Ununoctium (Uuo), also known as oganesson, is a synthetic element with atomic number 118. It is a noble gas, and therefore most similar to radon (Rn), which sits above it in the periodic table.

Here's why:

* Group 18 (Noble Gases): Both elements belong to Group 18, also known as the noble gases. Noble gases are known for their lack of reactivity due to their full outer electron shell.

* Similar Chemical Properties: While ununoctium is highly radioactive and has a very short half-life, its predicted chemical properties are very similar to those of radon.

However, it is important to note that ununoctium is a much heavier element than radon, and its properties may differ slightly due to relativistic effects.