Question

There is only one atomic number for each element. Why is there more than one atomic mass for each element?

Answer 1

The atomic mass is the average of various isotopes of each element.

Answer 2

The answer is neutrons!
Elements are determined by the number of protons residing within the nucleus, these protons are positively charged (+1) and will attract an equal number of electrons (-1) to balance this charge. The electrons will hang around on the surface of the atom and will determine the electronic interaction with other species (this is chemistry) .

However neutrons can also reside within the nucleus. Neutrons weigh about he same as protons but have no charge and do not affect the chemical reaction, .
As it turns out atoms can be stable(or unstable) with a range of number of neutrons.
These are called different isotopes of the element.

Answer 3

Due to presence of isotopes.