Given Ba, Li, Na, Cs, and Be, arrange the group 1 and 2 elements in order of increasing reactivity to water (H2O). 1. Be∠Na∠ Li∠ Ba∠ Cs
2. Be∠ Li∠ Ba∠ Na∠ Cs
3. Cs∠ Na∠ Be∠ Li∠ Ba
4. Li∠ Cs∠ Na∠ Be∠ Ba

I think it's the third option, but the Beryllium and Barium are messing me up! And a brief explanation too. Thanks!

Be∠ Li∠ Ba∠ Na∠ Cs

Explanation:

Beryllium does not react with water. It is the only alkaline earth metal that does not react with water because of its small size and high ionization energy. Beryllium differs considerably from other members of group two, its compounds when anhydrous show a considerable degree of covalent character.

As the atomic number of the group two elements increases, their ionization energies decreases and their electrode potentials become more negative hence their reactivity increases down the group. This implies that barium will have a very negative electrode potential comparable to that of the alkali metals, hence it reacts considerably with water.

The reactivity of alkali metals with water increases down the group. Lithium reacts quietly with water, sodium and potassium react with water with increasing vigour while rubidium and cesium react with water with exceptional violence.

This little explanation, is the reason behind the option chosen as the answer.

- lead + phosphoric acid = hydrogen + lead(ii) phosphate