Mullis 1

Nuclear chemistry

• Nuclide: Name for an atom in nuclear chemistry

• Nucleons: Protons and neutrons• Nuclear reaction: Reaction that affects the

nucleus of an atom• Transmutation: The change in the identity

of a nucleus due to change in number of protons

(Number of protons is an atom’s ID number)

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Isotopes in Nature

• Most elements in nature occur as mixtures of isotopes.

• All naturally occurring isotopes of elements with atomic numbers > 83 are radioactive.

• Beyond 83 amu, the repulsive force of protons is so great that no stable nuclides exist.

• However, some natural radioactive isotopes are also found among lighter elements.

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Band of Stability Unstable if neutron: proton ratio > 1.5:1

Source: http://www.sprawls.org/ppmi2/MATTER/#NUCLEAR%20STABILITY

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Types of Radioactive DecayType Symbol Charge Mass (amu)

Alpha particle

He 2+ 4.002 60

Beta particle

β 1- 0.000 548 6

Positron β 1+ 0.000 548 6

Gamma ray γ 0 0

4

4 2

0

- 1

0

+ 1

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Nuclear equations• Atomic numbers and mass numbers are

conserved.• Use the proper type of particle to balance.• If atomic number is 0, it is a neutron.• If atomic number is -1, it is an electron.• If atomic number is 1 or more it is an element,

except for a positron.• A positron increases the number of protons by 1

and does not change mass. ( β ) 0

+1

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Example nuclear equations• U ____ + Th

• U He + Th

• Ar + ____ Cl

• Ar + e Cl

• Be + He C + n

238

92

234

90

234

90

238

92

4

2

37

18

37

18

37

17

37

17

0

-1

4

2

1

0

12

6

9

4

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Half-life• Time required for half the atoms of a

RADIOACTIVE nuclide to decay.

• How many ½-lives have passed after 60 days if a nuclide has a given ½-life of 12 days?

Number of ½-lives = time elapsed x 1 half-life

given time

Number of ½-lives = 60 days x 1 half-life = 5 half-lives

12 days

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Using half-life dataMultiply the amount of a substance by (1/2)y to

find remaining substance after y half-lives have passed.

Ex.: Polonium-210 decays with a half-life of 138.4 days. How many milligrams remain after 415.2 days if you start with 2.0 mg of Po-210?

y = 415.2 days x 1 half-life = 3 half-lives

138.4 days

2.0 mg (1/2)(1/2)(1/2) = 0.25 mg

2.0 mg (1/2)3 = 0.25 mg

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Cobalt-60 decays with a half-life of 10.47 min. How many milligrams remain after 104.7 min if you start with 10.0 mg of Co-60?

y = 104.7 min x 1 half-life = 10 half-lives 10.47 min

10.0 mg (1/2)10 = 0.00977 mg

The half-life of carbon-14 is 5715 years. How long will it be until only half of the carbon-14 in a sample remains?

y = number of ½-lives 1/2 sample remains. 1 sample x (1/2)y = 1/2 sample

y = 1 half-life 1 half-life = 5715 years

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Another half-life exampleUranium-238 decays with a half-life of 4.46 x 109

years. How long would it take for 7/8 of a sample of U-238 to decay?

y = number of ½-lives

1/8 sample remains.

1 sample(1/2)y = 1/8 sample

yln(1/2) = ln(1/8) Or….

(1/2)(1/2)(1/2) = 1/8 y = 3 half-lives

3(4.46 x 109 years) = 1.34 x 1010 years

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How did Becquerel discover radioactivity?

• Fluorescence: Certain materials emit light when struck by radiant energy such as UV rays.

• He wrapped a photographic plate in black paper and left it beside a fluorescent mineral in a drawer during a period of several cloudy days. He had planned to place the mineral and the plate in the sun so that the minerals could fluoresce (emit X-rays).

• When the film was removed, it was exposed, just as if the mineral had been exposed to radiation and fluoresced.

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Ionizing Radiation

• Ionizing radiation – Higher energy waves that include UV, X-ray and

gamma rays. – Can eject electrons from atoms and molecules,

forming fragments and ions. These fragments can cause disruptions of normal cellular chemistry.

• Non-ionizing radiation– Electromagnetic radiation in the visible and lower

energy region of the spectrum.– Transfers energy to matter, causing molecules to

vibrate or move their electrons to a higher energy level.

