Isotopes are one of two or more forms of the same chemical element.
Explanation:
Different isotopes of an element have the same number of protons in the nucleus, giving them the same atomic number,but a different number of neutrons giving each element isotope of a different atomic weight.
If the formula for density=m/v, you can manipulate this formula to the the mass. After manipulation, you get the equation for mass to be: m=density * volume. With density and volume given, we can find the mass.