Chemistry Crossword Puzzle Help? 3 questions left before I'm done?

[Hannah]

12 down (6 letters)-- To metals and non-metals alike, this element bonds like epoxy--generally in a very exothermic manner (in other words, it makes things burn), but, oh, what trouble we would be in without those atoms, (both the pairs down here and the trios up there).

21 across (9 letters)-- An alloying agent in steel and an active ingredient in posion ivy lotion, its silicate may be cubic but it won't last forever (sorry, girls . . .)

10 across (8 letters)-- Not for sale at Arbee's, this alkali is a real go-getter in vacuum tubes and is known for its ruby-colored flame test.

Thanks!!! =)