Mr. ShieldsRegents Chemistry U02 L03

Nuclear Decay Series Uranium has an atomic number greater than 83. Therefore it is naturally radioactive. Most abundant isotope Alpha Particle

Thorium Decay Of course Thorium’s atomic number is also greater than 83. So it to is Radioactive and Goes through beta decay. 234 Pa + 0 e 91 Protactinium

U-238 Decay Series Protactinium decays Next and so on until we reach a stable Non-radioactive Isotope of lead Pb-206 Atomic No. 82

U-238 Decay Series

Decay Series U-238 IS NOT the only radioactive isotope that Has a specific decay series. All radioisotopes have specific decay paths they follow to ultimately reach stability

Decay Series Time Span The next Question you might consider asking is how long does this decay process take? The half life of U-238 is about 4.5 billion years which is around the age of the earth so only about half of the uranium Initially present when the earth formed has Decayed to date. Which leads us into a discussion of Nuclear Half life

Nuclear Half-life Unstable nuclei emit either an alpha, beta or positron particles to try to shed mass or improve their N/P ratio. But can we predict when a nucleus will Disintegrate? The answer is NO for individual nuclei But YES if we look at large #’s of atoms.

Nuclear Half-life Every statistically large group of radioactive nuclei decays at a predictable rate. This is called the half-life of the nuclide Half life is the time it takes for half (50%) of the Radioactive nuclei to decay to the daughter Nuclide

Nuclear Half-life The Half life of any nuclide is independent of: Temperature, Pressure or Amount of material left

Beanium decay 64 beans 32 beans 16 beans 8 beans 4 beans Successive half cycles % What does the graph of radioactive decay look like? This is an EXPONENTIAL DECAY CURVE

Loss of mass due to Decay Amount of beanium Fraction left1½¼1/81/16 Half life’s1234 If each half life took 2 minutes then 4 half lives would take 8 min. The equation for the No. of half lives is equal to: T (elapsed) / T (half Life) 32 minutes / 4 minutes = 8 half life’s

Carbon 14 is a radionuclide used to date Once living archeological finds Carbon–14 Half-life = 5730 years 22,920/5730 = 4 Half-life’s t0t0

Half-Lives In order to solve these half problems a table like the one below is useful. For instance, If we have 40 grams of an original sample of Ra-226 how much is left after 8100 years? ½ life period% original remaining Time Elapsed Amount left grams yrs 20 grams ? ? ? ? 10 grams 5 grams 2.5 grams 1.25 grams

Problem: A sample of Iodine-131 had an original mass of 16g. How much will remain in 24 days if the half life is 8 days? Step 1: Half life’s = T (elapsed) / T half life = 24/8 = 3 Step 2: 16g (starting amount) 842g Half lives 123

Problem: What is the original amount of a sample of H– 3 if after 36.8years 2.0g are left ? Table N tells us that the half life of H-3 is yrs yrs / yrs = 3 half lives. Now lets work backward Half life32 grams Half life 24 grams Half life 18 grams Time zero16 grams

Problem: How many ½ life periods have passed if a sample has decayed to 1/16 of its original amount? Time zero1x original amount First half life½ original amount Second half life¼ original amount Third half life1/8 Fourth half life 1/16

Problem: What is the ½ life of a sample if after 40 years 25 grams of an original 400 gram sample is left ? Step 1: 25 grams4 half lifes 503 half lifes 100 g2 half lifes 200 g1 half life 400 gtime zero Step 2: Elapsed time = # HL Half-life 40 years = 4 HL Half-life Half life = 10 years