Mr. Matthew TotaroLegacy High SchoolHonors Chemistry

Radioactivity &

NuclearChemistry

2

The Discovery of Radioactivity

• Antoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off x-rays.

3

Becquerel’s Plate

4

The Discovery of Radioactivity, Continued

• Bequerel discovered that certain minerals were constantly producing penetrating energy rays he called uranic rays.Like x-rays.But not related to fluorescence.

• Bequerel determined that: All the minerals that produced these rays

contained uranium.The rays were produced even though the

mineral was not exposed to outside energy.• Energy apparently being

produced from nothing?

5

Marie Curie• Marie Curie used an electroscope

to detect the radiation of uranic rays in samples.

• By carefully separating minerals into their components, she discovered new elements by detecting the radiation they emitted.Radium named for its green

phosphorescence. Polonium named for her homeland.

• Since the radiation was no longer just emitted from of uranium, she renamed it radioactivity.

6

Electroscope++

+ +++

When charged, the metalfoils spread apart due to

like-charge repulsion.

When exposed to ionizing radiation,

the radiation knocks electrons

off theair molecules,

which jump onto the foils and

discharge them, causing them to

drop down.

7

Properties of Radioactivity• Radioactive rays can ionize matter.

Cause uncharged matter to become charged.

Basis of Geiger counter and electroscope.

• Radioactive rays have high energy.

• Radioactive rays can penetrate matter.

• Radioactive rays cause phosphorescent chemicals to glow.Basis of scintillation counter.

8

What Is Radioactivity?• Radioactivity = release of tiny,

high-energy particles from an atom.• Particles are ejected from the

nucleus.

9

Types of Radioactive Rays• Rutherford discovered there were

three types of radioactivity:1. Alpha rays (𝝰):Have a charge of +2 c.u. and a mass

of 4 amu.What we now know to be helium

nucleus.2. Beta rays (β):Have a charge of -1 c.u. and

negligible mass.Electron-like.3. Gamma rays (𝜸):Form of light energy (not particle like

α and β).

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Rutherford’s Experiment

++++++++++++

--------------

a

gb

11

Penetrating Ability of Radioactive Rays

ab g

0.01 mm 1 mm 100 mm

Pieces of Lead

12

Facts About the Nucleus• Very small volume compared to

volume of the atom.• Essentially entire mass of atom.• Very dense.• Composed of protons and

neutrons that are tightly held together.Nucleons.

13

Facts About the Nucleus, Continued

• Every atom of an element has the same number of protons; equal to the atomic number (Z).

• Atoms of the same elements can have different numbers of neutrons.Isotopes.Different atomic masses.

• Isotopes are identified by their mass number (A).Mass number = number of protons +

neutrons.

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Facts About the Nucleus, Continued

• The number of neutrons is calculated by subtracting the atomic number from the mass number.

• The nucleus of an isotope is called a nuclide.Less than 10% of the known nuclides

are non-radioactive, most are radionuclides.

• Each nuclide is identified by a symbol.Element − mass number.238Uranium

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Important Atomic SymbolsParticle Symbol Nuclear

symbolProton p+

Neutron n0

Electron e-

Alpha a

Beta , b b-

Positron , b b+

p H 11

11

n10

e01

He α 42

42

e β 01

01

e β 01

01

16

Radioactivity• Radioactive nuclei spontaneously

decompose into smaller nuclei.Radioactive decay.We say that radioactive nuclei are unstable.

• The parent nuclide is the nucleus that is undergoing radioactive decay; the daughter nuclide are the new nuclei that are made.

• Decomposing involves the nuclide emitting a particle and/or energy.

• All nuclides with 84 or more protons are radioactive.

17

Transmutation• Rutherford discovered that during the

radioactive process, atoms of one element are changed into atoms of a different element—transmutation.

• In order for one element to change into another, the number of protons in the nucleus must change.

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Chemical Processes vs. Nuclear Processes

• Chemical reactions involve changes in the electronic structure of the atom.Atoms gain, lose, or share electrons.No change in the nuclei occurs.

