The least reactive elements are found in Group 18 (VIIIa) of the periodic table, also known as the noble gases. These elements have full outer electron shells, making them extremely stable and unreactive.

Here's why:

* Chemical reactions involve the sharing or transfer of electrons.

* Noble gases have a complete octet (or duet for Helium) of electrons in their outermost shell, meaning they have no tendency to gain, lose, or share electrons.

* This stability makes them extremely unreactive.

Examples of noble gases include:

* Helium (He)

* Neon (Ne)

* Argon (Ar)

* Krypton (Kr)

* Xenon (Xe)

* Radon (Rn)

While they are generally unreactive, it's important to note that some heavier noble gases like Xenon can form compounds under specific conditions.