1

Nuclear Chemistry

Chapter 22

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Nucleons

In nucleus of atom Protons and neutrons

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Nuclide

An atom Identified by the number of protons and

neutrons in its nucleus Example:

Sulfur-32 Has mass number of 32 Has 16 protons

S3216

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Mass defect

The difference between the mass of an atom and the sum of the masses of its protons, neutrons, and electrons.

Use isotopic mass to calculate, not average atomic mass.

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Nuclear binding energy

The energy released when a nucleus forms. Mass is converted to energy (E=mc2) when

the nucleus is formed.

Also the energy required to break apart the nucleus Measure of stability

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Binding energy per nucleon

Binding energy divided by number of nucleons

If high, nucleus is held together tightly

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Band of stability

Neutron-proton ratio Close to 1:1 for

smaller atoms Close to 1.5:1 for

larger atoms

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Stability

Protons repel each other through electrostatic forces

They attract each other through nuclear forces – but only over small distances

More neutrons are needed to increase nuclear force without increasing repulsive forces

Beyond bismuth (83), no stable nuclides exist

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Nuclear shell model

Nucleons exist in different energy levels, or shells, in the nucleus

Magic numbers – the numbers of nucleons that represent completed nuclear energy levels – 2, 8, 20, 28, 50, 82, and 126 Very stable nuclides

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Nuclear reaction

Affects the nucleus of an atom Atoms give off large amounts of energy

and increase their stability

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Transmutation

When a nucleus changes identity as a result in the change in its number of protons It becomes a different element

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Nuclear equations

The total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation.

Elements have atomic numbers 1 or greater

Neutrons have atomic numbers of 0 Electrons have atomic numbers of -1

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Examples

HeThU 42

23490

23892

PaeTh 23491

01

23490

RhePd 10045

01

10046

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Example

ClAr 3717

3718 _____

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You try

KrRb 8336

8337 _____

16

You try

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You try

TlHe 20881

42_______

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Radioactive decay

The spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of nuclear radiation (particles, electromagnetic radiation, or both).

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Radioactive nuclide

Unstable nucleus that undergoes radioactive decay.

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Alpha emission

Alpha particle (a) – two protons and two neutrons (a helium nucleus)

Emitted from the nucleus during some kinds of radioactive decay.

Restricted to very heavy nuclei – both protons and neutrons need to be reduced for stability

HeThU 42

23490

23892

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Beta emission

Decreases number of neutrons A neutron is converted into a proton and

an electron. Beta particle (b) – an electron emitted

from the nucleus during some kinds of decay.

PaTh 23491

01

23490

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Positron emission

Decreases number of protons A proton is converted into a neutron by

emitting a positron – a particle that has the same mass as an electron, but a positive charge

NO 157

01

158

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Electron capture

Increases number of neutrons A nucleus captures one of its inner

orbital electrons The electron combines with a proton to

form a neutron

RhePd 10045

01

10046

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Gamma emission

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

Supports the nuclear shell model Gamma emission usually occurs

immediately after other types of decay

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Decay series

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

Parent nuclide – the heaviest Daughter nuclides – produced by the

decay of the parent nuclide

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Artificial transmutation

Bombarding stable nuclei with charged and uncharged particles to create artificial radioactive nuclides

Great quantities of energy are needed Particle accelerator

Used to fill in the gaps in the periodic table and extend the table past uranium Transuranium elements – have more

than 92 protons

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Half-life, t1/2

How long it takes for half the atoms in a sample to disintegrate.

If we start with n atoms, after 1 half-life, we will

have 2

n atoms left.

After 2 half-lives, we will have 22or

4

nn atoms left.

After 3 half-lives, we will have 32or

8

nn

atoms left.

In general, x

n

2

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Half-life

We can’t predict when an individual atom will decay, only the rate of decay for a large number of atoms.

There is a table on page 708.

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Example

Uranium-238 decays through alpha decay with a half-life of 4.46 x 109 years. How long would it take for 7/8 of a sample of uranium-238 to decay?