• Nuclear reactions involve changes in the structure of the nucleus.When the number of protons in the

nucleus changes, the atom becomes a different element.

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Nuclear Equations• We describe nuclear processes using

nuclear equations.• Use the symbol of the nuclide to

represent the nucleus. • Atomic numbers and mass numbers are

conserved.Use this fact to predict the daughter nuclide

if you know parent and emitted particle.

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Alpha Emission (Decay)• An ά particle contains 2 protons and 2

neutrons.Helium nucleus.

• Loss of an alpha particle means:Atomic number decreases by 2.Mass number decreases by 4.

Rn He Ra 21886

42

22288

He α 42

42

21

ά Decay

22

Beta Emission (Decay)• A β particle is like an electron.

Moving much faster.Produced from the nucleus.

• When an atom loses a β particle, its:Atomic number increases by 1.Mass number remains the same.

• In beta decay, a neutron changes into a proton.

Pa e Th 23491

01

23490

e β 01

01

23

β Decay

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Gamma Emission• Gamma (𝜸) rays are high-energy

photons of light.• No loss of particles from the

nucleus.• No change in the composition of

the nucleus, however, the arrangement of the nucleons changes.Same atomic number and mass

number.• Generally occurs after the nucleus

undergoes some other type of decay and the remaining particles rearrange.

γ00

25

Positron Emission (Decay)• Positron has a charge of 1+ c.u.

and negligible mass.Anti-electron.

• When an atom loses a positron from the nucleus, its:Mass number remains the same.Atomic number decreases by 1.

• Positrons appear to result from a proton changing into a neutron.

Ne e Na 2210

01

2211

e β 01

01

26

β + Decay

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Nuclear Equations• In the nuclear equation, mass

numbers and atomic numbers are conserved.

• We can use this fact to determine the identity of a daughter nuclide if we know the parent and mode of decay.

28

Practice—Write a Nuclear Equation for Each of the Following:

• Alpha emission from Th-238.

• Beta emission from Ne-24.

• Positron emission from N-13.

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• Alpha emission from Th-238.

• Beta emission from Ne-24.

• Positron emission from N-13.

Practice—Write a Nuclear Equation for Each of the Following, Continued:

Th He U 23490

42

23892

Na β Ne 2411

01-

2410

C β N 136

01

137

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Decay Series• In nature, often one radioactive

nuclide changes in another radioactive nuclide. Daughter nuclide is also radioactive.

• All of the radioactive nuclides that are produced one after the other until a stable nuclide is made is called a decay series.

• To determine the stable nuclide at the end of the series without writing it all out:

1. Count the number of a and b decays.2. From the mass nunmber, subtract 4 for each a

decay.3. From the atomic number, subtract 2 for each a

decay and add 1 for each b.

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U-238 Decay Series

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Practice—Write All the Steps in the U-238 Decay Series and Identify the

Stable Isotope at the End of the Series.

• , , , , , , , , , , , , , a b b a a a a b a b a b ba

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Practice—Write All the Steps in the U-238 Decay Series and Identify the

Stable Isotope at the End of the Series, Continued.

• , , , , , , , , , , , , , a b b a a a a b a b a b ba a ab b

a a b a

Po-214 Pb-210 Bi-210 Po-210 Pb-206a ab b

a

b

Ra-226 Rn-222 Po-218 At-218 Bi-214

U-238 Th-234 Pa-234 U-234 Th-230

Daughter GranddaughterGreat

granddaughterGreat great

granddaughter

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Practice—Determine the Stable Isotope at the End of the U-238

Decay Series.

• , , , , , , , , , , , , , a b b a a a a b a b a b ba

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Practice—Determine the Stable Isotope at the End of the U-238

Decay Series, Continued.• , , , , , , , , , , , , , a b b a a a a b a b a b b

a

238

92U

8 a 238 - 32

92 - 16?

6 b 206 - 0

76 + 6?

206

82Pb=

Selected Types of Radioactive Decay

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Detecting Radioactivity• To detect when a phenomenon is

present, you need to identify what it does:

1. Radioactive rays can expose light-protected photographic film. Use photographic film to detect the

presence of radioactive rays — film badges.

