Which one of the following does not exist?

Explanation

To determine which of the following does not exist, we need to analyze the options provided, focusing on the properties of the elements in question, particularly the noble gases.

### Step-by-Step Solution:

1. **Identify the Elements**: The question likely provides a list of elements or compounds. For the sake of this solution, let's assume the options include noble gases like xenon (Xe) and neon (Ne), and possibly some compounds like F2 (fluorine gas).

2. **Understand Noble Gases**: Noble gases are known for their lack of reactivity due to having a complete valence shell. Neon (Ne) is a noble gas that is particularly non-reactive and exists only in atomic form.

...