Crossword Puzzle Unknown element

Clue: Unknown element

Plot element? (ACRE with 4 letters)
In one's element (ATHOME with 6 letters)
Element #27 (COBALT with 6 letters)
Key element (e.g. GIST with 4 letters)
NASA fuel element (LOX with 3 letters)
Element #76 (OSMIUM with 6 letters)
Element named for the German state where it was discovered (HASSIUM with 7 letters)
Element discovered by Marie and Pierre Curie (POLONIUM with 8 letters)
Unknown time letters (TBA with 3 letters)
Element of Dr. Seuss' work (WHIMSY with 6 letters)