Neutrons & isotopes...what's the significance

[janash]

Hi,

I am studying gen chem and have a couple of questions:

1. NEUTRONS: I understand what neutrons are, but what is the significance aside from contributing to the mass of an element and differentiating isotopes?

2. ISOTOPES: Can two DIFFERENT elements with the same number of protons, but different number of neutrons be considered isotopes, or do isotopes only exist within the same element? What is the significance of isotopes? How do different isotopes change the function of the element?

Thanks to whoever might answer these questions!