Is mass number the same as atomic number?
No. Atomic number refers to the number of protons present in the
nucleus of the atom whereas mass number refers to the sum of the
number of protons and neutrons in the nucleus.
For example, the isotope lithium-6 has an atomic number of 3
(for 3 protons) and a mass number of 6 (for 3 protons and 3
neutrons).