An isotope is an atom with a different number of neutrons. The charge is still 0, unlike ions. There are radioactive isotopes, which are used in nuclear reactions. In radioactivity, only the nucleus is important.
Add in extra neutrons, and the atom is heavier. It is still the same element. Take a neutron out, making it lighter, and it's still the same element. This is called an "isotope".
Isotopes are the reason why some elements have a relative atomic mass that is not a whole number. The RAM is calculated with the natural abundance of each isotope.
Chlorine has 2 main isotopes 35Cl occurring 75% of the time, and 37Cl occurring 25% of the time. Cl ((35*75)+(37*25))/100=35.5
RAM or Ar is simply how heavy atoms are compared with 1/12 Carbon-12
however isotopes do not beahve any different in chemical reactions
There are many types of Hydrogens e.g. deuterium and tritium. Tritium has an unstable nucleus - the forces holding the proton and neutrons together are weak. They will decay, so are radioactive. Radioactive isotopes are sometimes called radioisotopes.