Isotope frequency in a naturally occurring Isotope mixture. In nature, elements for which several isotopes exist occur in an Isotope mixture that – apart from a few specifically justified exceptions – is the same everywhere on earth. Several isotopes may occur in about the same ratio, e.g. Cu-63 with 69% and Cu-65 with 31% in the case of copper. However, one Isotope frequently predominates and the others are present only in traces in this case, e.g. for oxygen: 99.759% O-16; 0.0374% O-17; 0.2039% O-18.