Nuclear Chemistry Half-Life You will be able to predict amount of sample after certain amount of time has elapsed.

Chemical vs. Nuclear Changes Chemical reactions happen when bonds are broken and formed Same atoms, may be rearranged Involve electrons on outside of atom Relatively small ∆E Can change speed by temperature, pressure, concentration & catalysts Nuclear reactions happen when unstable nuclei releases particles and/or energy rays. Most atoms changed into another element Involve nucleus: protons, neutrons, electrons Enormous ∆E Cannot change speed of decay

What kind of reaction? Chemical Physical Nuclear H2O(l) → H2O(s) Atoms rearrange to form new things, changes chemical bonds Physical Same composition, change of state Nuclear New atoms are formed, nucleus different so transmutation H2O(l) → H2O(s) 2H2(g) + O2(g) → 2H2O(l) 2H(g) + 2H(g) → 4He(g) He(g) → He(l)

T1/2 → Measuring length of decay Half-lives measure the time it takes half of the nuclei in a radioactive sample to decay. Some are very short…fluorine-21 is 5 seconds; polonium-214 is 163.7 microseconds Others are longer…phosphorus-32 is 14.28 days; carbon-14 is 5730 years. Half lives are constant; radioisotopes are active/dangerous for 10 half lives.

For example Half-life of Sr-90 is 29 years How much would be left after 3 half-lives? Sr-90 was a waste product of nuclear bombs. Testing occurred often in the 1950s. How many half lives have past since then? Human body recognizes Sr-90 as calcium and stores it in bones…high rate of leukemia!

The T1/2 of fluorine-21 is 5 seconds. How much will remain after 30 seconds if you start with 21 grams? 21 ½ x ½ x ½ x ½ x ½ x ½ = (1/2)6 = 1/64 x 21 grams = 0.33 grams 0.329 10.5 5.25 2.63 1.32 0.658

Radiochemical Dating The age of an object can be determined by measuring the amount of radioisotope remaining. This can be done with living things by carbon dating. Click box to view movie clip.

Homework HW #2