The sun sum of protons and neutrons in the nucleus of an atom.

Answer is the mass of the atom. Protons + neutrons= mass
Let's say you have oxygen with a mass of 16.
Oxygen has 8 protons
To find neutrons subtract 16-8= 8
So it would also be 8 neutrons.
Example of how to find mass if you know protons + neutrons:
Sodium has 11 protons and 12 neutrons. Add the protons and neutrons
11 + 12= 23
Mass is 23

Ifound this on google “these two compounds have a different ionic structure. caesium ion is larger in size when compared to the sodium ion. the charge gets more area to spread out evenly. so caesium chloride has a lower melting point than sodium chloride.” i hope this you for what your looking for