Radioactive Decay
Types of Radioactive Decay
• Alpha Decay: nucleus eject an alpha particle
(made of 2 protons and 2 neutrons – a helium
nucleus)
Types of Radioactive Decay
• Beta Decay: nucleus emits an electron or positron
(electron with positive charge)
Types of Radioactive Decay
• Gamma Decay: nucleus emits highly energetic
photons
Types of Radioactive Decay
• Alpha Decay can be shielded by a
piece of paper
• Beta Decay can be shielded by
aluminum foil
• Gamma Decay can be shielded by
several centimeters of lead
Half-Life
• The half-life of a radioisotope is
the time required for one half
of the atoms in a sample to
degrade into a more stable
material
• The half-life for a given isotope
is always the same
• Half-lives vary from isotope to
isotope, ranging from fractions
of a second to millions of years
Half-Lives
• Ex: The half-life of Strogenium is 2 seconds. How
much of a 10 g sample will be left after 6 seconds?
1 half-life
6 seconds x
= 3 half-lives
2 seconds
•
•
•
•
After 0 half-lives, 10 g are left.
After 1 half-life, 5 g are left.
After 2 half-lives, 2.5 g are left.
After 3 half-lives, 1.25 g are left.
Practice Problems
1. The half-life of radon-222 is 4.0 days. How much
6.25 g
of a 100 g sample is left after 16.0 days?
2. Cesium-137 has a half-life of 30 years. If 4.0 g of
cesium-137 disintegrates over a period of 90
years, how many grams would remain? 0.5 g
3. What is the half-life of a 100.0 g sample of
nitrogen-16 that decays to 12.5 g in 33 seconds?11 s
4. How old is a bone if it presently contains 2.5 g of
carbon-14, but was estimated to originally
contain 80.0 g of carbon-14? The half-life of
carbon-14 is 5,700 years. 28,500 years
Practice Problem
Pd-100 has a half-life of 3.6 days. If there are
6.02 x 1023 atoms at the start, how many
atoms would be present after 20.0 days?