Argon have three electron rings 2,8,8 and has fullfilled its outer rings.
Krypton have four rings; 2,8,18(??),8 

How come Krypton have eighteen electrons in its third ring, while
argon have eight, and still is a noble gas if its outer ring can be
added further with electrons?

How does the periodic system work according to the numbering of rings?

as far as I know, a noble gas is an atom that has fullfilled its
outer electron ring. Which is why I find this third-ring argon/krypton
kinda frustrating :)

Thank You

Krypton is 2,8,8,18 not 2,8,18,8

Ian G.

If you examine a table of the electron configurations of the ground states
of the elements you will see that this occurs repeatedly:

   #18  A    2,8,8
   #19  K    2,8,8,1    Starts filling the N shell
   #20  Ca   2,8,8,2 
   #21  Se   2,8,9,2    Goes back and adds to the M shell
   #22  Ti   2,8,10,2

   #36  Kr   2,8,18,8   
   #37  Rb   2,8,18,8,1
   #38  Sr   2,8,18,8,2
   #39  Y    2,8,18,9,2

The ground state is the lowest state and although it seems that both 
A and Kr could add additional electrons to their outer shell, the energy
required to do this is large - the energy required to move an electron
from the ground state to an excited state is large so that makes them
inert gasses.

Sorry - I was wrong!

Ian G.

electrons in any atom are filled in accordance with the
Aufbau("building up") principle. follow the article on this principle
at :

http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/aufbau-principle.html

Then try to follow this,

Argon has 3 quantum states and a valency of 18 which gives it the
following configuration:

1s2        (2)
2s2 2p6    (8) 
3s2 3p6    (8)


Krypton, on the other hand has 4 quantum states and a valency of 36
which imparts it the following configuration:

1s2           (2)
2s2 2p6       (8) 
3s2 3p6 3d10  (18)
4s2 4p6       (8)

Try looking up more articles on Aufbau principle and these things
should be clear to you....bfn

elecrons set themselves according to the bohr-bury principle
there are three rules to be followed

first-the formula to calculate the number of electrns in a ring is
2(n^2),where n is the number of the ring, example-1 for first ring, 2
for second ring

second-the last ring cannot have more than 8 electrons

third-the second last ring cannot have more than 18 electrons.
so,
Krypton has 36 electrons
the first shell has 2(1^2)=2 electrons
the second shell has 2(2^2)=8 electrons
the third has 2(3^2)=18 electrons
the fourth is the last so it can hold just 8 electrons
2+8+18+8=36
so, krypton has 18 in the third shell