Crossword puzzle clue "element forming part of water and air"

Element forming part of water and air

Element forming poisonous compounds

River, rising in Venezuela, forming part of its border with Colombia and flowing into the Atlantic

River forming part of the boundary between Europe and Asia

1 part vermouth, 1 part campari, 1 part soda water, and some sweetened dairy beverage?

River forming part of the Germany-Netherlands boundary

Forming an essential part

River forming part of the European-Asian border