<< Chapter < Page Chapter >> Page >

In a gas, the atoms are spaced far apart and they do not influence each other. However, the atoms in a solid greatly influence each other. The forces that bind these atoms together in a solid affect how the electrons of the atoms behave, by causing the individual energy levels of an atom to break up and form energy bands. The resulting energy levels are more closely spaced than those in the individual atoms. The energy bands still contain discrete energy levels, but there are now many more energy levels than in the single atom.

In crystalline solids, atoms interact with their neighbors, and the energy levels of the electrons in isolated atoms turn into bands. Whether a material conducts or not is determined by its band structure.

Electrons follow the Pauli exclusion principle, meaning that two electrons cannot occupy the same state. Thus electrons in a solid fill up the energy bands up to a certain level (this is called the Fermi energy). Bands which are completely full of electrons cannot conduct electricity, because there is no state of nearby energy to which the electrons can jump. Materials in which all bands are full are insulators.

Metals

Metals are good conductors because they have unfilled spaces in the valence energy band. In the absence of an electric field, there are electrons traveling in all directions. When an electric field is applied the mobile electrons flow. Electrons in this band can be accelerated by the electric field because there are plenty of nearby unfilled spaces in the band.

Insulator

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.

Breakdown

A solid with filled bands is an insulator. If we raise the temperature the electrons gain thermal energy. If there is enough energy added then electrons can be thermally excited from the valence band to the conduction band. The fraction of electrons excited in this way depends on:

  • the temperature and
  • the band gap, the energy difference between the two bands.

Exciting these electrons into the conduction band leaves behind positively charged holes in the valence band, which can also conduct electricity.

Semi-conductors

A semi-conductor is very similar to an insulator. The main difference between semiconductors and insulators is the size of the band gap between the conduction and valence bands. The band gap in insulators is larger than the band gap in semiconductors.

In semi-conductors at room temperature, just as in insulators, very few electrons gain enough thermal energy to leap the band gap, which is necessary for conduction. For this reason, pure semi-conductors and insulators, in the absence of applied fields, have roughly similar electrical properties. The smaller band gaps of semi-conductors, however, allow for many other means besides temperature to control their electrical properties. The most important one being that for a certain amount of applied voltage, more current will flow in the semiconductor than in the insulator.

Conduction

  1. Explain how energy levels of electrons in an atom combine with those of other atoms in the formation of crystals.
  2. Explain how the resulting energy levels are more closely spaced than those in the individual atoms, forming energy bands.
  3. Explain the existence of energy bands in metal crystals as the result of superposition of energy levels.
  4. Explain and contrast the conductivity of conductors, semi-conductors and insulators using energy band theory.
  5. What is the main difference in the energy arrangement between an isolated atom and the atom in a solid?
  6. What determines whether a solid is an insulator, a semiconductor, or a conductor?

Questions & Answers

how to calculate resistance in grade 11
Ngomane Reply
last year memo in 2018 June exam
Ngomane
states the Newton's second law of motion
Shallin Reply
In words it says:" when a net force is applied to an object of mass, It accelerates in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the mass".
Buhle
And in symbols: Fnet = ma
Buhle
thank you
Shallin
pleasure
Buhle
hello everyone
Ngomane
how does hydrogen bonds differ from London force
Madzivha Reply
how come the resultant force is 0
Andrew Reply
It's when you have equivalent forces going different directions then your resultant will be equal to zero
Temosho
describe what john's experiment proves about water molecules?
Fanozi Reply
Newton's first law of motion
Ayabonga Reply
hy
Madzivha
hello
Ayah
hey
Dire
hey
Precious
am great n u
Precious
great
Dire
An unknown gas has pressure,volume and temprature of 0.9atm,and 120°C.how many moles of gas are present?
Chrislyn Reply
Can you, if possible send me more quizzes
Bradley Reply
What is selmon
heath Reply
how long it takes for 25ml ethanol to be evaporated?
Kgaugelo Reply
how to calculate acceleration
Sphe Reply
It depends
Mbongeni
Please state the Newton third low
Malwandla
hy
Nelito
Newton's Third law states that to every force applied, there's an equal but opposite reaction
Lala
difference between a head and tail methods
Zenande Reply
difference between a head-to-tail and tail-to-tail
Zenande
head to tail you draw each vector starting head of the previous vector and tail to tail you construct a parallelogram whereas you started the two vectors from the same axis(from their tails)and the diagonal between the vectors is the resultant vector,tail to tail only includes two vectors
Mbongeni
what is a normal force
Zenande Reply
a normal force is the force that the surface applies on the object. The force is perpendicular to the surface.
Minaa
Lewis structure for C2H2
Mthokozisi Reply
H-C-H
Koketso
what is the mathematical relationship between velocity and time?
Oyisa Reply

Get the best Siyavula textbooks: gr... course in your pocket!





Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 11 physical science' conversation and receive update notifications?

Ask