How is the mass number related to the number of protons and neutrons an atom has?
The mass number of an individual atom is the sum of the protons and the neutrons.
The proton has a mass of approximately 1 amu
The neutron has a mass of approximately 1 amu
The electron has a mass of several orders of magnitude less than 1 amu. The mass of the electrons can be ignored when calculating the total mass of the atom.