Band Gap I - Electrons in Atoms

The silicon atom has a total of 14 electrons in its 1, 2 and 3 shells.  The electrons are filled going from the lowest energy level 1s, to the highest energy level 3p.  Referring to the diagram, each horizontal line can hold 2 electrons, indicated by the arrows.  The arrows point in opposite directions because of Pauli exclusion principle, which is an important law in quantum mechanics.  It states that electrons have spins, which are rotations of charge, and there are 2 directions or quantum states - spin up and spin down.  These rotations induce magnetic dipoles - up and down.  Each horizontal line must hold a total of just 2 electrons spinning in opposite directions, never spinning in the same direction.  The horizontal line indicates an electron energy level.

The 4 electrons in the outermost shell 3 are the weakest bound to the nucleus, and will hence be the electrons that make bonds with other atoms in the molecules of liquids and gases, or in solids.  When used in bonds, the electrons will be arranged with one electron per electron energy level.

Reference:
2.2.1 Band Gap I - Electrons in Atoms, Delft University of Technology, https://www.youtube.com/watch?v=s-84yDC23HU