Well it’s difficult to say what the “largest reaction” is… Technically a nuclear bomb is a big reaction! But I think you mean which 2 chemicals react the most violently when introduced to one another, and that is probably caesium and water! Check this video out: https://www.youtube.com/watch?v=5Vd5pqjbWJU
Although water isn’t an element………….. Back to the drawing board maybe.
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