Nuclear ChemistryNuclear Chemistry

How are chemical reactionsand nuclear reactions

different?

ChemicalReactions

Nuclear Reactions

Chemical Reactions

1. Occur when bonds arebroken and formed.

2. Atoms remain unchangedthough they may be rearranged.

3. Involve only valence electrons.

4. Associated with smallenergy changes.

5. Reaction rate is influenced bytemp., pressure, concentration,

and catalysts.

Nuclear Reactions

1. Occur when nuclei emit particlesand/or rays.

2. Atoms are often converted into atomsof another element.

3. May involve protons, neutrons, and electrons.

4. Associated with large energychanges.

5. Reaction rate is not normally affectedby temp., pressure, or catalysts.

► Unstable atomic nuclei will spontaneously Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability. decompose to form nuclei with a higher stability.

► The decomposition process is called The decomposition process is called radioactivityradioactivity. .

► The energy and particles which are released The energy and particles which are released during the decomposition process are called during the decomposition process are called radiationradiation. .

► When unstable nuclei decompose in nature, the When unstable nuclei decompose in nature, the process is referred to as process is referred to as natural radioactivitynatural radioactivity. .

► When the unstable nuclei are prepared in the When the unstable nuclei are prepared in the laboratory, the decomposition is called laboratory, the decomposition is called induced induced radioactivity.radioactivity.

There are three major types There are three major types of natural radioactivity: of natural radioactivity:

Types of Radioactivity

Alpha Beta Gamma

Alpha RadiationAlpha Radiation

► Alpha radiation consists of a stream of Alpha radiation consists of a stream of positively charged particles, called positively charged particles, called alpha alpha particlesparticles, which have an atomic mass of 4 , which have an atomic mass of 4 and a charge of +2 (a helium nucleus). and a charge of +2 (a helium nucleus).

► When an alpha particle is ejected from a When an alpha particle is ejected from a nucleus, the nucleus, the mass numbermass number of the nucleus of the nucleus decreasesdecreases by fourby four units and the units and the atomic atomic number decreases by twonumber decreases by two units. units.

► For example: For example: 2382389292U -> U -> 44

22He + He + 2342349090Th Th

► The helium nucleus is the The helium nucleus is the alpha particle.alpha particle.

Beta RadiationBeta Radiation►Beta radiation is a stream of electrons, Beta radiation is a stream of electrons,

called called beta particlesbeta particles. . ►When a beta particle is ejected, a When a beta particle is ejected, a

neutronneutron in the nucleus in the nucleus is converted is converted to a protonto a proton, so the , so the mass numbermass number of of the nucleus the nucleus is unchangedis unchanged, but the , but the atomic number increases by oneatomic number increases by one unit.unit.

►For example: For example: 2342349090Th -> Th -> 00

-1-1e + e + 2342349191Pa Pa

►The electron is the The electron is the beta particlebeta particle. .

Gamma RadiationGamma Radiation►Gamma raysGamma rays are high-energy photons are high-energy photons

with a very short wavelength (0.0005 to with a very short wavelength (0.0005 to 0.1 nm). 0.1 nm).

►The emission of gamma radiation results The emission of gamma radiation results from an energy change within the from an energy change within the atomic nucleus. atomic nucleus.

►Gamma emission Gamma emission changes neither the changes neither the atomic number nor the atomic massatomic number nor the atomic mass. .

►Alpha and beta emission are often Alpha and beta emission are often accompanied by gamma emission, as an accompanied by gamma emission, as an excited nucleus drops to a lower and excited nucleus drops to a lower and more stable energy state. more stable energy state.

Bombardment ReactionsBombardment Reactions

► Alpha, beta, and gamma radiation also Alpha, beta, and gamma radiation also accompany induced radioactivity. accompany induced radioactivity.

► Radioactive isotopes are prepared in the lab Radioactive isotopes are prepared in the lab using bombardment reactions to using bombardment reactions to convert a convert a stable nucleus into one which is stable nucleus into one which is radioactive. radioactive.

► PositronPositron (particle with the same mass as (particle with the same mass as an electron, but a charge of +1 instead of -an electron, but a charge of +1 instead of -1) 1) emission isn't observed in natural emission isn't observed in natural radioactivity, but it is a common mode radioactivity, but it is a common mode of decay in induced radioactivity. of decay in induced radioactivity.

► Bombardment reactions can be Bombardment reactions can be used to used to produce very heavy elements, produce very heavy elements, including many which don't occur in including many which don't occur in nature.nature.

