Here's why:

* Inert means an element doesn't readily react with other elements.

* Helium is a noble gas, which are located in Group 18 of the periodic table.

* Noble gases have a full outer shell of electrons, making them very stable and unreactive.

While other noble gases like neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) are also generally considered inert, helium is the most unreactive of them all.