Every chemical element is identified with their number of protons (atomic number) in their nucleus. In addition to the protons, the nucleus also has neutrons. If the atomic number remains the same, and the number of neutrons varies to form different stable and radioactive (unstable) nucleus, then the element is said to have stable and radioactive isotopes. For example, helium has two stable form of isotopes, helium-3, and helium-4. They both have the same number of protons, but a different number of neutrons, i.e. one and two respectively.
Frequently Asked Questions
How does an isotope affect the atomic weight of an element?
Atomic mass is measured for individual isotopes. For an element in common, the average atomic mass of its isotopes is measured as its relative atomic mass. This is sometimes referred to as atomic weight. Therefore, the atomic mass of an isotope is the one that affects the atomic weight of an element in a periodic table.