What are isotopes?
Forms of atoms in which neutrons from the nucleus are gained or lost.
When an atom gains or loses neutrons, it becomes an isotope of it's original element. If too many neutrons are gained or lost it will become unstable, and begin to decay.
Atoms of the same element with different number of neutrons resulting in different masses and nuclear stability
Isotopes have the same number of protons and hence are the same element with the same chemical properties.
Isotopes have different numbers of neutrons which results in a different mass number. The different number of neutrons affects the nucleus of the atom. Some isotopes are very unstable and undergo significant nuclear decay. This makes many isotopes radioactive.
For example Carbon 12 ( the most common isotope of Carbon) is very stable and does not undergo measurable radioactive decay. Carbon 14 ( a very small percentage of Carbon atoms) is very unstable and undergoes radioactive beta decay becomingw a Nitrogen atom. The half life of Carbon 14 is about 5,700 years.
Isotopes are the same element with different mass numbers and nuclear stability.