An atom’s identity as an element depends only on its number of protons. Adding or removing neutrons changes the atom’s mass but not which element it is. Chemical bonding depends mainly on the number and arrangement of electrons.
Isotopes of an element have the same electron count, so they react the same way in chemical reactions. Most elements found in nature exist as a mixture of two or more isotopes. The atomic mass listed on the periodic table is not the mass of any single atom.
It is a weighted average based on the natural abundance of each isotope. For example, chlorine’s listed atomic mass of 35.45 reflects a mix of about 76% chlorine-35 and 24% chlorine-37. Some isotopes are stable and last essentially forever.
Others are radioactive, meaning their nuclei break down over time and release radiation. Radioactive isotopes have important uses. Carbon-14 lets scientists date ancient organic material, since it decays at a known, steady rate.
Other radioactive isotopes are used in medical imaging and cancer treatment. A common misconception is that isotopes of an element have different chemical properties. They do not. Only mass-dependent properties differ, along with how the isotope’s nucleus behaves during radioactive decay.