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Chapter 10

Section 10.1 What is Radioactivity?

• What happens when an element

undergoes radioactive decay?

• How does radiation affect the

nucleus of an unstable isotope?

• How do scientists predict when

an atom will undergo radioactive

decay?

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• Two years later, Marie and Pierre Curie discovered two new elements: polonium and radium (radioactive).

The Discovery of Radioactivity• In 1896, Henri Becquerel left uranium salt

on a photographic plate

• He hypothesized that the uranium salt had emitted some unknown invisible rays, or radiation, that had darkened the film.

Nuclear Radiation

•Radiation: The process of

emitting energy in the form of waves

or particles.

•Where does radiation

come from?•Radiation is generally produced when

particles interact or decay.

•The sun (solar) or radioactive

isotopes of the

elements (terrestrial).

Radiation is going through you at this very moment!

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• Background radiation comes from several sources.

Nuclear Radiation

It is low-level radiation created by

radioactive isotopes found in

Earth’s rocks, soils, and

atmosphere

Radioactivity

• The force is not large enough to hold a nucleus together tightly.

• Nuclei of certain isotopes are unstable.

• To become stable, they emit energy.

• Change into different isotope of same element or a completely different element.

• Release different types off radiation

• The process of nuclear decay is called radioactivity.

Nuclear Radiation

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Isotopes•What’s an isotope?

•Two or more varieties of an element having the

same number of protons but different number of

neutrons.

•Certain isotopes are “unstable” and decay to

lighter isotopes or elements.

• When an unstable nucleus decays, particles and energy called nuclear radiation are emitted from it.

• Alpha, beta and neutron radiation are particles.

• Gamma radiation is an electromagnetic wave.

Nuclear Radiation

Radiation type Symbol Mass (kg) Charge

Alpha particles

(αααα)

Beta particles(ββββ)

Gamma-rays(γγγγ)

Neutrons

6.646 x 10 -27

9.109 x 10 -31

none

1.975 x 10 -27

+2

-1,(+1)

0

0

Video

Types of radiation

Video

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Alpha Particles

• Made of two protons and two neutrons is emitted from the decaying nucleus.

• Alpha particle =helium nucleus

Nuclear Radiation

• Has an electric charge of

+2 and an atomic mass of

4 decreases the atomic number by 2 and the

mass number by 4

Same symbol

• Much more massive compared to others

• They also have the most electric charge.

Nuclear Radiation

Alpha Particles

• The least penetrating form of nuclear radiation.

• Can be stopped by a sheet of paper.

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Alpha Particles (α)

Radium

R226

88 protons138 neutrons

Radon

Rn222

Note: This is theatomic weight, whichis the number ofprotons plus neutrons

86 protons136 neutrons

+ nnp

p

αααα (4He)

2 protons2 neutrons

The alpha-particle (α) is a Helium nucleus.

It’s the same as the element Helium, with the electrons stripped off !

Beta Particles

• A neutron decays into a proton

• Emits a fast moving electron (beta particle).

Nuclear Radiation

• May be positively

charged (+), positrons

e0

-1

e0

1+

• Not as massive as alpha particles,

so they can pass through thicker

substances

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Beta Particles (b)CarbonC14

6 protons8 neutrons

NitrogenN14

7 protons7 neutrons

+ e-

electron(beta-particle)

Neutron decays into a proton and emits an electron.

Because the atom now has one more proton, it becomes the element

with an atomic number (proton number) one greater than that of the

original element.

The mass number of the new element is the same because the total

number of protons and neutrons does not change during beta decay

Atomic

Number

Changes

Try this!!

Identify the type of radiation?

A

C D

B

Beta particle

Positron particleAlpha particle

Alpha particle

e Ar K 0

1

38

18

38

19 ++→

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Gamma Rays

Nuclear Radiation

• γ radiation is electromagnetic radiation.

• The nuclear structure stays the same, it simply represents a loss of energy

Happens in conjunction with Beta or Alpha emissions. Rather than emit

another particle, excess energy is given off in gamma rays.

Gamma Ray=High Energy Photon

Nuclear Radiation

• Lead and concrete, are required

to stop gamma rays.

