Mass spectra of elements

A mass spectrum of an element displays one peak for each naturally occurring isotope, with the horizontal axis showing isotopic mass (m/z in amu for singly charged ions) and the vertical axis showing relative abundance.

Reading a Spectrum

• Provides two key pieces of information for each isotope: its mass (m/z) and how common it is in nature (its relative abundance)
• The number of peaks equals the number of naturally occurring isotopes
• The tallest peak identifies the most abundant isotope

Average Atomic Mass

The average atomic mass reported on the periodic table is calculated as the weighted average of all isotopic masses.

1. Multiply each isotope's mass by its fractional abundance
2. Sum the products to obtain the average atomic mass

Average atomic mass = Σ (isotopic mass × fractional abundance)

Because heavier and lighter isotopes contribute in proportion to their natural abundance, the average is always pulled toward the most abundant isotope. This weighted-average calculation is a core quantitative skill: be able to extract data from a mass spectrum and compute the average atomic mass efficiently and accurately.