500
This is why the atomic mass for each element on the periodic table usually has decimals.
What are isotopes?
Explanation: Each individual atom of an element will always have the same number of protons as any atom of that element, but not always the same number of neutrons nor natural abundance. Therefore, the mass of each atom must be calculated independently of the others. Since each isotope, or form of an element will have a different mass number, the mass number of all the isotopes must be averaged to come up with a number for the periodic table, which we call the atomic mass. This average is often a decimal.