Which metal will react least rapidly with water? A : Sr B : K C : Na D : Rb?

All the four metals in the four options are Group I (Group IA) elements.

Going down Group I, the elements are Li, Na, K, Rb, Cs and Fr.
For Group I elements, the reactivity increases when going down the group.
Hence, reactivity : Na < K < Rb < Cs
The least reactive metal among the four options is Na, and thus Na will react least rapidly with water.

The answer: C: Na

Reaction with water.....

The alkali metals (group 1) get their name from the fact that they react with water to produce the metal hydroxides. Potassium reacts more vigorously than does sodium and rubidium reacts more vigorously than either Na or K. Strontium is an alkaline earth metal (group 2), and these react with water as well, but not as vigorously as do the alkali metals.

Therefore, rubidium will take the cake for most reactive, and strontium is the least reactive.

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