The Allegory of the Cave. The dialogue between Socrates and Glaucon is probably fictitious and composed by Plato; whether or not the allegory originated with Socrates, or if Plato is using his mentor as a stand-in for his own idea, is unclear. In the dialogue, Socrates asks Glaucon to imagine a cave, in which prisoners are kept.
The d sublevel consists of five orbitals that can hold a maximum of  two electrons each. The total maximum number of electrons that the  d sublevel can hold is 10 electrons.
First shell contains 2 electrons since it can hold only 2 electrons, then the second shell can hold only 8 electrons, so it contains 8 electrons in second shell. Third shell can hold 18 electrons, but it contains only one electron, since it has 1 electron remaining.