Rates of Nuclear Decay

Chapter 10 Section 2Pg. 298-301

Half-life

• A nuclear decay rate describes how fast nuclear changes take place in a radioactive substance.

• Every radioisotope decays at a specific rate that can be expressed as a half-life.

• Half-life is the time required for one half of a sample of a radioisotope to decay.

Half-life

• After 1 half-life decay, half of the atoms in a radioactive sample have decayed, while the other half remain unchanged.

• After 2 half-lives, half of the remaining half decays, leaving one quarter of the original sample unchanged.

• Unlike chemical reaction rates, which vary with the conditions of a reaction, nuclear decay rates are constant

Half-life• Example:1 gram of iridium-182 undergoes beta decay to form osmium-182. The half-life of iridium-182 is 15 minutes. After 45 min., how much iridium-182 will remain in the sample?

½ elapsed = total time of decay = 45min = 3Half-life 15min

After 3 half-lives, the amount of iridium-182 has been reduced by half, 3 times

½ x ½ x ½ = ⅛So after 45 min, ⅛ x 1 gram, or 0.125 gram, of iridium-182 remains while 0.875 gram of the sample has decayed into osmium-182.

Radioactive Dating

• In radiocarbon dating, the age of an object is determined by comparing the object’s carbon-14 levels with carbon-14 levels in the atmosphere.

• Radiocarbon dating can be used to date any carbon-containing object less than 50,000 years old.