Atoms of an element that have different numbers of neutrons are isotopes of the same element. Isotopes of an element have the same atomic number but different mass numbers. Common examples are the isotopes of carbon:12C and 14C. 12C has six protons, six electrons, and six neutrons, while 14C has six protons, six electrons, and eight neutrons. Some isotopes are physically stable, while others, known as radioisotopes, are unstable. Radioisotopes undergo radioactive decay, emitting both particles and energy. If the decay leads to a change in the number of protons, the atomic number changes, transforming the isotopes into a different element.