Oxygen Isotopes - The BAS Library

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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.