ChemistryMapUnit 9, Nuclear chemistry
9.1Radioactive decay
Why some nuclei are unstable, what alpha, beta and gamma decay change, and what stops each.
1Parent and daughter
Radioactive decay is the set of various processes by which an unstable atomic nucleus spontaneously emits subatomic particles. Decay is said to occur in the parent nucleus and produce a daughter nucleus.
Which particles are in the nucleus? Protons and Neutrons.
2Why decay happens
- Protons are positively charged and repel each other.
- A certain number of neutrons is needed to keep the nucleus together.
- Too many or too few neutrons can lead to unstable nuclei, which will undergo radioactive decay to become more stable.
Carbon shows it. Every carbon atom has six protons; only the number of neutrons changes.
| Isotope | Neutrons | Stable? | Half-life |
|---|---|---|---|
| Carbon-10 | 4 | Unstable | 19 seconds |
| Carbon-11 | 5 | Unstable | 20 minutes |
| Carbon-12 | 6 | Stable | None |
| Carbon-13 | 7 | Stable | None |
| Carbon-14 | 8 | Unstable | 5,730 years |
| Carbon-15 | 9 | Unstable | 2.4 seconds |
3Alpha decay
- An alpha particle (α) is emitted from the unstable nucleus.
- Alpha particles consist of two neutrons and two protons, just like a helium nucleus.
- Sometimes shown as
- Has a mass of 4 amu and a charge of +2.
Mass number drops by 4, atomic number by 2. The atom becomes a different element.
Radium becomes radon. The mass numbers balance (226 = 222 + 4), and so do the atomic numbers (88 = 86 + 2).
4Beta decay
- A beta particle (β) is emitted from the unstable nucleus.
- During this form of radioactive decay, a neutron becomes a proton.
- The radioactive particle emitted after this process is a beta particle, which is an electron. It has negligible mass and a −1 charge.
Mass number stays the same, atomic number rises by 1. The atom becomes a different element.
Carbon-14 becomes nitrogen-14. The mass numbers balance (14 = 14 + 0), and so do the atomic numbers (6 = 7 + (−1)).
5Gamma decay
- A gamma ray (γ) is emitted from the unstable nucleus.An “m”, for “metastable”, is included next to the mass number of the higher energy form of the atom.
- A gamma ray is a high energy form of radiation with no mass and no charge.
Mass number and atomic number stay the same. The nucleus only loses energy.
Technetium-99m settles to technetium-99. The mass numbers balance (99 = 99 + 0), and so do the atomic numbers (43 = 43 + 0).
6What stops each kind
Energy and particles emitted by atoms during radioactive decay can cause damage to living tissues, but different materials can be used to protect against them.
| Radiation | Paper | Aluminum | Lead |
|---|---|---|---|
| Alpha α | Stopped | Stopped | Stopped |
| Beta β | Passes through | Stopped | Stopped |
| Gamma γ | Passes through | Passes through | Stopped |
What this topic builds
- Name the decay from its equationNot started
- Complete the products of a decayNot started
- Find the parent nucleusNot started
- Say why some nuclei are unstableNot started
- Say what each decay changes in the nucleusNot started
- Say what stops each kind of radiationNot started