4PH1

Properties of Radiation

Radioactivity & Particles · 2 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable
Low
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).