Explanation:
Atomic mass means the sum of total number of protons and neutrons present in an atom.
When an atom is neutral then number of protons equal the number of electrons.
Mass of a proton equals to 1.007276 u, mass of neutron equals to 1.008664 u.
For example, in a atom there are total 9 protons and number of neutrons is 19 - 9 = 10.
Since, it is a neutral atom so number of electrons will also be 9.
So, total mass will be calculated as follows.
Total mass = 9 (mass of electron + mass of proton)
= 9 (1.007825 u)
= 9.07043 u
Therefore, we can conclude that total mass of protons and electrons in is 9.07043 u.