Anewly discovered element, x, has two naturally occurring isotopes. 93.5 percent of the sample is an isotope with a mass of 268.9 u, and 6.5 percent of the sample is an isotope with a mass of 269.9 u. what is the weighted average atomic mass for this element? . 268.5 u . 269.0 u . 269.4 u . 269.8 u.