By the end of this section you should be able to...
  • Balance nuclear equations and identify the particles present.

Nuclear Equations

Elements revision

When elements are written down using their symbol it is usual to give also give the atomic number and atomic mass number of the substance.  The mass number is the number of protons and neutrons in the nucleus.  The atomic number is the number of protons  in the nucleus.  As shown below:

Helium has 2 protons in its nucleus and a total of 4 protons and neutrons.  This means that helium must have 2 protons and 2 neutrons.  

Nuclear decay

If an atomic nucleus is unstable then it may undergo decay to become more stable.  When this happens a particle is ejected from the nucleus.  The change in mass and atomic numbers gives clues to the type of particle that has been ejected.

Alpha decay

If Uranium 238 decays into Thorium 234 then the mass number has decreased by 4.  At the same time, the atomic number has decreased by 2.  This means that the particle ejected must have a mass number of 4 and an atomic number of 2; an alpha particle.

Beta decay

In the same way, Carbon 14 can decay into Nitrogen 14.  When this happens the atomic number increases by 1.  The particle ejected is therefore an electron with a mass number of zero and an atomic number of -1.  When this happens a neutrino is also released as shown below.


In nuclear equations it is vital that the mass numbers and atomic numbers on each side of the arrow are equal.  These numbers can be used to deduce what type of particle (and how many of them) has been produced.