Answer:
Mass number is defined as the number of protons plus the number of neutrons.
Explanation:
Every atom of an element have proton(s), neutron(s) and electron(s). The proton number of an element is the atomic number of that element. For an electronically neutral atom the proton number is equal to the electron numbers. The neutron and the proton contributes to the mass of every atom. The electron is more active when atoms are bonding.
Mass number of an element is the number of proton plus the number of neutron.
Atomic number of an atom is the number of proton present, so it can never be atomic number.
Isotopy talks about same element having different number of neutron but same number of protons in each atom. Example is hydrogen that exist as protium, deuterium and tritium. It cannot be isotopic number.
Ionic number talks about elements that possess a charge. The elements have been ionized.
The answer is Mass number because the sum of proton number and neutron number is equals to mass number.
