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Atomic number of 6 , the element is Carbon.

Normally  the total capacity of a shell is  2 n²  where n is the number of shell.

So K shell contains maximum of 2 electrons,  L shell contains maximum of 8 electrons and M shell contains a maximum of 18.

The way to fill an atom with 6 electrons is :  K shell contains 2 and L shell contains  4  :
   1 s²  ,  2 s²  2 p²
  Then  M shell which contains  3s , 3 p , 3d orbitals contains no electrons.
There are a number of rules to fillin the electrons in an atom.  Madelung energy ordering rule :  n + l determines the order in which electrons are filled in orbitals.   n is the principle quantum number (shell number).   l is the azimutal quantum number which tells the orbital s, p, d and f.   The maximum capacity of the orbital is given by  4 l + 2.

Aufbau principle states that electrons occupy the lowest energy level before filling higher energy level.   Also the order of filling is limited by Pauli's exclision principle. 

Further, there is Hund's rule that unoccupied orbitals are filled in before filling in already occupied orbitals.

There is a simple rule : the diagonal rule of filling in the orbitals.
Each shell contains a maximum of  2 n^2.

2 5 2