The Nucleus
• Remember that the nucleus is comprised of the two nucleons, protons and neutrons.
• The number of protons is the atomic number (Z)• The number of protons and neutrons together is
effectively the mass of the atom (A)
Isotopes (Nuclides)
• Different neutrons = different mass
• There are three naturally occurring isotopes of uranium:– Uranium-234– Uranium-235– Uranium-238
Nuclear Reactions
• Mass must be conserved = Law of Conservation of Mass
• Atomic Number must be conserved = Law of Conservation of Mass
• Larger nuclei can split into smaller
• Neutron can be a product
Radioactivity
• It is not uncommon for some nuclides of an element to be unstable, or radioactive.
• We refer to these as radionuclides.
Types of Radioactive Decay Alpha Decay
Loss of an -particle (a helium nucleus)
He42
U23892
Th23490 He4
2+
Types of Radioactive Decay Beta Decay
Loss of a -particle (a high energy electron)
0−1 e0
−1or
I13153 Xe131
54 + e0
−1
Types of Radioactive Decay Gamma Emission
Loss of a -ray
high-energy radiation
almost always accompanies the loss of a nuclear particle
00
Penetrating Ability
• Alpha least penetrating– Cannot penetrate skin– Can cause damage to mucous membranes– Cannot penetrate paper
• Beta = 100x more penetrating than alpha– Can penetrate paper & skin – Cannot penetrate lead or glass
• Gamma most penetrating– Can penetrate paper, skin, lead, glass and concrete
Radiation Exposure
• rem – the quantity of radiation that does damage to human tissue.
• Avg. exposure = 0.1 rem/yr
• Max. dose = 0.5 rem/yr
• Exposure long-term or in high dose = cancer and genetic mutations
• Detector = Geiger-Müller Counters
Half Life (t1/2)
• Time required for half of the atoms of a radioactive nuclide to decay.
• Ex: – Carbon-14 t1/2 = 5, 715 yrs
– Potassium-40 t1/2 = 1.3 x 109 yrs
– Polonium – 218 t1/2 = 3.0 min
– Uranium – 238 t1/2 = 4.46 x 109 yrs
– Astatine – 218 t1/2 = 1.6 sec
Ex: pg 709
Phosphorus-32 has a half life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with a 4.0 mg sample?
Nuclear Fission
• Radioactive nuclei bombarded with neutrons• Neutrons released in reaction go on to
bombard other radioactive nuclei• This process continues in what we call a
nuclear chain reaction.• Fission = “splitting”
Nuclear Reactors
Energy generated by fission = produces steam that turns a turbine connected to a generator.
Nuclear Fusion
• Fusion – “fusing”• Happens naturally at the center of our sun• Fusion would be a superior method of generating
power.– MUCH more energy produced– Products of the reaction are not radioactive.– Material used must be in the plasma state – Materials not readily available = more expensive