Why is lithium the least reactive alkali metal?

Lithium is the least reactive alkali metal because it has the smallest atomic radius and the highest ionisation energy.

Lithium, like all alkali metals, is located in Group 1 of the periodic table. These metals are known for their reactivity, which is due to their single electron in the outermost shell. However, lithium is the least reactive among them. This is primarily due to two factors: its small atomic radius and its high ionisation energy.

The atomic radius of an atom is the distance from the centre of the nucleus to the boundary of the surrounding electron cloud. Lithium, being the first alkali metal in the periodic table, has the smallest atomic radius. This means that the outermost electron is closer to the nucleus and therefore more strongly attracted to the positive charge of the protons in the nucleus. This strong attraction makes it harder for the electron to be lost, which is the main process in alkali metal reactions.

Ionisation energy is the energy required to remove an electron from an atom. Lithium has the highest ionisation energy among the alkali metals. This is again due to the small atomic radius, as the outermost electron is more tightly held by the nucleus. The higher the ionisation energy, the more energy is needed to remove the electron and cause a reaction. Therefore, lithium is less likely to react than the other alkali metals.

In summary, the small atomic radius and high ionisation energy of lithium make it the least reactive alkali metal. It is harder for lithium to lose its outermost electron, which is the key step in alkali metal reactions, making it less reactive than the other members of its group.