Order the following reactions of alkali metals with water, halogens, and hydrogen from the least reactive to the most reactive: A) Reaction with halogens B) Reaction with hydrogen C) Reaction with water D) Reaction with oxygen

The least reactive reaction is with oxygen (D), because alkali metals only form a thin oxide layer that slows further reaction. Next comes reaction with halogens (A); the metals give up electrons to form metal halides, but the reaction is not as vigorous as with water. Reaction with hydrogen (B) is weaker still, producing a metal hydride and hydrogen gas. The most reactive is with water (C), where the metal reacts violently, forming hydroxide and hydrogen gas. For example, sodium reacts with water to give NaOH and H₂, showing the strongest reaction among the four.