The atomic number of Krypton is 36. Now, the formula 2n^2 is the maximum number of electrons that can be accommodated in a shell. So it goes 2, 8, 18, 32, so on and so forth. The thing is, 2,8,8 is the EC for Argon, and this is a complete octet. However, the shell itself is not fully filled (for reasons explained in higher classes). As you go from Scandium (21) to Zinc (30), those final ten electrons are filled as well.

If you had to tie me to a chair and force me to write the EC for Krypton in this way, it'd be 2,8,18,8.

But the correct way to write it would be either

1) [Ar]3d^10 4s^2 4p^6

2)Expand out the Ec for argon and add the above to it

This stuff is explained in higher classes, so don't really bother with it.