Mass number decreases by 4. Atomic number decreases by 2.
Beta decay:
Parent nucleus → daughter nucleus + beta particle
A neutron becomes a proton + electron: ¹₀n → ¹₁p + ⁰₋₁e
Mass number unchanged. Atomic number increases by 1.
Alpha and Beta Decay Examples
ALPHA DECAY EXAMPLE:
Radium-226 decays by alpha emission:
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
Check: mass numbers: 226 = 222 + 4 ✓
Atomic numbers: 88 = 86 + 2 ✓
Radium (88) → Radon (86) — atomic number decreases by 2, so element changes.
BETA DECAY EXAMPLE:
Carbon-14 decays by beta emission:
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Check: mass numbers: 14 = 14 + 0 ✓
Atomic numbers: 6 = 7 + (−1) ✓
Carbon (6) → Nitrogen (7) — atomic number increases by 1.
GAMMA EMISSION:
Often follows alpha or beta decay — nucleus releases excess energy as gamma ray.
No change to mass number or atomic number.
No separate nuclear equation needed — just add + γ.
Effect on the Nucleus
ALPHA DECAY:
Mass number: decreases by 4
Atomic number: decreases by 2
Result: new element (two places lower in periodic table)
BETA DECAY:
Mass number: unchanged (a neutron converts to a proton + electron)
Atomic number: increases by 1
Result: new element (one place higher in periodic table)
GAMMA EMISSION:
Mass number: unchanged
Atomic number: unchanged
Result: same element, just releases energy
PATTERN TO REMEMBER:
Alpha → lose 4 mass, lose 2 charge
Beta → same mass, gain 1 charge
Gamma → no change to nucleus
EXAM STRATEGY:
Always check both mass AND atomic numbers balance.
Use the periodic table to identify the daughter element from its new atomic number.
⚠️ Common Mistake
In beta decay, the MASS NUMBER stays the same — a neutron turns into a proton plus an electron. The atomic number INCREASES by 1 (one more proton). Students often think beta decay is like alpha and reduces mass number.
📐 Key Equations
Alpha decay: A decreases by 4, Z decreases by 2
Beta decay: A unchanged, Z increases by 1
Gamma: no change to A or Z
📌 Key Note
Nuclear equations: mass numbers and atomic numbers must balance on both sides. Alpha (⁴₂He): A−4, Z−2. Beta (⁰₋₁e): A same, Z+1. Gamma (γ): no change. New element formed in alpha and beta (different Z). Always verify balance.
🎯 Matching Activity — Nuclear Equation Changes
Match each decay type to the change in mass number and atomic number. — drag the symbols on the right to match the component names on the left.
Alpha decay
Drop here
Beta decay
Drop here
Gamma emission
Drop here
Beta particle
Drop here
⁰₋₁e — electron emitted from nucleus when neutron converts to proton
Mass number unchanged, atomic number +1 — new element one place higher
No change to mass number or atomic number — same element, energy released
Mass number −4, atomic number −2 — new element two places lower
⚽ FIFA Worked Examples
Alpha Decay Equation
Write the nuclear equation for alpha decay of ²²⁶₈₈Ra (radium).
F
Alpha decay: A decreases by 4, Z decreases by 2. Product + ⁴₂He
I
A = 226 − 4 = 222; Z = 88 − 2 = 86. Z=86 is radon (Rn)
1. Thorium-234 (²³⁴₉₀Th) undergoes beta decay. What is the daughter nucleus?
2. In a nuclear equation, the mass numbers on both sides must balance. A nucleus of ²³⁸₉₂U emits an alpha particle. What is the mass number of the daughter nucleus?
⭐ How Well Do You Understand This Topic?
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