Argon and krypton are considered noble gases because they are in Group 18 (VIIIa) of the periodic table, also known as the "noble gas" group. They are called noble gases because they are generally unreactive due to their full outer electron shells. This makes them very stable and unlikely to participate in chemical reactions.

Here's why they are not usually found in compounds:

* Full Valence Shells: Noble gases have a complete outer shell of electrons. This electron configuration is extremely stable and doesn't readily accept or donate electrons to form chemical bonds.

* High Ionization Energy: It takes a significant amount of energy to remove an electron from a noble gas atom. This makes them resistant to forming cations.

* Low Electron Affinity: Noble gases have very low electron affinities, meaning they don't readily gain electrons to form anions.

However, it's important to note that while rare, compounds of heavier noble gases like krypton and xenon can exist under certain conditions:

* High Pressure and Temperature: Under extreme conditions of high pressure and temperature, some noble gases can be forced to react and form compounds.

* Specific Reactive Species: Very reactive molecules or compounds can react with noble gases to form unstable compounds, often only existing at low temperatures.

Therefore, while argon and krypton are generally considered unreactive and not found in common compounds, they can participate in chemical reactions under specific extreme conditions.