Real isotope masses and natural abundances for 30 elements displayed as mass spectra.
Atomic mass values on the periodic table are never whole numbers, and understanding exactly why requires connecting isotope abundance data to a weighted average calculation — a core AP Chemistry Unit 1 skill demonstrated directly through real mass spectrometry data.
A mass spectrometer separates isotopes of an element by mass-to-charge ratio (m/z), producing a spectrum with a distinct peak for each naturally occurring isotope, where peak height represents that isotope's relative abundance in a natural sample. Because different isotopes of the same element have different numbers of neutrons (and therefore different mass) but occur in fixed, characteristic natural proportions, each element's mass spectrum has a signature pattern of peaks.
The atomic mass reported on the periodic table is a weighted average: atomic mass = Σ(isotope mass × fractional abundance), summed across every naturally occurring isotope. Chlorine is the textbook example: Cl-35 (mass ≈ 34.97, abundance ≈ 75.8%) and Cl-37 (mass ≈ 36.97, abundance ≈ 24.2%) combine to a weighted average of approximately 35.45 — matching the periodic table value exactly, and explaining directly why chlorine's atomic mass isn't close to a whole number despite each individual isotope having a mass very near a whole number.
Copper shows the same principle with Cu-63 (abundance ≈ 69.2%) and Cu-65 (abundance ≈ 30.8%), weighting to approximately 63.55 — and because Cu-63 is more abundant, the weighted average sits closer to 63 than to 65, illustrating that the weighted average is always pulled toward whichever isotope is more common, not simply the midpoint between isotope masses.
Verifying a calculated weighted average against the accepted IUPAC atomic weight is a useful self-check built directly into working through these problems, confirming the calculation was set up correctly rather than just producing a plausible-looking number.
This tool displays real isotope mass and abundance data as mass spectra for 30 elements, lets you read m/z peaks directly, and calculates the weighted average atomic mass for comparison against the periodic table value.