Radioactive Decay
For a nucleus of an atom to be stable, it has to have a certain number of protons and neutrons. For example, a stable carbon atom will have 6 protons and 6 neutrons. Isotopes with too many or too neutrons do exist in nature, but these unstable nuclei will decay in a random process.
Types of Ionizing Radiation Emitted form Radioactive Decay
1. Alpha particle
- consists of two proton and two neutron
- essentially same as helium-4 nucleus
- causes the atomic number of the unstable nucleus to decrease by 2 and the mass number to decrease by 4
2. Beta particle
- released when a neutron of an unstable nucleus turns into a proton
- has almost 0 mass and electric charge of -1
- essentially the same as an electron
- causes the atomic number of the unstable nucleus to increase by 1
3. Gamma ray
- emitted when rearrangement of the particles in a nucleus moves the nucleus to a lower energy state
- consists of a photon of energy
- has no mass or charge
Balancing Nuclear Reactions
- mass number
- atomic number (or electric charge)
need to be balanced in nuclear reactions.
Half-life of Radioactivity
- time it takes for half of the existing unstable nuclei to decay
- time it takes for the activity to decrease to half
Example 1.
There is 100g of unstable nucleus A with half-life of 8 hours. Calculate the time it takes for the amount of A to decrease to 12.5g.
Solution.
I. After 1 half-life (8 hours), amount A will decrease to half of 100g, which is 50g.
2. After 2 half-lives (16 hours), amount of A will decrease to half of 50g, which is 25g.
3. After 3 half-lives (24 hours), amount of A will decrease to half of 25g, which is 12.5g.
It takes 24 hours for A to decrease from 100g to 12.5g.