Overview
Isotopes have the same number of protons and therefore belong to the same element, but they contain different numbers of neutrons. Their chemical behaviour is often similar while nuclear stability and mass can differ strongly.
How to use this idea
Same element, different neutron count: the essential idea behind isotopes.
One element can have several nuclear identities
A glass of water does not contain only one kind of hydrogen atom. Tiny fractions contain deuterium, whose nucleus carries a neutron as well as a proton. Chemically it is still hydrogen, but that extra neutron changes its mass and can make isotope effects measurable.
This is why the number printed beneath an element symbol deserves attention. For many elements it represents an average over naturally occurring isotopes, and nature does not always mix those isotopes in exactly the same proportions everywhere. Atomic weight can therefore carry a story about geology, atmosphere and origin—not just arithmetic.
Science is most useful when a number leads to a question, not when it ends one.
Same element, different neutron count
Isotopes have the same atomic number, so they are the same chemical element, but they contain different numbers of neutrons. Their mass numbers therefore differ. Chemical behaviour is usually similar because chemistry is governed primarily by electron structure, while nuclear stability can differ dramatically.
Some isotopes are stable and others radioactive. The relative abundances of naturally occurring isotopes are also why many standard atomic weights are not whole numbers and, for several elements, are expressed as intervals rather than a single conventional value.