14.4: Radioactivity and Nuclear Reactions

CHEMICAL REACTIONS: - take place in the outer valence shell of electrons NUCLEAR REACTIONS: - take place in the nucleus - involve huge amount of energy as described by the equation E = mc2

Nuclear Reactions Changes the nucleus of the atom. Changes an isotope of one element into an isotope of a different element Involve much more energy than a chemical reaction.

2 types of Nuclear Reactions: FUSION: joining together two nuclei. Occurs in stars, where hydrogen atoms are fused together to produce helium FISSION: splitting apart two nuclei. Occurs in nuclear power plants and nuclear bombs, where uranium is a reactant. Radioactive decay occurs when fission process repeats many times as the original radioactive nucleus gets smaller and smaller.

Radioactive Half life - the time required for 1/2 the nuclei in a radioactive sample to decay. This quantity can be predicted with great accuracy.

Each radioactive element has its own unique half life. Half lives vary from less than 1 second to billions of years. Radon-222 51.7 seconds Iodine-133 21 hours Carbon-14 5,730 years Uranium-238 4.5 billion years Thorium-232 14 billion years

FRACTIONAL AMOUNT REMAINING Carbon-14 has a half life of 5730 yrs. If a radioactive sample has 500 grams of carbon-14, about 250 grams will decay during the first half-life. That leaves 250 grams which have not decayed. During the next half-life, half of the remaining carbon-14 (about 125 grams) will decay. Below is a chart for this sample. Half life TIME (years) SAMPLE (grams) FRACTIONAL AMOUNT REMAINING 500 g 1 5,730 250 g 1/2 11,460 125 g 1/4 17,190 62.5 g 1/8 22,920 31.25 g 1/16

5730 yrs 10,460 yrs