• They have no mass and no charge and travel at the speed of light.

Gamma Rays

• The most penetrating form of nuclear radiation.

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Gamma Rays

NeonNe20 +

The gamma from nuclear decay is in the X-

ray/ Gamma ray part of the EM spectrum

(very energetic!)

NeonNe20

10 protons

10 neutrons

(in excited state)

10 protons

10 neutrons

(lowest energy state)

gamma

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Nucleus Symbol Explained

• A nucleus can be represented by a symbol that includes its atomic number, mass number, and the symbol of the element it belongs to.

• The symbol for the nucleus of the stable isotope of carbon is shown:

• Nuclear Equation – shows the

radioactive decomposition of an element

146C → 14

7N + 0-1e

Nuclear Decay

• Use the letter X to denote the unknown product.

• Note that the mass and atomic numbers of the

unknown isotope are represented by the letters A

and Z.

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Alpha Emission

He Th U 4

2

234

90

238

92 +→

parent

nuclide

daughter

nuclide

alpha

particle

Numbers must balance!!

Nuclear Decay

Alpha particle = helium nucleus

4

He Th U 4

2

234

90

238

92 +→

Alpha Emission Math Problem

• Mass number = sum of mass number of products.

• Atomic number = sum of atomic number of products.

238 = 234 +

92 = 90 + 2

Mass

number

Atomic

number

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Beta Emission

e Xe I 0

-1

131

54

131

53 +→

electron

e Ar K 0

1

38

18

38

19 ++→

positron

Nuclear Decay

Positron Emission

Beta particle – electron is given off

Beta Emission Math Problem

e Xe I 0

-1

131

54

131

53 +→

• Mass number before and after the decay does not change.

• Atomic number of the product increases by 1, so element changes.

Mass

number

Atomic

number

131 = 131 + 0

53 = +54 (-1)

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e Ar K 0

1

38

18

38

19 ++→

Positron Emission Math Problem

• Mass number before and after the decay does not change.

• Atomic number of the product decreases by 1, so element changes.

Mass

number

Atomic

number

38 = 38 + 0

19 = +18 (+1)

Electron Capture

Pd e Ag 106

46

0

-1

106

47 →+

electron

Nuclear Decay• Electron is captured by isotope and turns

into different element

Mass

number

Atomic

number

106 + 0 = 106

47 + (-1) = 46

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Nuclear Decay Practice Problems

Pd e Ag 106

46

0

-1

106

47 →+e Ar K 0

1

38

18

38

19 ++→

e Xe I 0

-1

131

54

131

53 +→He Th U 4

2

234

90

238

92 +→

Identify the unknown product.

Transmutation• Transmutation is the process of changing one

element to another through nuclear decay.

• In alpha decay, two protons and two neutrons are lost from the nucleus.

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• The new element has an atomic number (protons) two less than that of the original element.

• The new element has a mass (proton + neutron) number four less than the original element.

• In this transmutation, polonium emits an alpha particle and changes into lead.

Transmutation

Radioactive Decay Rates

� It is impossible to

predict the moment

when any particular

nucleus will decay, but

it is possible to predict

the time required for

half of the nuclei in a

given radioactive

sample to decay.

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½ ½ ½ ½

Ra

do

n

Po

lon

ium

Half-Life

Radioactive Decay Rates

• The “half-life” (h) is the time it takes for half the atoms of

a radioactive substance to decay.

Radioactive Half-Life

• Some radioisotopes decay to stable atoms in less than a second and some require millions of years.

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• The nucleus left after the isotope decays is called the daughter nucleus.

Radioactive Half-Life

Bill Nye Half Life

Ori

gin

al E

lem

en

t

New

Ele

men

t

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• Half-lives vary widely among the radioactive isotopes.

• The half-lives of some radioactive elements are listed in the table.

Radioactive Half-Life

Radioactive Decay of a Sample

of Uranium-238

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20,000

Half-Life Math Problem•For example, suppose we had 20,000 atoms of a

radioactive substance. If the half-life is 1 hour, how many

atoms of that substance would be left after:

10,000

5,000

2,500

1 hour

2 hours

3 hours

0 hours

1

2

3

Half-lives

100%

50%

25%

12.5%

Time % of atoms

remaining#atoms

remaining

0

– You just won $1,000,

but…

– …you can only spend half

of it in month 1…

– …half of the remainder in

month 2, etc.

