UNIT 1ATOMIC STRUCTURE

RADIOACTIVE DECAYNCCS 1.1.4

Radioactive decay is the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, electromagnetic radiation, or both.

Nuclear radiation is particles or electromagnetic radiation emitted from the nucleus during radioactive decay.

An unstable nucleus that undergoes radioactive decay is a radioactive nuclide.

All of the nuclides beyond atomic number 83 are unstable and thus radioactive.

Protons and neutrons are called nucleons.

An atom is referred to as a nuclide.

Radioactive Decay

Radioactive Nuclide Emissions

A nuclide’s type and rate of decay depend on the nucleon content and energy level of the nucleus.

Below are some examples of the type of radioactive decay that can occur.

Types of Radioactive Decay

Alpha EmissionAn alpha particle (α) is two protons and two neutrons

bound together and is emitted from the nucleus during some kinds of radioactive decay.

42He

Alpha emission is restricted almost entirely to very heavy nuclei.

Types of Radioactive Decay, continuedBeta EmissionA beta particle (β) is an electron emitted from the nucleus

during some kinds of radioactive decay.

To decrease the number of neutrons, a neutron can be converted into a proton and an electron.

0-1n p + β1 1

0 1The atomic number increases by one and the mass number

stays the same.

Types of Radioactive Decay, continuedPositron EmissionA positron is a particle that has the same mass as an electron,

but has a positive charge, and is emitted from the nucleus during some kinds of radioactive decay.

To decrease the number of protons, a proton can be converted into a neutron by emitting a positron.

0+1p n + β1 1

1 0The atomic number decreases by one and the mass number stays

the same.

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Types of Radioactive Decay, continuedElectron CaptureIn electron capture, an inner orbital electron is captured by

the nucleus of its own atom.

To increase the number of neutrons, an inner orbital electron combines with a proton to form a neutron.

0-1e + n 1 1

1 0p The atomic number decreases by one and the mass number

stays the same.

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Types of Radioactive Decay, continuedGamma Emission

Gamma rays () are high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state.

Comparing Alpha, Beta and Gamma Particles

+ 212 484 2Po He ? + 212 4

84 2Po He ?

+ 212 484 2Po He ?

What type of particle is emitted?

Identify the product that balances the following nuclear reaction.

NUCLEAR REACTION PROBLEM

Nuclear Reactions, continued

+ 212 484 2Po He ?

20882Pb.

20882 + Pb212 4

84 2Po He

mass number: 212 − 4 = 208 atomic number: 84 − 2 = 82

2. The nuclide has a mass number of 208 and an atomic number of 82,

3. The balanced nuclear equation is

This is an alpha emission type of reaction

Half-Life

Half-life, t1/2, is the time required for half the atoms of a radioactive nuclide to decay.

Each radioactive nuclide has its own half-life.

More-stable nuclides decay slowly and have longer half-lives.

Potassium-40 Half-Life

Rate of Decay

Half-Life, continuedSample Problem BPhosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope?

Sample Problem B SolutionGiven: original mass of phosphorus-32 = 4.0 mg

half-life of phosphorus-32 = 14.3 daystime elapsed = 57.2 days

Unknown: mass of phosphorus-32 remaining after 57.2 days

Half-Life, continuedSolution:

Step 1: find number of half livesStep 2:find amount of phosphorus remaining after time has lapsed

Step 1: 57.2 days = 4 half lives 14.3 days

Step 2: 4 mg x ½ x ½ x ½ x ½ = .25 mg

OR 4 mg (1/2) 4 = .25 mg

FORMULA: (original amount of substance) x (1/2 life) (number of ½ lifes)

FISSION

Fission is a reaction when the nucleus of an atom, having captured a neutron, splits into two or more nuclei, and in so doing, releases a significant amount of energy as well as more neutrons. These neutrons then go on to split more nuclei and a chain reaction takes place.

FUSIONFusion is a process where nuclei collide and join together to form a heavier atom, usually deuterium and tritium. When this happens a considerable amount of energy gets released at extremely high temperatures: nearly 150 million degrees Celsius. At extreme temperatures, electrons are separated from nuclei and a gas becomes a plasma—a hot, electrically charged gas.

DefinitionFission is the splitting of a large atom into two or more smaller ones.

Fusion is the fusing of two or more lighter atoms into a larger one.

Natural occurrence of the processFission reaction does not normally occur in nature.

Fusion occurs in stars, such as the sun.

Byproducts of the reactionFission produces many highly radioactive particles.

Few radioactive particles are produced by fusion reaction, but if a fission "trigger" is used, radioactive particles will result from that.

ConditionsCritical mass of the substance and high-speed neutrons are required.

High density, high temperature environment is required.

Energy RequirementTakes little energy to split two atoms in a fission reaction.

Extremely high energy is required to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion.

Energy Released

The energy released by fission is a million times greater than that released in chemical reactions, but lower than the energy released by nuclear fusion.

