Identify the reason why noble gases aren't reactive.

Noble gases are not reactive due to their full valence electron shells. Here are the steps to understand this:

1. The reactivity of an element is determined by its electron configuration, specifically the electrons in the outermost shell, also known as valence electrons.

2. Elements react with each other to achieve a stable electron configuration. Most elements do this by gaining, losing, or sharing electrons to achieve a full valence shell, which usually consists of 8 electrons. This is often referred to as the "octet rule."

3. Noble gases, which are the elements in the far right column of the periodic table, already have a full valence shell. For example, helium has 2 valence electrons (filling its only electron shell), while neon, argon, krypton, xenon, and radon each have 8 valence electrons.

4. Because noble gases already have a full valence shell, they do not need to gain, lose, or share electrons to achieve stability. This makes them inert, or non-reactive.

5. This is not to say that noble gases can't ever form compounds and react with other elements, but it takes extreme conditions for this to happen. Under normal conditions, they are very stable and do not readily react with other elements.