elements in group 18 have complete outer electron shells on the periodic table of elements & helium / neon / argon / krypton / xenon / radon belongs to noble gases family , group 18 on the periodic table ->  helium, neon, argon, krypton, xenon, and radon have complete outer electron shells  helium, neon, argon, krypton, xenon, and radon have complete outer electron shells & if the outermost electron shell of an atom is complete, then the element of those atoms will not react with other elements easily -> helium, neon, argon, krypton, xenon, and radon will not react with other elements easily