Complex sentence: Its extreme reactivity is such that it was not isolated (via electrolysis) until 1886; it was not isolated chemically until 1986; and its occurrence in an uncombined state in nature was not reported until 2012, and is contentious.
Split into simpler sentences: Its extreme reactivity is such that it was not isolated (via electrolysis) until 1886 and was not isolated chemically until 1986. Its occurrence in an uncombined state in nature was reported until 2012, but is contentious.