Complex sentence: Indium-115 makes up 95.7% of all indium: thus indium is one of only three elements (the others being tellurium and rhenium) whose stable isotopes are less abundant in nature than their long-lived primordial radioisotopes.
Split into simpler sentences: Indium-115 makes up 95.7% of all indium. Indium is one of three known elements (the others being tellurium and rhenium) of which the stable isotope is less abundant in nature than the long-lived primordial radioisotopes.