This topic accounts for approximately 8% of your exam marks.
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Stable8%
Properties of alpha, beta and gamma radiation and nuclear equations tested consistently.
Alpha decay
In alpha decay, the nucleus emits an alpha particle (a helium-4 nucleus)
Effects on the parent nucleus:
decreases by 4
decreases by 2
A new element is formed (since Z has changed)
General equation:
ᴬ_Z X → ᴬ⁻⁴_Z₋₂ Y + ⁴₂α
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
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 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 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
Worked example
Completing an alpha decay equation
A nucleus of radon-220 (atomic number 86) undergoes alpha decay to form a polonium nucleus. Complete the nuclear equation by finding the missing mass number and atomic number of the daughter nucleus and the alpha particle.
Solution:
An alpha particle has mass number 4 and atomic number 2, written ⁴₂α.
Mass number check: 220 = daughter mass number + 4, so daughter mass number = 216.
Atomic number check: 86 = daughter atomic number + 2, so daughter atomic number = 84.
Element with Z = 84 is polonium (Po).
Completed equation: ²²⁰₈₆Rn → ²¹⁶₈₄Po + ⁴₂α
Exam tip
Completing a beta decay equation
What comes up: given an incomplete nuclear equation for beta decay with blank boxes for the mass or atomic numbers, fill in the missing values.
Write (two marks): (1) The mass number stays the same (a neutron converts to a proton, so total nucleons are unchanged). (2) The atomic number increases by 1 (one extra proton in the nucleus), so identify the new element at Z + 1.
Watch out: a common slip is decreasing the atomic number or changing the mass number. In beta decay a neutron becomes a proton — the nucleon count is unchanged and the proton count goes up by one. The beta particle itself carries mass number 0 and atomic number −1 (written ⁰₋₁β).