Which of the following pairs of 'number - composition' is correct? I. Atomic number - number of protons II. Mass number - Sum of number of neutrons and protons

Explanation

Atomic number Z counts protons in the nucleus. Mass number A counts nucleons: protons plus neutrons, A = Z + N. Both pairs are correct. Mass number is not the same as the measured atomic mass, and it need not increase monotonically with atomic number across arbitrary nuclides.