Atomic number - Gives the number of protons (Z)
Mass number - Gives the number of protons and neutrons (A)
Isotopes - When atoms of same element have same number of protons (Z) but different number of a neutrons (s) such atoms are called isotopes.
Isobars - When atom of different element have same mass number (A) but different atomic number (Z) such atoms are called isobars.
Valency - It is the combining capacity of an atom.