isotopes are versions of the same element (ie. atomic number) but with different numbers of neutrons (ie. atomic mass). For example, chlorine exists as two common isotopes, Cl-35 and Cl-37. This is also why most elements do not have integer atomic masses (chlorine is often written on periodic tables as being atomic mass 35.5, but how can u have half a proton or neutron?!?!??!). The reason is the aforementioned chlorine isotopes exist in an approximate 3:1 ratio (75% Cl-35, 25% Cl-37), so on average, the atomic mass works out to be 35.5
as they are the same element (and by extension, have the same no. of electrons), the chemical properties of isotopes are identical.
but as their atomic masses are different, u will see different physical properties and nuclear properties. many isotopes are highly unstable (ie. radioactive) and will decay.