Chemistry – The Periodic Table - Group I properties | e-Consult

The reactivity of alkali metals increases down Group I because the outermost electron is held less tightly by the nucleus.

Alkali metals have the electronic configuration of *ns¹*, meaning they have one electron in their outermost shell. This electron is relatively easily removed. As you move down the group, the number of electron shells increases. However, the shielding effect of the inner electrons increases as well. This means that the effective nuclear charge experienced by the outermost electron decreases.

A lower effective nuclear charge means the outermost electron is held less strongly, making it easier to remove and form a positive ion (cation). Therefore, the ease of losing an electron and forming a positive ion increases down the group, leading to increased reactivity. This increased reactivity is reflected in their greater tendency to react with water and other substances.