Calculate average atomic mass from abundances.
The atomic mass on the periodic table is a weighted average over the isotopes as they occur naturally, not the mass of any single atom. Each isotope contributes its own mass in proportion to its abundance, so chlorine — three parts Cl-35 to one part Cl-37 — averages 35.45 u even though no chlorine atom weighs that. Because the two abundances must sum to 100%, entering one fixes the other, which is why only the first is asked for. The average always sits nearer the more abundant isotope, and the distance it shifts from the lighter mass is a direct read-out of how much heavy isotope is present.
Abundance-weighted average atomic mass
Average atomic mass = (mass1 x abundance1 + mass2 x abundance2) / 100, where abundance2 = 100 - abundance1
Because the mix is roughly 76% Cl-35 and 24% Cl-37. The weighted average lands close to the abundant lighter isotope, which is exactly the arithmetic performed here.
Not directly — it handles two. For three or more, weight each mass by its own abundance and divide by 100; the principle is identical, only the number of terms changes.
Because of nuclear binding energy. Mass is lost as energy when nucleons bind, so an isotope weighs slightly less than the sum of its protons and neutrons — the mass defect that makes nuclear energy possible.