01What the spectrum shows
A mass spectrometer ionizes atoms and sorts them by mass-to-charge ratio. Each peak is an isotope; peak height is its relative abundance.
The atomic mass on the periodic table is a weighted average of those isotopes, which is why it's rarely a whole number.
02Notebook box
Average mass = Σ (isotope mass × fractional abundance).
Worked example: Chlorine is 75.8% Cl-35 and 24.2% Cl-37 → (0.758)(35) + (0.242)(37) = 35.5 amu.