3 half-lives, or 1.34 x 1010 years

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Example

The half-life of polonium-210 is 138.4 days. How many milligrams of polonium-210 remain after 415.2 days if you start with 2.0 mg of the isotope?

0.25 mg

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Example

The half-life of iodine-131 is 8.040 days. What percentage of an iodine-131 sample will remain after 40.2 days?

3.12 %

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Nuclear Radiation

Alpha particles, beta particles (positive or negative), and gamma rays.

Have different penetrating powers

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Alpha particles

Large mass (4 amu) and charge (+2). Can’t travel far in air Low penetrating power

Cannot penetrate skin Can be stopped by a sheet of paper

Harmful if ingested or inhaled

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Beta particles

Travel close to the speed of light Penetrate about 100 times as much as

alphas Can travel a few meters in air Can be stopped by lead or glass

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Gamma rays

Travel at the speed of light Greatest penetrating ability Can travel indefinitely through air or

empty space Can only be stopped by thick layers of

lead or concrete.

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Roentgen

Unit used to measure nuclear radiation The amount of radiation that produces

2 x 109 ion pairs when it passes through 1 cm3 of dry air.

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rem

Roentgen equivalent man The quantity of ionizing radiation that

does as much damage to human tissue as is done by 1 roentgen of high-voltage X-rays.

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Radiation exposure damage

DNA mutations Cancer Genetic effects

Can come from direct radiation exposure or by interaction with previously ionized molecules

In the US, average yearly exposure is 0.1 rem.

Up to 0.5 rem is permissible.

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Radiation detection Film badges

Used by people working with radiation Film is exposed by radiation

Geiger-Müller counters Count electric pulses carried by ionized

gas Best for beta particles

Scintillation counters Used when radiation causes materials to

emit visible light

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Radioactive dating

Determining the age of a substance based on the amount of radioactive nuclides present

Carbon-14 is used for organic materials up to 50 000 years old

Others used for older materials and minerals up to 4 billion years old

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Radioactive medicine

Used to destroy cancer Used to detect cancer and other

diseases Radioactive tracers

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Radioactive agriculture

Tracers can be used to determine fertilizer effectiveness

Radiation can be used to extend shelf life by killing bacteria and insects

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Nuclear waste containment

Waste can have a half life from a few months to thousands of years.

It must be contained to protect living organisms

Can be on-site storage or off-site disposal

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Nuclear Waste storage

Usually for rods from power plants. Can be stored in pools of water or dry

casks (concrete and steel). Usually a temporary solution

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Nuclear waste disposal

Materials are never meant to be retrieved.

Careful planning is needed. There are currently 77 disposal sites in

the US.

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Nuclear fission

A very heavy nucleus splits into more-stable nuclei

Mass of products is less than mass of reactants Releases enormous amounts of energy

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Chain reaction

The material that starts the reaction is one of the products and can start another reaction.

Critical mass – minimum amount of nuclide that is needed to sustain a chain reaction

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Nuclear reactors

Use controlled fission chain reactions to produce energy or radioactive nuclides.

Uncontrolled fission chain reactions – atomic bombs

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Nuclear power plants

Use heat from nuclear reactors to produce electrical energy

Main components Shielding Fuel Control rods Moderator Coolant

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Shielding

Radiation-absorbing materials that decreases amount of gamma rays leaving the reactor.

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Fuel

What powers the chain reaction Usually Uranium-235

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Control rods

Neutron absorbing rods Control reaction by limiting number of

free neutrons

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Moderator

Used to slow down fast-moving neutrons Fission of uranium is more efficient with

slower neutrons

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coolant

Absorbs heat from the reaction to produce electricity

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Nuclear fusion

Light-mass nuclei combine to form a heavier, more stable nucleus

Releases more energy per gram of fuel than fission

Takes place in stars (including the sun) Hydrogen to helium

Uncontrolled reactions – hydrogen bombs

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Fusion requirements

High heat and pressure needed Right now, no known material can

withstand the initial temperatures (100 million K) needed for controllable fusion.