What defines the mass number of an isotope

The mass number of an isotope is determined by the number of neutrons in the isotope plus the number of protons.
For instance Carbon-13 is an isotope of carbon. The "13" is the mass number of the isotope.
So, in this example, 13 = #protons + #neutrons

Cr(oh)3 -> cr(3+) + 3oh-

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