The gas used to fill party balloons contains only helium atoms. This makes an inert gas.

Here's why:

* Helium is a noble gas: Helium belongs to Group 18 of the periodic table, known as the noble gases.

* Noble gases are unreactive: Noble gases have a full outer shell of electrons, making them very stable and unlikely to form chemical bonds with other elements. This is why they are called "inert" gases.

So, the helium in party balloons is an inert gas.