The number of protons in the nucleus, the atomic number , determines the atom's chemical properties. For instance, a nucleus containing a single proton will be some form of hydrogen . These elements have very different properties from one another. In a neutral atom the number of protons in a nucleus is balanced by an equal number of electrons surrounding it. The sum of the protons and neutrons gives the atomic mass number .
Some elements are unstable and undergo radioactive decay .
The unstable elements usually have too many or too few neutrons in their nuclei and are known as radioactive isotopes . When an isotope is radioactive it emits particles or energy in an effort to become stable. It will usually achieve stability by changing into another element. This occurs predominantly with elements which contain a greater number of protons than lead. Fusion can take place between isotopes of elements lighter than iron.