The electron arrangement of an atom

You will write a function that fills the shells of an atom from its atomic number, for the first twenty elements. Then you will use it to find the outer electrons, and every atom that needs four shells.

The idea. Fill the shells from the inside: at most $2$ in K, then $8$ in L, $8$ in M, and the rest in N. Stop when no electrons are left.

Your tasks.

1. In arrangement, put min(left, limit) electrons in each shell, and stop when none are left.
2. In outer, return the number of electrons in the outermost shell.
3. Find four_shells, the list of atomic numbers from $1$ to $20$ whose atoms use four shells.