The energy released by fusion is three to four times greater than the energy released by fission.

Nuclear weaponOne class of nuclear weapon is a fission bomb, also known as an atomic bomb or atom bomb.

One class of nuclear weapon is the hydrogen bomb, which uses a fission reaction to "trigger" a fusion reaction.

Energy production Fission is used in nuclear power plants.Fusion is an experimental technology for producing power.

FuelUranium is the primary fuel used in power plants.

Hydrogen isotopes (Deuterium and Tritium) are the primary fuel used in experimental fusion power plants.

FISSION VS FUSION

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Nuclear Binding Energy

According to Albert Einstein’s equation E = mc2, mass can be converted to energy, and energy to mass.

This is the nuclear binding energy, the energy released when a nucleus is formed from nucleons.

The nuclear binding energy is a measure of the stability of a nucleus.

Mass StabilityThe difference between the mass of an atom and the sum of the

masses of its protons, neutrons, and electrons is called the mass defect.

The binding energy per nucleon is the binding energy of the nucleus divided by the number of nucleons it contains

Elements with intermediate atomic masses have the greatest binding energies per nucleon and are therefore the most stable. The elements with an atomic number of 82 or less have stable isotopesSUPPLEMENTAL

INFORMATION

Band of Stability

The graph has a plot of stable elements, this part is called band of stability. At the higher end of the band of stability lies alpha decay, below is positron emission or electron capture, above is beta emissions and elements beyond the atomic mass of 83 are unstable radioactive elements.

Alpha particle

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Alpha decay is located at the top of the plotted line, because the alpha decay decreases the mass number of the element in order to keep the isotope stable. This is done by using the element helium (He). An unstable isotope's protons are decreased by 2 and its neutrons are decreased by 4, and because the isotope was originally unstable before it went through alpha decay, the elements are still considered unstable.

Alpha particle

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Beta decay accepts protons so it changes the amount of protons and neutrons. the number of protons increase while neutrons decrease. To make things easier to understand think of the ratio of the isotope: there are too many neutrons compared to the number of protons therefore it is above the band of stability.

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Positron emission and electron capture is when the isotope gains more neutrons. Positron emission and electron capture are below the band of stability because the ratio of the isotope has more protons than neutrons, think of it as there are too few protons for the amount of neutrons and that is why it is below the band of stability.

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The band of stability can be explained by the relationship between the nuclear force and the electrostatic forces between protons.

Stable nuclei tend to have even numbers of nucleons.

This is referred to as the even-odd rule

According to the nuclear shell model, nucleons exist in different energy levels, or shells, in the nucleus.

The numbers of nucleons that represent completed nuclear energy levels—2, 8, 20, 28, 50, 82, and 126—are called magic numbers.

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•Nuclides containing odd numbers of both protons and neutrons are the least stable •means more radioactive.

•Nuclides containing even numbers of both protons and neutrons are most stable •means less radioactive.

•Nuclides contain odd numbers of protons and even numbers of neutrons are less stable than nuclides containing even numbers of protons and odd numbers of neutrons.

•In general, nuclear stability is greater for nuclides containing even numbers of protons and neutrons or both

EVEN ODD RULE AND MAGIC NUMBERS

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Magic numbers are natural occurrences in isotopes and are stable. Below is a list of numbers of protons and neutrons; isotopes that have these numbers occurring in either the proton or neutron are stable. In some cases there the isotopes can consist of magic numbers for both protons and neutrons; these would be called double magic numbers. But the double numbers only occur for isotopes that are heavier, because the repulsion of the forces between the protons.  The magic numbers:proton: 2, 8, 20, 28, 50, 82, 114neutron: 2, 8, 20, 28, 50, 82, 126, 184Also, there is the concept that isotopes consisting a combination of even-even, even-odd, odd-even, and odd-odd are all stable. There are more nuclides that have a combination of even-even than odd-odd. (See chart.)

MAGIC NUMBERS

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Half-Lives of Some Radioactive Isotopes

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Decay Series

A decay series is a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached.

The heaviest nuclide of each decay series is called the parent nuclide.

The nuclides produced by the decay of the parent nuclides are called daughter nuclides.

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Uranium-238 Decay

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Artificial Transmutations

Artificial radioactive nuclides are radioactive nuclides not found naturally on Earth.

They are made by artificial transmutations, bombardment of nuclei with charged and uncharged particles.

Transuranium elements are elements with more than 92 protons in their nuclei.

Artificial transmutations are used to produce the transuranium elements.

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Radiation Exposure

Nuclear radiation can transfer the energy from nuclear decay to the electrons of atoms or molecules and cause ionization.

The roentgen (R) is a unit used to measure nuclear radiation exposure; it is equal to the amount of gamma and X ray radiation that produces 2 109 ion pairs when it passes through 1 cm3 of dry air.

A rem is a unit used to measure the dose of any type of ionizing radiation that factors in the effect that the radiation has on human tissue.

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