Protons and neutrons are all approximately 1amu or atomic mass unit. The mass number of lithium with 3 protons and 4 neutrons is 7 amu.
Work Step by Step
1 proton = 1 amu 1 neutron = 1 amu 3 protons + 4 neutrons = 3 amu + 4 amu = 7amu, making the mass number of lithium 7. This is very close to the actual atomic mass of lithium which is 6.941