Argon is unreactive mainly because it is a noble gas. This group of elements, which also includes helium, neon, krypton, xenon, and radon, is characterized by having a full valence shell of electrons. In the case of argon, it has eight electrons in its outer shell, satisfying the octet rule. This complete electron configuration makes argon stable and less likely to lose, gain, or share electrons with other atoms.
Furthermore, the lack of available electrons for bonding means that argon does not readily form compounds with other elements. As a result, argon exists primarily in its monatomic form (as individual atoms) in the atmosphere rather than as part of any chemical compound. Therefore, the intrinsic properties of argon, due to its electron configuration, lead to its unreactivity and make it a suitable choice for applications that require an inert atmosphere.