Alpha decay
- In alpha decay, the nucleus emits an alpha particle (a helium-4 nucleus)
- Effects on the parent nucleus:
- Mass number decreases by 4
- Atomic number decreases by 2
- A new element is formed (since Z has changed)
- General equation:
ᴬ_Z X → ᴬ⁻⁴_Z₋₂ Y + ⁴₂α
Example — radium-226 undergoes alpha decay. Write the decay equation.
- ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α
- Check the totals: mass numbers 226 = 222 + 4 ✓; atomic numbers 88 = 86 + 2 ✓
- Radium has decayed to radon (a different element with Z = 86)
Beta decay
- In beta decay, a neutron inside the nucleus transforms into a proton plus an electron. The electron is ejected as the beta particle; the proton stays in the nucleus
- Effects on the parent nucleus:
- Mass number stays the same (a neutron has become a proton, but the total nucleon count is unchanged)
- Atomic number increases by 1 (one more proton)
- A new element is formed
- General equation:
ᴬ_Z X → ᴬ_Z₊₁ Y + ⁰₋₁β
Example — carbon-14 undergoes beta decay. Write the decay equation.
- ¹⁴₆C → ¹⁴₇N + ⁰₋₁β
- Check the totals: mass numbers 14 = 14 + 0 ✓; atomic numbers 6 = 7 + (−1) ✓
- Carbon has decayed to nitrogen, which is the basis of carbon-14 dating
Gamma decay
- In gamma decay, the nucleus emits a burst of high-energy electromagnetic radiation (a gamma photon)
- Effects on the parent nucleus:
- Mass number stays the same
- Atomic number stays the same
- No new element is formed; the nucleus has simply lost some energy
- Gamma emission often happens immediately after another decay (alpha or beta) leaves the daughter nucleus in an excited state. The excited nucleus then sheds the excess energy as a gamma photon
- General equation:
ᴬ_Z X → ᴬ_Z X + ⁰₀γ
Neutron emission
- A handful of very neutron-rich isotopes can decay by ejecting a single neutron directly
- Effects on the parent nucleus:
- Mass number decreases by 1 (one nucleon has gone)
- Atomic number stays the same (no protons lost)
- The result is a different isotope of the same element
- General equation:
ᴬ_Z X → ᴬ⁻¹_Z X + ¹₀n
Balancing nuclear equations
- Two simple rules let you check or complete any decay equation:
- The sum of mass numbers on the left must equal the sum on the right
- The sum of atomic numbers on the left must equal the sum on the right
- A balanced equation conserves both mass and charge across the reaction