺

 Electrons and positrons have a mass of almost 0  The mass number of a beta particle is 0 because it has no protons

or neutrons

Two Types of Beta Decay  Not all isotopes of an element decay in the same way.

 Isotopes are atoms that have the same number of protons as

other atoms of the same element, but have different numbers of neutrons  Example: Carbon-14 and Carbon-11

Carbon-14  A neutron breaks down into a proton and an  The nucleus becomes a nucleus of a different element

(Nitrogen-14)  Both mass number and charge are conserved

http://www.windows.ucar.edu/physical_science/physics/atom_particle/c14_decay_to_n14_big.jpg

Carbon-11  A proton breaks into a positron and a neutron  The nucleus becomes a nucleus of a different element

(Boron-11)  Both mass number and charge are conserved

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http://www.hcc.mnscu.edu/chem/abomb/Positron_Disc.jpg

Gamma Decay  Some of the energy given off during alpha and beta decay is

in the form of light with very high energy: gamma rays

 Gamma decay – release of gamma rays from the nucleus  Happens as the particles in the nucleus shift places  Gamma rays have no mass or charge  Therefore, there is no change into another element

Radioactive Decay

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The Penetrating Power of Radiation  Difference in penetration is due to their mass and charge

(Alpha, Beta, Gamma particles)  Effects of radiation on matter: Atoms that are hit by nuclear radiation can give up

electrons Chemical bonds between atoms can break This can cause damage to living and non-living matter

Damage to Living Matter  Can cause burns  Radiation sickness: fatigue, loss of appetite, hair loss  Destruction of blood cells

 Death  Risk of cancer

Damage to Non-Living Matter  Metal is weakened  Metal structures of buildings and nuclear power plants can

become unsafe  High levels of radiation from the sun can damage spacecraft

Damage  Gamma rays go through matter easily; cause damage deep

within matter  Beta particles cause damage closer to the surface

 Alpha particles cause damage very near the surface  Larger and more massive  Cause the most damage in an organism

Finding Date by Decay  Iceman found in the Italian Alps in 1991  He was 5,300 years old  The decay of radioactive carbon was the key!

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Oetzi the Iceman

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Carbon-14 It’s in YOU!  Carbon atoms are found in all living things  A portion of those carbon atoms are radioactive carbon-14

atoms  During life, the percentage of carbon-14 atoms stays the

same (atoms that are decayed are replaced)  When an organism dies, the carbon-14 is no longer replaced

A Steady Rate of Decay  Every 5,730 years, half of the carbon-14 in a sample decays

 The rate of decay is constant (temperature, pressure or other

conditions do not affect the rate)  Half-life: the amount of time it takes one-half of the nuclei of

a radioactive isotope to decay

Radioactive Decay & Half-Life

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Half-Life continued…  Original sample  1 half-life: 50% (½) of the sample has decayed, 50% (½) is

unchanged  2 half-lives: 75% (¾) of the sample has decayed, 25% (¼) is

unchanged  3 half-lives: 87.5% (7/8) of the sample has decayed, 12.5%

(1/8) is unchanged

Examples of Half-Lives  Table 1 pg. 453 in text

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Determining Age  Iceman’s age:  Scientists measured the # of decays in his body each minute  ½ of the carbon had decayed (5,730 years)  Carbon can be used to find the age of objects up to 50,000

years old  To find the age of older things, other elements such as

potassium-40 (1.3 billion years) – used for dinosaur fossils!

Uses of Radioactivity  Ex: Smoke detectors  Tracers: radioactive elements whose paths can be followed

through a process or reaction

Radioactivity in Healthcare  Used to diagnose medical problems

 Short half-lives  Injected into the patient; followed through the body  Treat illnesses (cancer)  Prevent illnesses: sterilization of food products

Quick Quiz  Compare alpha, beta, and gamma radiation in terms of their

size and mass

 Why is a half-life called a half-life?