The first twenty elements

The place of an element follows from its electron shells. You will fill the shells for any atomic number up to $20$, find the period and the group, and then the program draws the first twenty elements in the shape of the table.

The idea. Shells fill $2$, $8$, $8$, $2$. The period is the number of shells. Groups 1 and 2: the outer electrons. Groups 13 to 18: $10$ plus the outer electrons. Helium goes to group 18.

Your tasks.

1. In shells, return the list of electrons in each shell, such as [2, 8, 7] for chlorine.
2. In period, return the number of shells.
3. In group, return the group number, with helium in group 18.