Nuclear decay

The emission of radiation from an unstable nucleus may cause changes to the nucleus, including:

Changes due to radioactive decay can be represented by balanced nuclear equations. The changes that happen depend on the type of radiation emitted.

Alpha decay

Alpha particles consist of two protons and two neutrons. Protons are positively charged. When a nucleus emits an alpha particle, these changes happen:

  • the mass number decreases by 4
  • the atomic number decreases by 2
  • the nuclear charge decreases by 2

Example

Radon-219 decays into polonium-215 by emitting an alpha particle. This is the balanced equation for the reaction:

_{86}^{219}Rn \rightarrow _{84}^{215}Po + _{2}^{4}\alpha

Notice that:

  • the mass number of the nucleus decreases by 4, from 219 to 215
  • the atomic number of the nucleus decreases by 2, from 86 to 84
  • the numbers at the top and bottom give the same totals on both sides

Polonium nuclei have 84 protons, so their nuclear charge is +84. They have two fewer protons than radon nuclei, which have 86 protons and so a nuclear charge of +86.

Beta decay

A beta particle forms when a neutron changes into a proton and a high-energy electron. The proton stays in the nucleus but the electron leaves the atom as a beta particle. When a nucleus emits a beta particle, these changes happen:

  • the mass number stays the same
  • the atomic number increases by 1
  • the nuclear charge increases by 1

Example

Carbon-14 decays into nitrogen-14 by emitting a beta particle. This is the balanced equation for the reaction:

_{6}^{14}C \rightarrow _{7}^{14}N + _{-1}^{~0}\beta

Notice that:

  • the mass number of the nucleus stays the same, 14
  • the atomic number of the nucleus increases by 1, from 6 to 7
  • the numbers at the top and bottom give the same totals on both sides, as (7 - 1) = 6

Nitrogen nuclei have 7 protons, so their nuclear charge is +7. They have one more proton than carbon nuclei, which have 6 protons and so a nuclear charge of +6.

Gamma decay

Gamma rays are a type of electromagnetic radiation. There is no change in mass or charge for this type of decay.

Gamma radiation transfers energy from the nucleus, so the symbol _{0}^{0}\gamma may appear in some balanced equations.