Atoms of an element may have different masses. An isotope is all the atoms of an element having the same mass.
All atoms have masses that are close to whole numbers. The whole number closest to the mass of the atoms of an isotope is the mass number of that isotope.
Mass numbers of stable isotopes
|8||Oxygen||16, 17, 18|
|10||Neon||20, 21, 22|
|12||Magnesium||24, 25, 26|