Discovered - or rather manufactured - in 2010, element 117 is the last element to be placed on the periodic table as of this writing. The heavier atom oganesson was prepared a few years before by smashing atoms of californium with calcium. A similar approach but with berkelium instead of californium should have yielded tennessine but there was no Bk available. Instead, they simply waited til the sample of Cf used in oganesson had decayed into Bk. The experiment was a success and seven atoms of tennessine, and all the backup data to prove it, was published in 2012 by the Russian-American team cooking up new atoms in their Dubna lab.

But, as a postscript, I have to ask why wasn't the symbol made Tn as the state is usually abbreviated?