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Detecting Radioactivity, Continued

2. Radioactive rays cause air to become ionized. An electroscope detects radiation by its

ability to penetrate the flask and ionize the air inside.

Geiger-Müller counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays.

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Detecting Radioactivity, Continued

3. Radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical. A scintillation counter is able to count the

number of flashes per minute.

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Natural Radioactivity• There are small amounts of

radioactive minerals in the air, ground, and water.

• It’s even in the food you eat!• The radiation you are exposed to from

natural sources is called background radiation.

Radioactivity in Medicine

• An isotope scan Technetium-99 is often used as the radiation source for bone scans such as this one.

• Phosphorus-32 is used to image tumors because it is preferentially taken up by cancerous tissue.

• Iodine-131 is used to diagnose thyroid disorders.

Radiotherapy for Cancer • Treatment involves exposing a malignant tumor to

gamma rays, typically from radioisotopes such as cobalt-60.

• The beam is moved in a circular pattern around the tumor to maximize exposure of the cancer cells while minimizing exposure of healthy tissues.

43

Half-Life• Each radioactive isotope decays at a

unique rate.Some fast, some slow.Not all the atoms of an isotope change

simultaneously.Rate is a measure of how many of them

change in a given period of time.Measured in counts per minute, or grams

per time.• The length of time it takes for half of

the parent nuclides in a sample to undergo radioactive decay is called the half-life, t1/2.

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Half-Lives of Various Nuclides

Nuclide Half-life Type of decay

Th-232 1.4 x 1010 yr Alpha

U-238 4.5 x 109 yr Alpha

C-14 5730 yr Beta

Rn-220 55.6 sec Alpha

Th-219 1.05 x 10–6 sec Alpha

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How “Hot” Is It? • When we speak of a

sample being hot, we are referring to the number of decays we get per minute.

• For samples with equal numbers of radioactive atoms, the sample with the shorter half-life will be hotter.That is, more atoms will

change in a given period of time.

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Half-Life• Half of the radioactive atoms decay each

half-life.Radioactive decay

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Time (half-lives)

Pe

rce

nta

ge

of

ori

gin

al s

am

ple

47

Decay of Au-198half-life = 2.7 days

0

10000

20000

30000

40000

50000

60000

0 2 4 6 8 10 12 14 16 18 20 22

Time (days)

Rad

ioac

tivity

(cpm

.)

48

How Long Is the Half-Life of this Radionuclide?

• All things that are alive or were once alive contain carbon.

• Three isotopes of carbon exist in nature, one of which, C-14, is radioactive.– C-14 radioactive with

half-life = 5730 years

• Atmospheric chemistry keeps producing C-14 at nearly the same rate it decays.

Radiocarbon Dating

• While an organism is still living, C-14/C-12 is constant because the organism replenishes its supply of carbon.– CO2 in air is the ultimate source of

all C in an organism.

• Once the organism dies the C-14/C-12 ratio decreases.

• By measuring the C-14/C-12 ratio in a once-living artifact and comparing it to the C-14/C-12 ratio in a living organism, we can tell how long ago the organism was alive.

• The limit for this technique is 50,000 years old.– About 9 half-lives, after which

radioactivity from C-14 will be below the background radiation

Radiocarbon Dating

51

Practice—Radon-222 Is a Gas that Is Suspected of Causing Lung Cancer as It Leaks into Houses. It Is Produced by Uranium Decay. Assuming No Loss or Gain from Leakage, if There Is 1024 g of Rn-222 in the House Today, How Much Will There

be in 5.4 Weeks? (Rn-222 Half-Life Is 3.8 Days.)

