The noun form of the adjective reactive is reactiveness.
The related nouns are reactant (The material that reacts), reactor (The apparatus which contains the reaction), and reaction (The process the reactant undergoes).
Reaction is a noun, reactive is an adjective, but react itself is a verb.
It is as reactive as indium. Just kidding, it is reactive. Hoe reactive, more reactive than silver, but not as reactive as tin.
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Caesium is an extremely reactive chemical element.
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Reactivity, like most (if not all) words that end in "ivity", is a noun. It is a noun formed from the adjective "reactive". Other words ending in "ivity" include creativity, connectivity, activity, and sensitivity.
Hydrogen IS reactive
chromium is quite reactive :)
Cerium is reactive but not highly reactive.
helium is non reactive.