An unknown element gives two major isotopic peaks at 79 amu (52 %) and 81 amu (48 %)
CHEMISTRY · Single correct · A Level · Chemistry · Atomic structure · Atomic structure
Problem
An unknown element gives two major isotopic peaks at 79 amu (52 %) and 81 amu (48 %). A student calculates an average mass of 79.96 amu. Which statement best justifies identifying the element as bromine?
(a) The peak at 79 amu corresponds to the element’s atomic number, which identifies bromine (atomic number 79).
(b) The nearly 1 : 1 ratio of the 79 and 81 amu peaks is unique to bromine, so the equal abundances confirm its identity.
(c) The weighted average of the 79 and 81 amu peaks (79.96 amu) matches bromine’s tabulated atomic mass to within typical experimental uncertainty.
(d) The two peaks reflect Br₂ molecules containing different isotope combinations, and only bromine forms diatomic molecules under mass-spectrometric conditions.