Answer: potassium and rubidium
Explanation:
1) The group 1 of the periodic table contain six elements namely Lithium(Li), Sodium(Na),Potassium(K),Rubidium(Rb),Cesium(Cs) and Francium(Fr). These metals are called alkali metals because they form alkalies( i.e. strong bases capable of neutralizing acids) when they react with water.
2) The heavier alkali metals (K, Rb, and Cs) react so vigorously that they invariably explode.
3) This trend, which is not consistent with the relative magnitudes of the reduction potentials of the elements, serves as another example of the complex interplay of different forces and phenomena—in this case, kinetics and thermodynamics.