Ne =[He]2s2 2p6
Ar = [Ne]3s2 3p6
Kr = [Ar]4s2 3d10 4p6
As it can be seen that for all three elements, their outermost orbital are completely filled, that is it has both s orbital, p orbital and d orbital fulfilled. Noble or inert gas atoms like Neon, Argon, Krypton have fulfilled valence shell. Fulfilled outermost orbital is the most stable electronic state, hence all elements tends to achieve such stability. These noble gas elements are called inert gas because of their fulfilled outermost shell. This means they don't react easily or take part in eletron donating, receiving or sharing. This is because, for all other elements except inert gas atoms, their valence shell is incomplete and they tend to react by other atoms so as to complete their outermost shell , which we call as duplet (in case of Helium like) or Octet state. Such elements either donate some electrons or receive some to acheive such stable state..
