Final answer:
The mass number of the most common type of atom in a natural sample of neon with a relative atomic mass of 20.2 is 20, corresponding to the most abundant isotope, Ne-20.
Step-by-step explanation:
The mass number of the most common isotope in a natural sample of neon with a relative atomic mass of 20.2 can be determined by looking at the different isotopes of neon and their natural abundances. Neon has three naturally occurring isotopes: Ne-20, Ne-21, and Ne-22. Their mass numbers are 20, 21, and 22, respectively, which correspond to their number of nucleons (protons plus neutrons). Given that the relative atomic mass of neon is a weighted average of the masses of these isotopes based on their abundance, and the most common isotope in neon is Ne-20 (90.48% abundance), it is also the isotope that contributes most to the average atomic mass of neon. Therefore, the mass number of the most common type of atom in the sample of neon is 20.