Nuclear Chemistry and Radioactive Decay

Isotopes Remember, not all atoms of the same element have the same mass. These are called Isotopes – forms of the same element but different # of neutrons. Some isotopes are RADIOACTIVE.

H H H Radioactive isotope http://education.jlab.org/glossary/isotope.html

Radio isotopes Have an Unstable nucleus - Isotopes that are unstable/radioactive often have too many or too few neutrons. they often become more stable by ejecting one or more parts of the atom in order to reach a stable proton/neutron ratio. - This ejection is known as Radiation

A. Types of Radiation 2+ 1- Alpha particle () helium nucleus paper 2+ Aluminum foil Beta particle (-) electron 1- lead Gamma () high-energy wave

Alpha Decay Emission of alpha particles a : helium nuclei two protons and two neutrons can travel a few inches through air can be stopped by a sheet of paper, clothing. Very dangerous if consumed

Alpha Decay Example Transmutation One element becomes another.

Another Example Numbers must balance!! Alpha Emission parent daughter nuclide daughter nuclide alpha particle Numbers must balance!!

Beta radiation Beta Emission electron Beta emission occurs in elements with more neutrons than protons/too many neutrons. a neutron splits into a proton and an electron. The high speed electron leaves the nucleus. The proton stays in the nucleus while the electron is emitted as “beta radiation”.

Gamma radiation Gamma Emission Gamma radiation occurs when an excited nucleus ejects energy out of the atom. A gamma ray has no mass and no charge/it’s energy not matter. Like light, it is a form of electromagnetic radiation. Gamma rays often are the by-product of other nuclear changes/reactions

Practice Nuclear Equations The alpha decay of Uranium-238: The beta decay of Iodine-131: U 238 92  Th 234 90 He 4 2 + I 131 53 Xe 54  + e −1

Nuclear Fission Nuclear fission is the type of reaction carried out in nuclear reactors. Neutrons bombard atoms of Uranium and particles and energy are released.

Nuclear Fission Neutrons released strike other nuclei, causing their decay and the production of more neutrons. This process continues in what we call a nuclear chain reaction. The heat and energy released is used to make a turbine spin which produces the desired electricity.

Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

Nuclear Reactors The reaction is kept in check by the use of control rods. These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

FUSION (not fission!) Fusion reactions do not occur naturally on our planet but are the principal type of reaction found in stars. The sun fuses hydrogen atoms to produce helium, subatomic particles, and vast amounts of energy.

Radioactive Half-Life (t1/2 ): The time it takes for half of a radioactive sample to decay/ change into another element.

Radioactive Half-Life After one half life there is 1/2 of original sample left. After two half-lives, there will be 1/2 of the 1/2 = 1/4 the original sample. After 84.0 grams of Sr-85 undergoes 1 half-life how much will be left? After 2? After 3?

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C. Half-life Half-life (t½) Time required for half the atoms of a radioactive nuclide to decay. Shorter half-life = less stable/more radioactive

C. Half-life mf: final mass mi: initial mass n: # of half-lives

C. Half-life t½ = 5.0 s mf = mi (½)n mi = 25 g mf = (25 g)(0.5)12 Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s? GIVEN: t½ = 5.0 s mi = 25 g mf = ? total time = 60.0 s n = 60.0s ÷ 5.0s =12 WORK: mf = mi (½)n mf = (25 g)(0.5)12 mf = 0.0061 g

Example You have 100 g of radioactive C-14. The half-life of C-14 is 5730 years. How many grams are left after one half-life? How many grams are left after two half-lives? 50 grams 25 grams

Learning Check! The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours? 8mg