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Nuclear Equations, Radioactivity, and Fission/Fusion

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Facts About the NucleusVery small volume compared to volume of the whole atom

Essentially entire mass of atomVery denseComposed of protons and neutrons that are tightly held togetherNucleons

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Facts About the Nucleus, Con’t Every atom of an element has the same

number of protons; equal to the atomic number

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

Isotopes are identified by their mass number Mass number = number of protons + neutrons

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Facts About the Nucleus, Con’t The number of neutrons is calculated by

subtracting the atomic number from the mass number.

The nucleus of an isotope is called a nuclide. Over 90% of isotopes are radioactive.

Therefore, their nucleus is called a radionuclide

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

238Uranium

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Radioactivity Radioactive nuclei (radionuclides)

spontaneously decompose into smaller nuclei Is called 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 (α, β, etc.) and/or energy

All nuclides with 84 or more protons are radioactive

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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.

Sample Nuclear Equations:

U Th + He

Th Pa + e 234

23490

90

23892

42

23491 -1

0

Alpha decay:

Beta decay:

SAME ON BOTH SIDESMass numbers: 238Atomic numbers: 92

SAME ON BOTH SIDESMass numbers: 234Atomic numbers: 90

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What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?

Alpha emission giving a daughter nuclide with

9 protons and 7 neutrons.

11 p+

9 n0

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What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued

Beta emission giving a daughter nuclide with10 protons and 11 neutrons.

9 p+

12 n0

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What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued

Positron emission giving a daughter nuclide with4 protons and 5 neutrons.

5 p+

4 n0

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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.

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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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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, Con’t2. 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, Con’t3. 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 Able to measure alpha and beta particles only

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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.

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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.

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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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Half-Life Half of the radioactive atoms decay each half-

life.

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Perc

enta

ge o

f orig

inal

sam

ple

Time (half-lives)

Radioactive decay

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0

10000

20000

30000

40000

50000

60000

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

Rad

ioac

tivity

(cp

m.)

Time (days)

Decay of Au-198half-life = 2.7 days

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How Long Is the Half-Life of this Radionuclide?

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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.)

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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 0512 g 1 3.8

256 g 2 7.6128 g 3 11.464 g 4 15.232 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 22.88 g 7 26.6

4 g 8 30.42 g 9 34.21 g 10 38

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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?

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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 0 064 g 1 1032 g 2 2016 g 3 308 g 4 404 g 5 502 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 Fission Chain Reaction is the process

by which neutrons from one reaction cause the fission process to keep continuing Only small number of neutrons needed

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.

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Fission Chain Reaction, Con’t

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Fusion+ +

21H 31H 42He 10n

deuterium + tritium helium-4 + neutron