Anything that has weight is matter. As per the theory of atom, all matter, whether it is solid, liquid, or gas is composed of atoms. An atom contains a central part called nucleus, which holds the neutrons and the protons. Normally, protons are positively charged particles and neutrons are neutrally charged particles. Electrons which are negatively charged particles are arranged in orbits around the nucleus in a way similar to the array of planets around the Sun. The following figure shows the composition of an atom.
Atoms of different elements are found to have different number of protons, neutrons, and electrons. To distinguish one atom from another or to classify the various atoms, a number which indicates the number of protons in the nucleus of a given atom, is assigned to the atoms of each identified element. This number is known as the atomic number of the element. The atomic numbers for some of the elements which are associated with the study of semiconductors are given in the following table.
Normally, an atom has an equal number of protons and planetary electrons to maintain its net charge at zero. Atoms frequently combine to form stabilized molecules or compounds through their available valence electrons.
The process of combining of free valence electrons is generally called bonding. Following are the different kinds of bonding that takes place in atom combinations.
Let us now discuss in detail about these atomic bondings.
Each atom is seeking stability when the atoms bond together to form molecules. When the valence band contains 8 electrons, it is said to be a stabilized condition. When the valence electrons of one atom combine with those of another atom to become stable, it is called ionic bonding.
If an atom has more than 4 valence electrons in the outer shell it is seeking additional electrons. Such atom is often called an acceptor.
If any atom holds less than 4 valence electrons in the outer shell, they try to move out from these electrons. These atoms are known as donors.
In ionic bonding, donor and acceptor atoms frequently combine together and the combination becomes stabilized. Common salt is a common example of ionic bonding.
The following figures illustrate an example of independent atoms and ionic bonding.
It can be seen in the above figure that the sodium (Na) atom donates its 1 valence electron to the chloride (Cl) atom which has 7 valence electrons. The chloride atom immediately becomes overbalanced negatively when it obtains the extra electron and this causes the atom to become a negative ion. While on the other hand, the sodium atom loses its valence electron and the sodium atom then becomes a positive ion. As we know unlike charges attract, the sodium and chloride atoms are bound together by an electrostatic force.
When the valence electrons of neighboring atoms are shared with other atoms, covalent bonding takes place. In covalent bonding, ions are not formed. This is a unique dissimilarity in covalent bonding and ionic bonding.
When an atom contains four valence electrons in the outer shell, it can share one electron with four neighboring atoms. A covalent force is established between the two linking electrons. These electrons alternately shift orbits between the atoms. This covalent force bonds the individual atoms together. An illustration of covalent bonding is shown in the following figures.
In this arrangement, only the nucleus and valence electrons of each atom are shown. Electron pair are created due to individual atoms are bonded together. In this case, five atoms are needed to complete the bonding action. The bonding process widens out in all directions. Each atom is now linked together in a lattice network and a crystal structure is formed by this lattice network.
The third type of bonding generally occurs in good electrical conductors and it is called as metallic bonding. In metallic bonding, an electrostatic force exists between the positive ions and electrons. For example, the valence band of copper has one electron in its outer shell. This electron has a tendency to roam around the material between different atoms.
When this electron leaves one atom, it instantly enters the orbit of another atom. The process is repetitive on a nonstop basis. An atom becomes a positive ion when an electron leaves it. This is a random process. It means that one electron is always linked with an atom. It does not mean that the electron is associated with one particular orbit. It is always roaming in different orbits. As a consequence, all atoms are likely to share all the valence electrons.
Electrons hang around in a cloud that covers the positive ions. This hovering cloud bonds the electrons randomly to the ions. The following figure shows an example of the metallic bonding of copper.