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dysprosium a rare-earth metal chemical element of the lanthanide series that has sixty-sixprotons in each nucleus. (symbol: Dy)
erbium a rare-earth metal chemical element of the lanthanide series that has sixty-eight protons in each nucleus. (symbol: Er)
lanthanide any of the lanthanide series of chemical elements.
rare-earth metal any of the series of chemical elements, all metals, that have from fifty-seven to seventy-one protons per atom; lanthanide series.