Isotopes are the atoms of same element (same atomic number) with different mass no.
e.g.
Hydrogen have three isotopes
1H1, 1H2, 1H3
Number of protons 1 1 1
(atomic number)
Mass number 1 2 3
(Number of Proton + neutron)
Number of neutron 0 1 2
So isotopes differ in number of neutron.
Uranium (92U) have three naturally occurring isotopes, these are 92U238, 92U235, 92U234 . all these isotopes are radioactive.