Nuclear ChemistryText Chapter 18

Tomotherapy machine for radiation treatment of cancer at Johns Hopkins

Nuclear Submarine

Glow in the Dark Stars

Nuclear ChemistryWhat does nuclear refer to?

• Nucleus

• Protons & neutrons (nucleons).

• Overall positive charge.***

• Strong force holds nucleons together.

• Neutrons space positive charges and add stability.

Who discovered the very dense, positive nucleus?

Isotopes

• Same atm #(protons) but diff # of neutrons (diff atomic mass).

• Most isotopes are stable but many are unstable.

• If unstable, the neutrons can’t balance all protons and the nucleus spontaneously decays, emitting radiation and/or particles (radioactivity).

• Radioactive isotopes are called radioisotopes.

Predicting Nuclear Stability

• Stability of isotopes is based on the ratio of neutrons and protons in its nucleus.

• Low atomic #’s(<20) stable atoms have a ratio of neutrons/protons=1.

• High atomic #’s stable atoms have ratio of neutrons/protons=1.5.

• Above atomic #83, no atoms are stable. How shown on Ref Tables?

Radioactivity or Radioactive Decay

• Process by which an unstable nucleus emits particles and/or radiant energy.

• If emitted particles are protons, what will happen?• The atomic # is altered and one element is changed to

another due to the nuclear change (transmutation).

Emanations:emission of particles and/or energy from nucleus

• Types of Emanations differ from each other in mass, charge, penetrating power and ionizing power.

• Reference Table O.

Alpha decay ()• Alpha particle (helium

nucleus) is given off as a result of nuclear disintegration.

• High energy, relative velocity.

• Shielding: stopped by thickness of a sheet of paper, skin

Beta decay (-)• Beta particle (high speed electron) is given off

as the result of nuclear disintegration.• High velocity, low energy.• Beta particles have virtually no mass.• Shielding: stopped by 1cm of aluminum,

average thickness of book.

A neutron is converted to a

proton by emitting an electron.

Gamma Radiation ()

• Gamma rays are similar to high energy x-rays.

• Travel @ speed of light like all other forms of electromagnetic energy.

• Do not have charge or mass. Type of radiation (photons) , not particles.

• Shielding: 13cm of lead

Positron decay(+)• Positrons are given off as

result of nuclear disintegration.

• Positrons are antiparticles of electrons.

• When a positron hits an e-, they annihilate each other, forming 2 gamma rays (high penetration and high ionizing power). A proton is

converted to a neutron.

Separating , , and emissions

• Gamma rays and alpha and beta particles can be separated using an electric or magnetic field.

Geiger Counters are used to measure radioactivity.

Review

Radioactivity Equations• Notice that the sum of the mass numbers

(superscripts) on both sides of equation are equal (226=222+4). Why?

• Law of Conservation of Mass• The sum of the atomic numbers (subscripts) on both

sides of equation are also equal (88=86+2). Why?• Law of Conservation of Charge

Balancing Radioactivity Equations• Use both Reference Table O & (PT).

• If the atomic number changes, remember the identity of the element changes.

• What 2 quantities must be balanced?

#1

#2 The breakdown of Co60 in cancer radiation therapy.

More Balancing of Radioactivity Equations

• First use Reference Table N to determine the decay mode.

• Continue and finish similar to other problems.

Radioactive Orchestra

ArtificialRadioactivity

• Elements can be made radioactive by bombarding their nuclei with high energy particles.

• Use particle accelerators.

• Most elements from 93 and up (transuranium elements) were created with the use of particle accelerators.

CERN Particle Accelerator in France & Switzerland

+

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NuclearReactions

• Mass is converted to energy.

• Think E=mc2.• Produce tremendous

amounts of energy!!!• 2 Types: Fission and Fusion

Nuclear explosion at sea

Nuclear Fission• Type of nuclear rxn.• Splitting of nucleus of

a large atom into two or more fragments.

• Produces additional neutrons and a lot of energy.

• *Think binary fission or fissure. (splitting)

ChainReactions

• Each nucleus emits 3 neutrons that can cause the fission of another radioactive nucleus and so on.

• Continues until a stable compound forms.

• Ex: Atomic Bomb.• Nuclear reactors can

control fission chain reactions and convert released energy into electric power.