– After how many months

would you be left with

less than $1?

– What is the half life for

this prize?

Radioactive Decay Rates

Half live AmountTime

1,0000 months0

1

2

3

4

5

6

7

8

9

10

11

500

250

125

62.5

31.25

15.625

7.8125

3.906

1.953

0.976

1 month

2 months

3 months

4 months

5 months

6 months

7 months

8 months

9 months

10 months

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1. How many grams of iodine 131 (half life- 5 days)

would be left after 20 days if you start with 25 grams?

The half life is

Number of half-lives

passedAmount of Matter Time

0 Started with

1How Much is

left

2How Much is

left

3How Much is

left

4How Much is

left

5How Much is

left

Answer: 1.56 g

Radioactive Half-Life Practice Problems

25 g 0 { days}

5 days

12.5g

6.25 g

3.12 g

1.56 g

5 days

10 days

15 days

20 days

How long will it take 600 grams of Plutonium 239 (half

life 24,000 years) to decay to 18.75 grams?

The half life is

Number of half-lives

passedAmount of Matter Time

0 Started with

1How Much is

left

2How Much is

left

3How Much is

left

4How Much is

left

5How Much is

left

24,000 yrs

600 g

300 g

0 yrs

24,000 yrs

150 g 48,000 yrs

75 g 72,000 yrs

37.5 g 96,000 yrs

18.75 g 120,000 yrs

120,000 yrs

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K-42 has a half-life of 15.5 hrs. If 13.125g of K-42

remains undecayed after 62.0 hours, what was the

original sample size?

The half life is

Number of half-lives

passedAmount of Matter Time

0 Started with

1How Much is

left

2How Much is

left

3How Much is

left

4How Much is

left

5How Much is

left

15.5 hrs

31 hrs

15.5 hrs

0 hrs

46.5 hrs

62 hrs13.125 g

26.25 g

52.5 g

105 g

210 g

210 g

Your turn!

1. Thallium-208 has a half-life of 3 min. How long

will it take for 120.0 g to decay to 7.50 g?

2. An isotope of cesium (cesium-137 has a half -life

of 30 years. If 20 mg of cesium-137 disintegrates

over a period of 90 years, how many mg of

cesium-137 would remain?

3. If 60 g of Lithium-9 has a half-life of 100 years,

how long will it take for lithium-9 to decay to 15 g?

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The half life is

Number of half-lives

passedAmount of Matter Time

0 Started with

1 How Much is left

2 How Much is left

3 How Much is left

4 How Much is left

5 How Much is left

Secton 2: Nuclear Fission and Fusion

• What holds the nuclei of atoms

together?

• What is released when the nucleus of

a heavy atom is split?

• What happens when the nuclei of

small atoms are joined?

Bill Nye Video (25)

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Nuclear Forces

� The stability of a nucleus depends on the nuclear

forces that hold the protons and neutrons

(nucleus) together.

� The number of neutrons determines the stability

of the nucleus. If it has too many neutrons, the

nucleus becomes unstable.

Nuclear Fission

�Splitting of atom into two or more smaller

fragments, releasing neutrons and energy.

Nuclear Forces

Tremendous amounts

of energy are released

during fission

Tremendous amounts

of energy are released

during fission

Same amount of energy as

6.7 million TNT molecules

do when they explode!

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• The products of a fission reaction usually include several individual neutrons in addition to the smaller nuclei.

Nuclear Fission

Nuclear Forces

• Neutrons released

by fission can start

a chain reaction,

each cause the

division of another

nucleus

� critical mass -the minimum amount of a substance than can undergo a fission reaction and can sustain a chain reaction.

Nuclear ForcesNuclear Fission

• Nuclear chain reaction - the continuous series of nuclear fission due to neutrons dividing other nucleus from the same sample.

The chain reaction

principle is used in

the atomic bomb.

Mouse trap Video

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How it worksHow it works

� 2 or more masses of U-235 are surrounded by an explosive.

� The explosion forces the masses together to form a critical mass and a fission chain reaction occurs rapidly.

