Nuclear Reactions

After reading this section you will be able to do the following:

  • Describe an atom using its chemical notation.
  • Begin to understand nuclear reaction equations.

What is the difference between chemical reactions and nuclear reactions?

Nuclear reactions can be described mathematically in much the same way as chemical reactions. We commonly express these reactions by equations, although there is a unique difference in the nature of the reactions. The principle difference between them lies in how the reaction occurs, specifically how the atom is affected. Chemical reactions involve an atom’s electrons while nuclear reactions involve the atom’s nucleus.

Writing a nuclear reaction equation

In order to write an equation for a nuclear reaction, we must first establish some basic rules. Each of the elements involved in the reaction is identified by the chemical symbol. Two numbers are attached to the symbol. The number at the upper right is the mass number, also known as the ‘A’ number. The 'A' number describes the atomic weight of the atom and identifies the number of protons and neutrons in the nucleus. The number at the lower left is the atomic number, or ‘Z’ number. The 'Z' number describes the number of protons in the nucleus and determines the type of atom.

The symbol for Uranium-238 is 92 238 U ^{238}_{92}U

This shows you that Uranium has a mass number of 238 and an atomic number of 92. Symbols are also utilized to represent alpha and beta particles. The symbol for an alpha particle is 2 4 H e ^{4}_{2}He . The symbol for a beta particle is 1 0 e ^{0}_{-1}e . The chemical symbol for a neutron is 0 1 n ^{1}_{0}n  . Can you determine the mass number and atomic number of the neutron?

Now that we know what these symbols represent, let's see how they can be applied to a nuclear equation. Uranium-238 is an isotope, which undergoes alpha decay to produce Thorium and gamma rays. This is expressed mathematically by the following equation:

92 238 U 90 234 T h + 2 4 H e + G a m m a ^{238}_{92}U \rightarrow ^{234}_{90}Th + ^{4}_{2}He + Gamma R a y s Rays

Note that when the mass numbers on each side of the equation are added together that they are equal. The same principle is true for the atomic numbers, and it shows that none of the atomic particles have been lost. One way to check to see if you have written the proper nuclear equation is to make sure both sides of the equation have the same number or atomic particles represented.


  1. A nuclear reaction can be described by an equation, which must be balanced.
  2. The symbol for an atom or atomic particle includes the symbol of the element, the mass number, and the atomic number.
  3. The mass number, which describes the number of protons and neutrons, is attached at the upper left of the symbol.
  4. The atomic number, which describes the number of protons in the nucleus, is attached at the lower left of the symbol.