Here's why:

* Rn is the chemical symbol for Radon, a noble gas.

* O is the chemical symbol for Oxygen.

* The subscript 3 indicates that there are three oxygen atoms bonded to each radon atom.

However, it's important to note that Radon trioxide is a hypothetical compound and has not been synthesized or observed experimentally. Radon's reactivity is very low due to its stable electron configuration, and it doesn't readily form stable compounds.