� Control rods are inserted that absorb neutrons.

Nuclear Fission

• High levels of exposure cause radiation

sickness.

• Radiation toxic waste

Energy produced from fission is used to provide electrical energy to

millions of homes and businesses.

AnimationRisk Video

Electricity

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Nuclear Fusion

�Light nuclei

combine at

extremely high

temperatures

forming heavy

nuclei and

releasing energy.

�Light nuclei

combine at

extremely high

temperatures

forming heavy

nuclei and

releasing energy.

Nuclear Forces

• The energy released from fusion is 3-4 times greater than the energy released from fission.

Nuclear Forces

Nuclear Fusion

• The sun and other stars are natural fusion reactors.

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Why aren’t we using Fusion instead of Fission?

Why aren’t we using Fusion instead of Fission?

�Ignition temperatures are 100 million Kelvin, and no manmade container can hold this without melting.

�Ignition temperatures are 100 million Kelvin, and no manmade container can hold this without melting.

�The reactants are hydrogen, a very abundant gas, and the product is helium, which is harmless.

Runaway reactions aren’t

a problem.

Fusion Reactors

� Not yet sustainable

Tokamak Fusion Test Reactor

Princeton University

National Spherical

Torus Experiment

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Fission vs. Fusion

�235U is limited

� danger of meltdown

� toxic waste

� thermal pollution

� fuel is abundant

(Hydrogen)

� no danger of meltdown

� no toxic waste (Helium)

� not yet sustainable

F

I

S

S

I

O

N

F

U

S

I

O

N

Section 3: Nuclear Radiation

Today

Section 3: Nuclear Radiation

Today

�Where are we exposed to radiation?

�What are some beneficial uses of nuclear radiation?

�What factors determine the risks of nuclear radiation?

�How is the energy produced by nuclear fission used?

�Where are we exposed to radiation?

�What are some beneficial uses of nuclear radiation?

�What factors determine the risks of nuclear radiation?

�How is the energy produced by nuclear fission used?

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Beneficial uses of Nuclear Radiation

� Some common applications of

nuclear radiation

– medical diagnosis and treatment

– smoke detectors

– manufacturing

– and agriculture

� Radioactive tracers- short lived

isotopes that tend to

concentrate in affected cells

and are used to locate tumors.

Radioactive Tracers show Alzheimer’s Disease in a PET

scan.

• Examples of tracers include carbon-11, iodine-131, and sodium-24.

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Medicine• Medical imaging – Radioactive

tracers can be ingested and

then viewed in the body

through imaging equipment.

• Cancer Treatment – radiation

kills cancerous cells more easily

than healthy cells

Using Nuclear Reactions in Medicine

Tracer Showing Cancer

Treating Cancer with Radioactivity• Radiation can be used to stop some types of

cancerous cells from growing.

Using Nuclear Reactions in Medicine

• An intense beam of gamma rays is focused on the tumor for a short period of time.

• The gamma rays pass through the body and into the tumor.

• Healthy cells in the body that grow quickly are also damaged.

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Smoke detectors

• A radioactive source ionizes the air between two

electrodes. Thus current flows between them

Using Nuclear Reactions in Medicine

• If smoke particles enter

this space they stick to the

ions and the current is

reduced.

• This reduced current

triggers the alarm

• Sterilization –used to

kill germs and sterilize

food and plastic

equipment

• Food Irradiation-

Food irradiation is a

process in which

radiation is used to

keep food fresh longer

and kill germs.

Using Nuclear Reactions in Medicine

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Geiger CounterGeiger counters are devices that display

(measurements of) radiation levels by detecting the presence of radioactive

particles in an area.

Carbon dating• Once a living organism dies, it is no longer taking

in any Carbon.

• C14 is radioactive, and decays over time.

• By measuring the activity of C14 in an object and comparing it with the amount of C14 which was present initially you can estimate when the organism died

Radioactive Dating

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Radioactive Dating

• Used in aging of rocks and fossils found on Earth.

• The ages of these materials can be determined using radioactive isotopes and their half-lives.

Carbon Dating• Plants use carbon dioxide when they make

food, so all plants contain carbon-14.