There are 3 types of subatomic particles in an atom, which includes proton, neutron, and electrons.
However, in the nucleus of the atom, only proton and neutron is found. Electrons are moving in a very high speed around the nucleus.
In addition, the protons is positive, meanwhile a neutron doesn't carry either positive or negative charge.
When we're calculating atomic mass, we only add up the number of protons and neutrons, but we can neglect the electrons since they're too light.