Why do alkali metals react vigorously with water?

Alkali metals react vigorously with water due to their low ionisation energies and high reactivity.

Alkali metals, found in Group 1 of the Periodic Table, are known for their high reactivity. This is primarily due to their electronic configuration. They have one electron in their outermost shell, which is relatively far from the positive nucleus and thus held quite loosely. This makes it easy for alkali metals to lose this electron and form a positive ion, a process known as ionisation. The energy required to remove this electron is called ionisation energy. Alkali metals have low ionisation energies, meaning they can easily lose their outermost electron.

When alkali metals come into contact with water, they readily donate their outermost electron to a water molecule. This results in the formation of a hydroxide ion (OH-) and a hydrogen gas (H2). This reaction is exothermic, meaning it releases energy in the form of heat and light, which is why we observe a vigorous reaction. The reaction can be represented as follows: 2M(s) + 2H2O(l) → 2MOH(aq) + H2(g), where M represents an alkali metal.

The reactivity of alkali metals increases down the group. This is because the outermost electron gets further from the nucleus and is less tightly held as we move down the group. Therefore, it becomes even easier for the alkali metal to lose this electron and react with water. For example, potassium (K) reacts more vigorously with water than sodium (Na).

In conclusion, the vigorous reaction of alkali metals with water is due to their low ionisation energies and high reactivity. This makes them readily lose their outermost electron to water, resulting in a vigorous, exothermic reaction.