Answer:
Number of neutrons and stability
Step-by-step explanation:
An isotope of an element is basically the same element but with different number of neutrons. For example here, boron can exist in the forms of boron-10 and boron-11, and so the latter would have one more neutron than the former one.
Adding an extra neutron may or may not disrupt the strong force that much, and so the half-life and stability of the new isotope can be slightly different than its most stable one.