How Do Valence Electrons Form Silicon Bonds?

• Copper example with one outer electron
• A silicon atom: four valence electrons
• The loosely bound electron can easily leave as a free electron

What you will learn

• Say what a valence electron is and why the number of them matters.
• Describe how a silicon atom bonds with its four neighbours.
• Explain what happens when a bond breaks or when a crystal is doped.

Before you start

You only need to know that an atom has a nucleus and electrons arranged in shells. No formulas are needed.

What is a valence electron?

A valence electron is an electron in the outermost shell of an atom. An electronics lecture from Harvey Mudd College takes copper as an example: a copper atom has only one electron in its outermost shell. That electron is loosely bound to the atom, so it easily becomes a free electron that moves under an applied voltage.

Insulators sit at the opposite end. According to the lecture, they do not conduct electricity because no free electrons exist in the material. Semiconductors lie between the two groups, and the number of valence electrons is part of the reason.

• Schematic inner shell
• Outermost shell
• Valence electrons

How are silicon's bonds formed?

Silicon shares its valence electrons with its neighbours. The Harvey Mudd lecture says elements with four valence electrons in the carbon group conduct less well than conductors but better than insulators, and are called semiconductors. Silicon and germanium both have four valence electrons.

In their crystal lattice, each atom shares one valence electron with each of its four neighbouring atoms to form covalent bonds. The lattice has a tetrahedral pattern. Each atom both shares an electron and receives a shared electron back from its neighbours.

• Silicon atom at the centre
• Four neighbouring atoms
• Shared electron pair between two atoms

What if a bond breaks or changes?

A covalent bond can break. If an electron gains enough thermal energy, it may break the bond and become a negatively charged free electron. The place it leaves behind is a positively charged hole. At room temperature relatively few electrons gain enough energy for this, so pure silicon has low conductivity.

• The broken bond
• Free electron escaping
• The hole it leaves behind

The second way is doping, which adds an impurity element with three or five valence electrons. A donor atom with five valence electrons uses four to form bonds and leaves one extra free electron, giving an N-type semiconductor. An acceptor atom with only three valence electrons forms three bonds and leaves a hole in the lattice, giving a P-type semiconductor.

• Donor atom: one electron extra
• The extra free electron
• Acceptor atom: one electron short
• Hole in the lattice

Real-world example

Use an illustrative model of clay balls and connecting sticks. Put one ball in the centre and connect it to four neighbours to remember silicon's coordination. Each stick represents a shared bond; it is not a track for an electron. To represent an electron leaving a bond, use a loose bead and mark the vacancy in the model.

The comparison is only a memory aid. Ask yourself: if an extra person joined the circle, would that be like a donor or an acceptor atom? A donor atom, because it brings one extra electron.

Common misunderstandings

A valence electron is not every electron of an atom. Only those in the outermost shell count.

A covalent bond is not an electron taken from one atom. The electrons are shared between two atoms.

A hole is not a ready-made particle. It is the place where an electron has left, behaving as a positive charge.

Frequently asked questions

How many valence electrons does silicon have?

Four. Germanium also has four valence electrons.

Why does pure silicon conduct poorly?

At room temperature relatively few electrons gain enough energy to break a bond and become free electrons.

How do N-type and P-type doping differ?

N-type doping uses atoms with five valence electrons, leaving one extra free electron. P-type doping uses atoms with three valence electrons, leaving a hole.

Summary

• Valence electrons are the outermost-shell electrons, and silicon has four of them.
• Each silicon atom shares one electron with each of its four neighbours to form covalent bonds.
• Breaking a bond or doping the crystal produces a free electron or a hole.