Here's why:

* Electronegativity: Lithium has the highest electronegativity among the alkali metals. This means it has a stronger hold on its valence electron, making it less likely to lose it and form a positive ion.

* Ionization Energy: Lithium has the highest ionization energy in group 1. This means it requires more energy to remove an electron from its outermost shell.

* Reactivity Trends: Reactivity increases as you move down group 1. This is because the atomic radius increases, meaning the valence electron is farther from the nucleus and easier to remove.

While Lithium is still considered a reactive metal, it is the least reactive among the alkali metals.