Complex sentence: Its existence was predicted by Dmitri Mendeleev in 1869, though hafnium was not identified until 1923, making it the penultimate stable isotope element to be discovered (rhenium was identified two years later).
Split into simpler sentences: Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable isotope element to be discovered (rhenium was identified two years later).