Question

Select all correct statements about the Covalent Bond?

◻ It is a strong bond

◻ Could have ionic characteristics

◻ Common in semiconductor materials

◻ Results in materials with high melting points

◻ It can be polar or non-polar

Answer 1

1. It is a strong bond
2. Could have ionic characteristics
3. Common in semiconductor materials
4. Results in materials with high melting points
5. It can be polar or non-polar

For covalent bond following statements are correct

1. It is a strong bond
2. It can be polar or non-polar

A covalent molecular compound consists of individual molecules that contain only covalent bonds. The covalent bonds within the molecules are very strong and highly directional, so the molecules usually have definite shapes and retain their identities during physical changes. The forces between the molecules are by comparison very weak. It's these weak intermolecular forces that determine many of the properties of covalent molecular compounds. Compounds that contain both metals and non-metals are usually ionic, not covalent. So KBr and Na2SO4 are easily recognized as ionic compounds; CO2 and CH4 are covalent

Properties of Covalent Molecular Compounds.

1. Low melting points and boiling points. A relatively small amount of energy is required to overcome the weak attractions between covalent molecules, so these compounds melt and boil at much lower temperatures than metallic and ionic compounds do. In fact, many compounds in this class are liquids or gases at room temperature.
2. Low enthalpies of fusion and vaporization These properties are usually one or two orders of magnitude smaller than they are for ionic compounds
3. Soft or brittle solid forms. The weak intermolecular forces make the solid form of covalent molecular compounds easy to distort or break.
4. Poor electrical and thermal conductivity. Ionic compounds conduct electricity well when melted; metallic solids do as well. Covalent molecular compounds do not.

Why it is not common in semiconductors?

The chemical activity of an atom is determined by the number of electrons in its valence shell. When the valence shell is complete, the atom is stable and shows little tendency to combine with other atoms to form solids. Only atoms that possess eight valence electrons have a complete outer shell. These atoms are referred to as inert or inactive atoms. However, if the valence shell of an atom lacks the required number of electrons to complete the shell, then the activity of the atom increases.

Silicon and germanium, for example, are the most frequently used semiconductors. Both are quite similar in their structure and chemical behaviour. Each has four electrons in the valence shell. Consider just silicon. Since it has fewer than the required number of eight electrons needed in the outer shell, its atoms will unite with other atoms until eight electrons are shared. This gives each atom a total of eight electrons in its valence shell; four of its own and four that it borrowed from the surrounding atoms. The sharing of valence electrons between two or more atoms produces a COVALENT BOND between the atoms. It is this bond that holds the atoms together in an orderly structure called a CRYSTAL. A crystal is just another name for a solid whose atoms or molecules are arranged in a three-dimensional geometrical pattern commonly referred to as a lattice.

As a result of this sharing process, the valence electrons are held tightly together. The +4 in the circles is the net charge of the nucleus plus the inner shells (minus the valence shell). Because every atom in this pattern is bonded to four other atoms, the electrons are not free to move within the crystal. As a result of this bonding, pure silicon and germanium are poor conductors of electricity. The reason they are not insulators but semiconductors is that with the proper application of heat or electrical pressure, electrons can be caused to break free of their bonds and move into the conduction band. Once in this band, they wander aimlessly through the crystal.

Energy can be added to electrons by applying heat. When enough energy is absorbed by the valence electrons, it is possible for them to break some of their covalent bonds. Once the bonds are broken, the electrons move to the conduction band where they are capable of supporting electric current. When a voltage is applied to a crystal containing these conduction band electrons, the electrons move through the crystal toward the applied voltage. This movement of electrons in a semiconductor is referred to as electron current flow. There is still another type of current in a pure semiconductor. This current occurs when a covalent bond is broken and a vacancy is left in the atom by the missing valence electron. This vacancy is commonly referred to as a "hole." The hole is considered to have a positive charge because its atom is deficient by one electron, which causes the protons to outnumber the electrons. As a result of this hole, a chain reaction begins when a nearby electron breaks its own covalent bond to fill the hole, leaving another hole. Then another electron breaks its bond to fill the previous hole, leaving still another hole. Each time an electron in this process fills a hole, it enters into a covalent bond.