Here's why their properties are similar despite the large gap in their atomic numbers:
* Electron Configuration: The defining characteristic of noble gases is their completely filled outermost electron shell, which makes them very stable and unreactive.
* Argon has the electron configuration [Ne]3s²3p⁶
* Krypton has the electron configuration [Ar]3d¹⁰4s²4p⁶
* Xenon has the electron configuration [Kr]4d¹⁰5s²5p⁶
Notice how the outermost shell (the valence shell) is filled with 8 electrons (or 2 for Helium).
* Similar Reactivity: Because of their full valence shells, noble gases have a very low tendency to gain or lose electrons. They are therefore very unreactive and exist as single atoms, not forming molecules readily.
Why the Gap?
The gap in atomic numbers between these elements is due to the filling of the d-orbitals (transition metals) in the periodic table. As you move down the periodic table, the d-orbitals become progressively filled with electrons. This is why there are 18 elements between Argon and Krypton, and 18 between Krypton and Xenon.
Summary
Despite the seemingly large jump in atomic numbers, the noble gases Argon, Krypton, and Xenon share similar chemical properties due to their stable, filled valence electron shells. This common feature leads to their very low reactivity and makes them distinct from other elements.
