What is the mass number of an atom made up of?

The mass number is the sum of the number of protons and neutrons in the nucleus of a given atom.

Hello Tracey, Inside the Atom there is a nucleus made up of both Protons and Neutrons. The atomic mass of an atom refers to the number of Protons and Neutrons within the nucleus of the atom. The Atom also has an Atomic Number, which refers to just the number of Protons in the nucleus. If you look on the periodic table, you will see the Atomic Mass displayed above each substance, and the Atomic Number below. For example: Na (Sodium) has an Atomic mass of 23, and an Atomic number of 11. This means it has 11 Protons and therefore 12 Neutrons. 11+12= 23 (the Atomic mass).

I hope this answers your question, but if you're still not sure, just let me know, and we can go over the answer again.