Isotopes are atoms of the same element that have different masses. They have the same number of protons but a different number of neutrons. The isotope number is the number of protons plus the number of neurons.

The Half-life is the length of time required for half of a given number of initial number of atoms of that isotope to decay. Each individual radioactive substance has a characteristic and different half-life.

An example of three different isotopes of uranium and its half-life period is given below.
uranium-234: half life = 244 thousand years.
uranium-235: half life = 704 million years.
uranium-238: half life = 4.5 billion years.