1-10
The concept of energy bands is particularly important in classifying materials as conductors,
semiconductors, and insulators. An electron can exist in either of two energy bands, the conduction band
or the valence band. All that is necessary to move an electron from the valence band to the conduction
band so it can be used for electric current, is enough energy to carry the electron through the forbidden
band. The width of the forbidden band or the separation between the conduction and valence bands
determines whether a substance is an insulator, semiconductor, or conductor. Figure 1-6 uses energy level
diagrams to show the difference between insulators, semiconductors, and conductors.
Figure 1-6.—Energy level diagram.
The energy diagram for the insulator shows the insulator with a very wide energy gap. The wider this
gap, the greater the amount of energy required to move the electron from the valence band to the
conduction band. Therefore, an insulator requires a large amount of energy to obtain a small amount of
current. The insulator "insulates" because of the wide forbidden band or energy gap.
The semiconductor, on the other hand, has a smaller forbidden band and requires less energy to
move an electron from the valence band to the conduction band. Therefore, for a certain amount of
applied voltage, more current will flow in the semiconductor than in the insulator.
The last energy level diagram in figure 1-6 is that of a conductor. Notice, there is no forbidden band
or energy gap and the valence and conduction bands overlap. With no energy gap, it takes a small amount
of energy to move electrons into the conduction band; consequently, conductors pass electrons very
easily.
Q9. What term is used to describe the definite discrete amounts of energy required to move an electron
from a low shell to a higher shell?
Q10. What is a negative ion?
Q11. What is the main difference in the energy arrangement between an isolated atom and the atom in a
solid?
Q12. What determines, in terms of energy bands, whether a substance is a good insulator,
semiconductor, or conductor?
COVALENT BONDING
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