Parts of a NuclearReactor

• Fuel (U235 & Pu239)• Moderator (slow down

speed of neutrons, H2O, Be or graphite)

• Control Rods (absorb neutrons, B & Cd)

• Coolants (lowers temp,H2O)

• Shielding (protects the reactor and people from radiation, steel or concrete)

NuclearFusion

• Two nuclei unite to form a heavier nucleus with release of enormous amounts of energy.

• High temps and High pressures are necessary.

• Occurs in the Sun and stars and the Hydrogen Bomb.

• Think Unite, Fusion.

1st aerial test of H-bomb makes Bikini Atoll unlivable

Compare & Contrast Fission andFusion

• Differences• Fission:splitting

Fusion: uniting• Fusion releases much more

energy than fission.• Fission produces

radioactive waste, fusion only produces He.

• Similarities• Both release a lot of

energy.• Both convert mass

into energy.

Half-lives

• Each radioisotope has a specific mode and rate of decay (half-life).

• Ref Table N• Half-life is the time

required for one-half of the nuclei of a given sample of an isotope to disintegrate.

Half-life Problems can be used to find the following 4:1. Fraction of

radioisotope remaining (left)

2. Half-life3. Initial amount

(original amount) of radioisotope

4. Age of sample containing radioisotope (Radioactive Dating)

Decay animation

Benefits of Radioisotopes• Tracers • Medical Diagnosis or Treatment• Radiation of food• Radioactive Dating• Nuclear Power• Industrial Measurement• Industrial Applications

Tracers

• Radioisotopes can be used to follow the course (trace/track) of a chemical or biological reaction.

• This is one way scientists learn about the many steps involved in reactions.

• For example, C-14 has been used as a tracer to learn the steps of respiration (Kreb’s cycle)

Medical Diagnosis & Treatment• Isotopes with very

short half-lives and which will be quickly eliminated from the body are used in detecting and treating diseases.

• Has created field of medicine called “Nuclear Medicine.”

A PET scan using radiotracers to identify heart disease

Medical Diagnosis & Treatment Examples

• Tc-99 is used for pinpointing brain tumors and bone scans.

• Radium and Cobalt-60 are used to attack cancer.

• I-131 is used for diagnosis and treatment of thyroid disorders.

A CT scan of the brain using Tc-99

Irradiation of Food

• Radiation kills bacteria, molds and yeast. It permits food to be stored for a longer time.

Symbol for Irradiated Food

Radioactive Dating

• Comparing the ratio of radioactive to stable isotopes in a rock sample can give the age of the rock or geologic formation (mountains, etc.) (ie: U-238 to Pb-206

• Ratio of C-14:C-12 can be used to find age of organic materials.

Nuclear Power

• Nuclear reactors are used to produce electrical energy or electricity.

Industrial Measurement

• A beam of subatomic particles (α,β, or γ) is blocked by a certain thickness of metal. Measuring the fraction of the beam that is blocked gives a precise measurement of the thickness of the metal.

Industrial Applications

• A variety of radioisotopes are used in everyday applications.

• Am-251 is used in smoke detectors.

• The neutron activation analysis method can be used to detect artwork forgeries.

Risks of Radioisotopes• Biological Damage• Long Term Storage• Accidents• Pollution

Uranium Implosion Little Boy and Plutonium Implosion Fat Man

Biological Damage

• Radiation exposure can damage or destroy cells of organisms. Examples are burns, cataracts, cancer, etc.

• When reproductive cells are damaged, the damage is passed on to offspring.

Radiation burns from A-bomb in Hiroshima

Long Term Storage

• Fission products from nuclear reactors are very radioactive and dangerous.

• These products must be stored in special containers underground for hundreds of thousands of years until radioactively decayed.

Yucca Mtn. Storage Project

Accidents• Nuclear reactor

accidents can cause fuel and wastes to escape and harm the environment and biosphere.

• Example: Chernobyl, Ukraine (1986) in the former USSR. Uncontrolled chain reaction and fire allowed winds to spread radioactivity across Europe.

Chernobyl Meltdown Solidified

Pollution

• Traces of radioactive materials are present in air, water, food and soil either naturally or released by human activities. People can be harmed if there is too much radioactive material.