Answer is: The highest occupied s and p sublevels are completely filled.
Noble gases (group 18) are in group 18: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). They have very low chemical reactivity.
Noble gases have very stable electron configuration and does not need to gain electrons, only when they gain energy.
1) For example, krypton is a chemical element with symbol Kr and atomic number 36, which means it has 36 protons and 36 electrons.
Electron configuration of krypton atom:
₃₆Kr 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶.
2) Another example, xenon (Xe) is noble gas with atomic number 54, which means it has 54 protons and 54 electrons.
Electron configuration of xenon atom:
₅₄Xe 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶.
