Chemistry / Unit 2: Atomic Structure
2.5 Radioactive Decay
Notes
Some nuclei are unstable and give off radiation to become more stable. This changes the nucleus, so one element can become another.
| Type | Symbol | Change to nucleus | Stopped by |
|---|---|---|---|
| alpha | $^{4}_{2}\text{He}$ | $A - 4$, $Z - 2$ | paper |
| beta | $^{0}_{-1}\text{e}$ | $A$ same, $Z + 1$ | aluminum foil |
| gamma | $^{0}_{0}\gamma$ | no change | thick lead or concrete |
- In a nuclear equation, mass numbers and atomic numbers must add up the same on both sides.
- Example: $^{238}_{92}\text{U} \rightarrow\ ^{234}_{90}\text{Th} +\ ^{4}_{2}\text{He}$.
Practice
What is produced when $^{222}_{86}\text{Rn}$ undergoes alpha decay?
Alpha decay removes 4 from the mass number and 2 from the atomic number: $222 - 4 = 218$, $86 - 2 = 84$, which is polonium.
Carbon-14 undergoes beta decay. What is the product?
Beta decay turns a neutron into a proton: mass number stays 14, atomic number goes from 6 to 7 (nitrogen).
Which type of radiation is the most penetrating?
Gamma rays are high-energy light with no mass or charge and need thick lead or concrete to stop them.
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