ChemAtlas

Chapter 2

Radioisotopes

Nuclear equations balance by conserving mass number and atomic number — nothing else. Half-life follows from repeated halving.

Alpha decay23892Uemits23490Th+42HeBeta decay146Cemits147N+−0−1e
Fig. 2Alpha decay sheds a helium nucleus; beta decay converts a neutron into a proton. Mass number and atomic number are conserved across the arrow in both.
Decay mode

Try one

The nucleus ejects a helium-4 nucleus. Mass number drops by 4 and atomic number by 2, so the element changes.

Balanced nuclear equation

23892U→23490Th+42He

The daughter is Thorium-234.

Working

  1. 01Mass number: 238 − 4 = 234. Mass number is conserved across the arrow.
  2. 02Atomic number: 92 − (2) = 90. Atomic number is conserved across the arrow.
  3. 03Atomic number 90 is Thorium, so the daughter nuclide is 234Th.