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In this video, we will talk about the bonding model of single crystal silicon.

Single crystal (crystalline) silicon has a diamond lattice structure. Each atom has four nearest neighbors. As we have introduced in the last video, each isolated silicon atom has 14 electrons in total, 10 of them are tightly bound to the nucleus and normally don’t involve in chemical reactions or atom-to-atom interactions. But the outer most four electrons are heavily involved and are called the valence electrons.

The combination of 10 inner electrons and the nucleus is called the core, and is represented by a circle. The four valence electrons are represented by four dots around the core.

In an ideal 100% pure single crystal silicon, each atom shares four of its valence electrons with four closest neighbors, it also accepts four shared electrons from these neighbors. These sharing are called valent bonds and manifest as attractions between each atom and its four nearest neighbors.

The valent bonds are represented as two dots between a pair of neighbors. We can simplify this model each more, by representing the core as a circle, and each valent bond as a line. Each line represents one shared valence electron. Since each atom both gives and accepts electrons with its four neighbors, there are eight lines on each atom.

This simplified bond model represents the relationship between atoms in the space. But please note that although it is drawn as two dimensional flat on a paper, this is only intended for our mental image. In reality, these are all 3-dimensional atoms and bonds.

We can use this bond model to illustrate some applications to show its usefulness.

The first example shows a missing atom, which is a point defect in the crystal.

The second example shows the breaking of an valent bond and the release or freeing of an electron.