52

Practice—Radon-222 Is a Gas that Is Suspected of Causing Lung Cancer as It Leaks into Houses. It Is Produced by Uranium Decay. Assuming No Loss or Gain from Leakage, if There Is 1024 g of Rn-222 in the House Today, How Much Will There be in 5.4

Weeks? ( Rn-222 Half-Life Is 3.8 Days.), Continued

Amount of Rn-222

Number of Half-lives

Time(days)

1024 g 0 0

512 g 1 3.8

256 g 2 7.6

128 g 3 11.4

64 g 4 15.2

32 g 5 19.0

5.4 weeks x 7 days/wk = 37.8 38 days

Amount of Rn-222

Number of Half-lives

Time(days)

16 g 6 22.8

8 g 7 26.6

4 g 8 30.4

2 g 9 34.2

1 g 10 38

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Practice — How Much of a Radioactive Isotope, Rn-224 (with Half-Life of 10 Minutes)

Did You Start with if, After One Hour if You Have 2 g?

54

Practice—How Much of a Radioactive Isotope, Rn-222(with Half-Life of 10 Minutes) Did You Start with if, After One Hour if You Have 2 g?, Continued

Amount of Rn-222

Number of half-lives

Time(min)

128 g 0 0

64 g 1 10

32 g 2 20

16 g 3 30

8 g 4 40

4 g 5 50

2 g 6 60

Fill in the “Number of half-lives” and “Time…” columns first, then work backwards up the “Amount…” column.

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Nonradioactive Nuclear Changes

• A few nuclei are so unstable, that if their nuclei are hit just right by a neutron, the large nucleus splits into two smaller nuclei. This is called fission.

• Small nuclei can be accelerated to such a degree that they overcome their charge repulsion and smash together to make a larger nucleus. This is called fusion.

• Both fission and fusion release enormous amounts of energy.Fusion releases more energy per gram than

fission.

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Fission+ energy!!

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Fission Chain Reaction• A chain reaction occurs when a

reactant in the process is also a product of the process.In the fission process it is the neutrons.So you only need a small amount of

neutrons to start the chain.• Many of the neutrons produced in the

fission are either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238.

• Minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass.

58

Fission Chain Reaction, Continued

59

Fissionable Material• Fissionable isotopes include U-235,

Pu-239, and Pu-240.• Natural uranium is less than 1% U-

235.The rest is mostly U-238.Not enough U-235 to sustain chain

reaction.• To produce fissionable uranium the

natural uranium must be enriched in U-235:To about 7% for “weapons grade.”To about 3% for “reactor grade.”

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Nuclear Power• Nuclear reactors use fission to

generate electricity.About 20% of U.S. electricity.The fission of U-235 produces heat.

• The heat boils water, turning it to steam.

• The steam turns a turbine, generating electricity.

61

Nuclear Power Plants vs. Coal-Burning Power Plants

• Use about 50 kg of fuel to generate enough electricity for 1 million people.

• No air pollution.

• Use about 2 million kg of fuel to generate enough electricity for 1 million people.

• Produces NO2 and SOx that add to acid rain.

• Produces CO2 that adds to the greenhouse effect.

62

Nuclear Power Plant

63

Nuclear Power Plants—Core• The fissionable material is stored in

long tubes, called fuel rods, arranged in a matrix.Subcritical.

• Between the fuel rods are control rods made of neutron absorbing material.B and/or Cd.Neutrons needed to sustain the chain

reaction.• The rods are placed in a material to

slow down the ejected neutrons, called a moderator.Allows chain reaction to occur below

critical mass.

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PLWR

Core

Containmentbuilding

Turbine

Condenser

Coldwater

Boiler

65

PLWR—Core

Coldwater

Fuelrods

Hotwater

Controlrods

66

Problems with Nuclear Reactors

Chernobyl

Fukushima

67

Nuclear Fusion• Fusion is the combining of light nuclei

to make a heavier one.• The sun uses the fusion of hydrogen

isotopes to make helium as a power source.

• Requires high input of energy to initiate the process.Because need to overcome repulsion of

positive nuclei. • Produces 10x the energy per gram as

fission.• No radioactive byproducts.• Unfortunately, the only currently

working application is the H-bomb.

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Fusion+ +

21H 3

1H 42He 1

0n

deuterium + tritium helium-4 + neutron