How to shield against How to shield against radiationradiation

► In general, In general, the shorter the EM wavelength, the shorter the EM wavelength, the thicker and denser the shield material the thicker and denser the shield material must be.must be.

► Ultraviolet (UV) can be blocked simply by a Ultraviolet (UV) can be blocked simply by a sufficiently opaque sheet of plastic. We are all sufficiently opaque sheet of plastic. We are all familiar with tinted sunglasses that promise to familiar with tinted sunglasses that promise to block some 97% of solar UV rays. Not much block some 97% of solar UV rays. Not much additional protection is required in space. additional protection is required in space.

► X-rays and gamma rays are another matter. X-rays and gamma rays are another matter. Where intense x-rays and gamma rays occur, it Where intense x-rays and gamma rays occur, it requires several inches or centimeters of lead requires several inches or centimeters of lead and/or concrete to provide adequate shielding. and/or concrete to provide adequate shielding.

Blocking Alpha particlesBlocking Alpha particles

►Alpha particles are very large Alpha particles are very large particles. As such they don't penetrate particles. As such they don't penetrate very deeply into many things. very deeply into many things.

► In fact, alpha particles will not even In fact, alpha particles will not even penetrate the epidermal (dead) layer penetrate the epidermal (dead) layer of skin, and so of skin, and so present no special present no special hazard to humans.hazard to humans.

►A sheet of reasonably thick paper A sheet of reasonably thick paper will block all alpha particles. will block all alpha particles.

Blocking Beta particlesBlocking Beta particles

► Beta particles are very small and Beta particles are very small and can can penetrate centimeters into the body. penetrate centimeters into the body.

► But luckily they're But luckily they're too small to cause too small to cause much damage if they hit anything. much damage if they hit anything.

► But there's a special problem here. When But there's a special problem here. When beta particles hit large atoms, the impact beta particles hit large atoms, the impact causes those atoms to give off x-rays. Metal causes those atoms to give off x-rays. Metal atoms are usually quite heavy, and so are atoms are usually quite heavy, and so are especially susceptible to this kind of re-especially susceptible to this kind of re-radiation which is known by its German name radiation which is known by its German name "Bremsstrahlung". In fact, this is how x-rays "Bremsstrahlung". In fact, this is how x-rays are produced intentionally for medical are produced intentionally for medical applications. applications.

►The best materials to shield against beta The best materials to shield against beta particles have lots of hydrogen atoms in particles have lots of hydrogen atoms in them. them.

►Hydrogen atoms are light, and so absorb Hydrogen atoms are light, and so absorb the particles without giving off x-rays. the particles without giving off x-rays.

►Plain old water works very well. In fact, 4 Plain old water works very well. In fact, 4 inches (10 centimeters) of water will inches (10 centimeters) of water will block almost all background beta block almost all background beta particles. particles.

►But water is impractical for shielding in But water is impractical for shielding in space, so high-density polyethylene space, so high-density polyethylene (HPDE, chemical formula CH(HPDE, chemical formula CH22CHCH22...) is ...) is frequently used instead. This also frequently used instead. This also effectively blocks protons. effectively blocks protons.

Blocking Gamma raysBlocking Gamma rays► Gamma rays are a form of electromagnetic Gamma rays are a form of electromagnetic

radiation, so they have no mass, carry radiation, so they have no mass, carry energyenergy and and travel the speed of light.travel the speed of light.

► Where intense gamma rays occur, it requires Where intense gamma rays occur, it requires several inches or centimeters of lead and/or several inches or centimeters of lead and/or concreteconcrete to provide adequate shielding. to provide adequate shielding.

Radiation Source  Dose Radiation Source  Dose (rems)(rems)

► Chest x-ray Chest x-ray  0.010    0.010    ► Mammogram Mammogram  0.030    0.030    ► Cosmic raysCosmic rays 0.031 annually   0.031 annually   ► Human body Human body  0.039 annually  0.039 annually 

    ► Household radon Household radon  0.200 annually  0.200 annually 

    ► Cross-country airplane flight Cross-country airplane flight  0.005  0.005 

► Are there any legal limits for radiation Are there any legal limits for radiation exposure? exposure?

► Worker Category Legal Limit Worker Category Legal Limit       ► 18-year old male 18-year old male  5 rem/year    5 rem/year    ► Pregnant womanPregnant woman 0.500 rem 0.500 rem