How to find mass number of an element without neutrons

- How do you find the mass number of an element without neutrons
- How to find the number of neutrons in an element with atomic mass
- How to find the number of neutrons in an element
- How to calculate the number of neutrons in an atom
- How to calculate number of neutrons in an isotope
- How to find number of protons neutrons and electrons
- How to find atomic number