A mass spectrum shows two major peaks for an element at m/z 69 and 71, with the lighter peak about 1.5 times taller than the heavier one. Which element best matches this data?

Core Concept

Weighting the two isotope masses by their roughly 3-to-2 abundance ratio gives (3 × 69 + 2 × 71) ÷ 5 = 69.8 amu, essentially gallium's tabulated average atomic mass of 69.72. Peak positions give isotope masses while relative heights give abundances, and combining them identifies the element. None of the other listed elements averages anywhere near 69.8 amu.