27. State the requirements for chemical stability of an atom.
28. Almost every conductor material possesses a POSITIVE coefficient of
resistance; that is, the resistance of the conductor INCREASES as
temperature INCREASES. Temperature can change the total number of
electrons available for conduction, and it can change their average speed of
motion in an electric field. A conductor is an element whose atoms have
from one to three valence electrons (see copper on the periodic chart).
Since a!! the valence electrons of such materials are used in the conduction
of current, the total number of electrons for conduction cannot be appreciably
changed. The dominant effect, then, is the scattering of the current-carrying
electrons by collision with atoms of the crystal. This means that the average
velocity of the electrons used for conduction decreases as the temperature
Thus, the resistance increases as the temperature increases, which shows a
positive temperature coefficient of resistance.