Final Answer:
The Group 2 elements become more reactive as the group is descended due to a decrease in ionisation energies.
Step-by-step explanation:
The reactivity of Group 2 elements, also known as alkaline earth metals (beryllium, magnesium, calcium, strontium, barium, and radium), with water is influenced by their ionisation energies. Ionisation energy is the energy required to remove one mole of electrons from one mole of atoms in their gaseous state. As we move down the Group 2 elements, the atomic radius increases due to the addition of extra electron shells. This increase in atomic size results in a weaker attraction between the outer electrons and the nucleus.
Ionisation energy is inversely proportional to atomic size; as atomic size increases, ionisation energy decreases. The outer electrons in Group 2 elements are further away from the nucleus in the descending order of the group, making them easier to remove. Thus, the decrease in ionisation energy down the group facilitates the loss of electrons, leading to more vigorous reactions with water.
When these Group 2 elements react with water, they undergo displacement reactions, producing metal hydroxides and hydrogen gas. The trend in ionisation energies explains why the reactivity increases down the group, with elements like calcium and barium reacting more vigorously than beryllium or magnesium. This reactivity trend is fundamental to understanding the chemical behavior of alkali earth metals and provides insights into their practical applications, such as in the removal of hardness from water.