The unrounded masses given on the periodic table are weighted averages of all isotopes of an atom. Weighted averages are based on percentages much like you calculate your grades at school. For example, 50% tests, 25% quizzes, 25% homework. Sulfur has a mass of 32.07 according to the periodic table, what could this tell you about the percentage of sulfur-32 compared to other sulfur isotopes?