Question

How can cobalt-60 and cobalt-59 be the same element but have different mass numbers?

Answer 1

The 'element' is based on its properties, which are primarily the electrons in balance with the protons. The isotopes are based on the mass, which is varied by the number of neutrons.

Explanation:

So, changing the number of protons changes the type of element, changing the neutrons creates different isotopes of the same element, and the interaction (loss, gaining, sharing) of electrons gives an element its properties.