 Reactions that affect the nucleus  Can change the identity of the element (if number of protons change)

 When the number of protons in the nucleus of an atom are changed, resulting in the formation of a different element ◦ Identity of the element changes  Elements and isotopes that are unstable and break apart are said to be naturally RADIOACTIVE

 Natural transmutation – when an unstable atom (radioisotope) spontaneously breaks apart into a more stable atom (ONE reactant)  Artificial transmutation – when one splits an atom by bombarding the nucleus with a high energy particle (TWO reactants)

 Very large nuclei are unstable ◦ Elements with atomic # higher than 83 are unstable because they are too big  Stable nuclei have an equal number of protons and neutrons ◦ Isotopes of elements that do NOT have a 1:1 proton:neutron ratio are also unstable

 As radioactive elements and isotopes break apart to turn into more stable elements, they give off a lot of different types of particles ◦ Table O in the Reference Table

 Consider a proton from Table O 1 p 1 Mass Charge Mass = A X Charge = Z

 Alpha particles – Helium nucleus ◦ Most massive; low penetrating power  Beta particles – electron  Gamma Radiation – pure energy ◦ No mass and no charge (not affected by an electric field) ◦ Highest penetrating power  Proton – + charged  Neutron – neutral ◦ No charge (not affected by an electric field)  Positron – like an electron, but has a + charge

 Alpha ◦ Can be stopped by a thin Al foil or piece of paper  Beta ◦ Can get through several cm of Al or a piece of ply- wood  Gamma ◦ Can get through about 30cm of lead

 Type of particle given off for different isotopes during nuclear decay is given in Table N  Write nuclear decay equations using Table N  Mass and Charge are always conserved (stay the same)

 Since Mass and Charge are always conserved, the top and bottom numbers on each side of the equation must add to up be the same!!  Example 238 U  + 92 Look up Atomic # of U Mass and charge MUST add up to the same on both sides of the equation Th Look up element with atomic # of 90 2 Look up decay mode in Table N He 4  worksheet

 Nuclear reactions release huge amounts of energy  End up with a little less mass than you started with  Consider Einstein’s equation E = mc 2  Mass and energy can be converted from one to the other ◦ Nuclear reactions convert a small amount of mass into a huge amount of energy

 A very large (heavy) nuclei breaks apart into smaller pieces  Typical reaction with Uranium  Products are very radioactive  Chain reaction – needs to be controlled well U + 1 n  142 Ba + 91 Kr n + ENERGY

 Very small (lighter) nuclei combine to form a larger nucleus  Difficult because nuclei are + charged, and + charges repel each other  Requires very high temperatures and pressures  Reaction occurs on the surface of the sun 2 H + 3 H  4 He + 1 n + ENERGY

 The amount of time it takes for one half of an original amount of a naturally radioactive isotope to decay  Values of half life are given in Table N  Values do NOT depend on temperature, pressure, or the original amount Original sample (100g) ½ life (50 g) ½ life (12.5 g) ½ life (25 g)

 First figure out how many half lives have gone by; draw that many arrows  Figure out the original amount  Figure out how much is left Original sample (100g) (1 g) ½ life (50 g) (½ g) ½ life (12.5 g) (1/8 g) ½ life (25 g) (¼ g)

 If the half-life of a radioisotope is 3 hours, what fraction of an original amount remains unchanged after 15 hours?  An original sample of K-40 has a mass of 25.00g. After 3.9 x 10 9 years, grams of the original sample remains unchanged. What is the half-life of K-40? Total time = # of half lives Half life 15 hours = 5 3 hours 1  1  1  1  1_ /32 nd of the sample remains

 Carbon-14 is used in “Carbon dating” ◦ All living things have Carbon in it, and one of the isotopes of Carbon, C-14, is radioactive ◦ Can determine the age of once living material by examining the amount of radiation given off by the sample ◦ Amount of radiation decreases over time  Uranium238/Lead-206 ratio ◦ Unstable U-238 decays to a stable Pb-206 ◦ As U-238 decreases, Pb-206 increases ◦ Can date rocks and other geological formations by looking at the ratios of U-238 and Pb-206