Isotopes Number of Protons Number of Neutrons Abundance within the Earth (weight percent)
O-16 or 16O 8 8 99.76%
O-17 or 17O 8 9 0.04%
O-18 or 18O 8 10 0.20%
Isotopes are defined by their atomic mass = number of protons + number of neutrons.
Isotope abundances are measured using a mass spectrometer.
All three oxygen isotopes are found in calcite (CaCO3, calcium carbonate), the primary mineral in limestone.