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Exposure to Ionizing Radiation• Background radiation from natural

sources:• Decay of radioactive elements(U-238, U-

235 and Th-232) found in soil and water• Very high-energy particles from outer

space• Radioactive isotopes in the atmosphere

(Rn-222) and its decay products such as polonium.

• Radioactive isotopes in foods such as carbon-14 and potassium-40.

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Units of Radiation exposure• gray (Gy): The SI unit used to measure the

quantity of ionizing radiation delivered to a sample such as human tissue.

1 Gy = 1 J/kg• Sievert (Sv): The SI unit that expresses the ability

of any radiation to cause ionization in human tissue.

1 Sv exposure = same effect as 1 Gy of gamma rays

• rad measures absorbed dose of radiation– 1/100th of 1 gray

• rem measures the ionizing effect on living organisms.– 1/100th of 1 Sv

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Tissue Damage Due to Ionizing RadiationTwo factors: 1. Radiation density (# radiations within a given

volume)2. Dose (the quantity of radiation received)Gamma rays and X-rays break bonds in molecules—at

low levels, the body can repair these molecules.• Damage to nucleic acid and proteins are big

concerns with high levels of radiation.• Proteins make up enzymes, molecules that control

the rates of chemical reactions in the body.• Nucleic acids in DNA may incur mutations, or

changes in DNA structure which may result in production of altered proteins.

• Cancer: cell growth and metabolism are out of control.

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Geiger-Mueller Counter• Used to detect radiation• Detector tube is filled with gas such as argon.• If ionizing radiation enters the end of tube, it will

ionize the gas particles.• When ions are formed, (-) ions are drawn to (+)

charged center, and the (+) ions go to the (-) outer wall.

• Moving of charged particles makes a pulse of electric current.

• Each pulse is counted to indicate the INTENSITY of radiation.

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Scintillation Counter• When radiation strikes the detector,

flashes of light are emitted.

• These flashes are detected electronically by a photomultiplier tube.

• The photomultiplier tube converts light to an electron pulse and increases the pulse many times.

• The detector is a solid whose atoms are excited by ionizing radiation. An example is NaI.

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Solid State Detector

• Monitors change in movement of electrons when they are exposed to radiation.

• Counts the changes electronically.

• Electron movement is monitored through silicon or another semiconductor.

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Types of particles in radioactive decay

• Alpha particle 4 2

– equal to He atom– Easy to stop from penetrating (large mass)– Very damaging to living tissue if not stopped (large mass

and high energy,charge)

• Beta particle 0

-1

– A neutron decays to a proton and an electron.– Net effect to the nuclide is that neutron changes to proton.

The electron is ejected at high speed = beta emission.– Lighter and faster than alpha– Not as damaging to living tissue, but harder to block than

alpha.

He

β

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Gamma rays (γ)• No charge, no mass• Most penetrating (need barrier such as lead to

block)• Result of alpha or beta decay leaving nuclei in

an excited and metastable state• Energy released from this state is high energy

electromagnetic radiation.• Least damage to tissue over comparable

distances: Damage is related to the extent of ionization created by the radiation.

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Nuclear Fission

• Nucleus of one heavy atom splits into 2 or more nuclei.

• Bombardment usually used to start fission.• Products are nuclei and nuclides from decay.• Used for powering nuclear reactors,

submarines and missiles• Chain reaction: Product of one reaction

starts another• Produces more nuclear waste than fusion

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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.

• Hydrogen is usually the fuel for fusion.

• Fusion reaction control is limited by the high initial temperature.

• The sun is a good example of fusion: – 4 H nuclei combine to form He nucleus. – Loss of mass = huge energy release– 4 H nuclei He nucleus + 2 β particles

42

11

0+1

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Nuclear Reactor Power Plants• Fuel rods: Contain uranium dioxide pellets about the

size of chalk sticks. These contain ~3% fissionable U-235—enough to sustain a chain reaction, but too little for a nuclear explosion.

• Shielding: Radiation-absorbing material used to contain radiation

• Control rods: Neutron-absorbing rods that control reaction by limiting free neutrons (boron and cadmium)

• Moderator: Slows down fast neutrons produced by fission (water and graphite)