The Electron Shell

- The Electron Shells

An electron shell, or main energy level, is the part of an atom where electrons are found orbiting the atom's nucleus. In chemistry and atomic physics, an electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. All atoms have one or more electron shell(s), all of which have varying numbers of electrons. 

Each electron shell has one or more electron sub-shells, or sub-levels. Electron shells make up the electron configuration of an atom. The number of electrons that can be in a certain shell is equal to 2n². 

The name for electron shells came from the Bohr model, which states that electrons orbit the nucleus at certain distances so that their orbits form "shells". This term was presented by Niels Henrik David Bohr.

- The Valence Shell

The valence shell is the outermost shell of an atom in its neutral state. The valence shell contains the electrons most likely to be involved in reactions. In a noble gas, an atom tends to have eight electrons in its outer shell (except helium, which is only able to fill its shell with two electrons). Noble gases serve as the model for the octet rule, which states that atoms tend to form bonds or create ions so that they can have eight electrons in their outermost shell. This is mostly applicable to the main-group elements of the second and third periods.

(Stanford University - Jaclyn Chen)

- Sub-shells

Electron sub-shells are identified by the letters s, p, d, f, g, h, i, etc., corresponding to the azimuth quantum numbers: 0, 1, 2, 3, 4, 5, 6, etc. Each shell is able to hold 2, 6, 10, 14, and 18 electrons respectively.

- Silicon Atoms

To understand how semiconductors work, you must first understand a little about how electrons are organized in an atom. The electrons in an atom are organized in layers. These layers are called shells. The outermost shell is called the valence shell. The electrons in this shell are the ones that form bonds with neighboring atoms. Such bonds are called covalent bonds. Most conductors have just one electron in the valence shell. Semiconductors, on the other hand, typically have four electrons in their valence shell. 

If all the neighboring atoms are of the same type, it’s possible for all the valence electrons to bind with valence electrons from other atoms. When that happens, the atoms arrange themselves into structures called crystals. Semiconductors are made out of such crystals, usually silicon crystals. 

Here, each circle (as shown in the figure above) represents a silicon atom, and the lines between the atoms represent the shared electrons. Each of the four valence electrons in each silicon atom is shared with one neighboring silicon atom. Thus, each silicon atom is bonded with four other silicon atoms.

The number of valence electrons in the electron shell of an elemental semiconductor determines its conductivity. This unique feature makes semiconductors pivotal to modern technology. Without semiconductors, integrated circuits, transistors, solar cells, and many other computing materials simply would not exist. Because of this, semiconductor materials have a significant impact on the computing and electronic product supply chain.