mass of the nucleus

Because electrons do not contribute to the atom mass, the mass of atoms is concentrated in the nucleus. Knowing the number of nuclear particles, automatically you know the mass of that atom in U(nits).
Then taking one MOL atoms of that substance (6.1023 atoms), you know the mass of it in grammes.

Example: Calcium ( In the table you find the value of atom mass of Calcium 40,1 (consequence of isotopes).
40 is the number of nuclear particles of a normal Calcium isotope with 40 nucleons.
Now applying the number 40,1 (atom mass) in micro and on macro level: