Alkali metals react with water to promote H2(g) and metal hydroxides. Li floats and reacts slowly, whereas Cs reacts more vigorously. This is because of which one of the following properties ?

The reactivity of alkali metals with water is governed by how easily the metal atom loses its outermost valence electron to form a positive ion (cation). This ease of electron loss is directly related to the First Ionization Energy.

• Periodic Trend: As we move down Group 1 from Lithium (Li) to Cesium (Cs), the atomic size increases and the outer electron becomes further shielded from the nucleus. Consequently, the first ionization energy decreases significantly.
• Chemical Reaction: Because Cesium has a much lower ionization energy than Lithium, it requires far less energy to initiate the reaction, making the process much more rapid and vigorous.
• Other Options: While melting point (A) affects the reaction rate (as metals with lower melting points liquefy and increase surface area contact), the fundamental chemical property driving the trend in reactivity across the group is the ionization energy. Electronegativity and electron affinity are more relevant to non-metals or the formation of anions.