Which alkali metal tarnishes the quickest in reaction to oxygen?

Potassium tarnishes the quickest in reaction to oxygen due to its position in the alkali metal group of the periodic table. Alkali metals are known for their reactivity, which increases as you move down the group. This is because the outermost electron, which is relatively far from the nucleus, experiences less nuclear attraction and is more easily lost in chemical reactions.

Potassium has a larger atomic radius than lithium and sodium, and as a result, it reacts more vigorously with oxygen. When exposed to air, potassium reacts with oxygen to form potassium oxide, which leads to the rapid tarnishing of its surface. The formation of this oxide layer imparts a dull appearance, indicating the quick tarnishing characteristic of potassium.

In contrast, lithium and sodium react with oxygen at a slower rate and therefore tarnish less quickly. Rubidium, although it is larger and more reactive than potassium, is generally less commonly observed in a laboratory setting due to its instability and reactivity, making potassium the representative alkali metal that tarnishes the fastest under normal conditions.