The Unstable Nucleus
Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation. This process, called transmutation, changes the atom into a different nuclide or element.
Types of Radioactive Decay
1.Alpha (α) Decay:
The nucleus emits an alpha particle, which is a helium nucleus (2 protons and 2 neutrons).
The resulting nucleus has an atomic number that is 2 less and a mass number that is 4 less than the original.
Alpha particles are large and have low penetrating power; they can be stopped by a sheet of paper.
2.Beta (β) Decay:
A neutron in the nucleus is converted into a proton, and an electron (the beta particle) is ejected from the nucleus.
The resulting nucleus has an atomic number that is 1 greater and the same mass number as the original.
Beta particles are more penetrating than alpha particles but can be stopped by a thin sheet of aluminum.
3.Gamma (γ) Decay:
The nucleus emits a gamma ray, which is a high-energy photon of electromagnetic radiation.
This process does not change the atomic number or mass number of the nucleus; it only lowers the energy state of the nucleus.
Gamma rays are extremely penetrating and require thick shielding, like lead or concrete, to be stopped.
Half-Life
Radioactive decay is a random process at the level of a single atom, but for a large number of atoms, the rate of decay is predictable.
The half-life (t₁/₂) of a radioactive isotope is the time it takes for half of the atoms in a given sample to decay.
Each isotope has its own unique, constant half-life, which is unaffected by external conditions.
Half-Life Calculations:
After n half-lives, the amount of the original substance remaining is (1/2)ⁿ of the initial amount.
Example: A sample of Iodine-131 has a half-life of 8 days. If you start with 100 grams, how much is left after 24 days?
Number of half-lives: n = 24 days / 8 days = 3.
Amount remaining = (1/2)³ 100 g = (1/8) 100 g = 12.5 g.
Radiometric Dating
This technique uses the known, constant decay rates of radioactive isotopes to determine the age of rocks and fossils.
Principle: By measuring the ratio of the parent radioactive isotope to the stable daughter product in a sample, and knowing the half-life of the parent isotope, scientists can calculate the age of the sample.
Example: Carbon-14 dating is used for dating organic materials up to about 50,000 years old. Uranium-lead dating is used for dating very old rocks.