A. In order to determine the atomic number of an element we need to know the number of protons in an atom. As we know that atom is neutral we can also take the number of electrons as atomic number.
B. Mass number of an element can be found out by adding the total number of protons with the total number of neutrons present in an atom. So mass number = Number of protons +Number of neutrons.
C. As we know that one Dalton ≡ mass of one neutron or one proton, so to calculate the weight of one atom of an element in Daltons, we need to know the mass of one proton or one neutron of the particular atom.