Which of the following is an example of an alkali metal?

Rubidium is an example of an alkali metal because it belongs to Group 1 of the periodic table, which is characterized by its highly reactive metals that have one electron in their outermost shell. Alkali metals, including lithium, sodium, potassium, rubidium, cesium, and francium, are known for their low density and tendency to lose that single outer electron easily, leading to their high reactivity, especially with water and halogens.

Calcium is a member of the alkaline earth metals, found in Group 2 of the periodic table. Krypton and radon, on the other hand, are noble gases located in Group 18, which are known for their lack of reactivity due to their complete valence electron shells. Each of these elements falls into different categories based on their properties and positions on the periodic table, which is why rubidium stands out as the only alkali metal among the options.