Answer the following question given this paragraph: 

The mass number is the number of protons plus the number of neutrons in an atom. For example, most atoms of helium have 2 neutrons, so their mass number is 2 + 2 = 4. This mass number means that an atom of helium has a mass of about 4 amu.


Q: What makes up the mass number of an atom?


A:
protons and neutrons