In solids, atomic orbitals merge into energy bands; gaps between bands determine conductor, semiconductor or insulator behavior.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
Band theory extends molecular orbital ideas to solids, where a huge number of atomic orbitals overlap and merge into nearly continuous energy bands. The highest filled band is the valence band, and the lowest empty band is the conduction band. The energy gap between them, called the band gap, determines whether a material conducts. In metals the bands overlap or the band is partly filled, so electrons move freely. In semiconductors the gap is small enough for thermal energy to promote electrons, and in insulators the gap is large. Band theory explains the electronic behaviour of solids from first principles.
Key ideas: Count N atomic orbitals producing N levels per band. Positions of Fermi level and gap decide behaviour: overlapping or partly filled bands are conductors, gap under about 3 eV gives semiconductors, and above about 4 eV gives insulators. Photon energy needed to excite across a gap: wavelength in nm equals 1240 divided by gap in eV.
Common mistakes and connections: Mistakes include saying electrons in an insulator cannot move at all, rather than saying there are no available states nearby, and mixing up valence and conduction bands. Another error is describing the gap as the width of a band. Links to MO theory, semiconductors, doping, LEDs, solar cells and colours of materials.
Orbitals merge into bands. In silicon a 1.12 eV gap separates the full valence band from the empty conduction band. The interactive lesson shows this as a live animation or chart.
Diamond has a band gap of about 5.5 eV, making it an insulator, whereas silicon has a gap of only about 1.1 eV.
In solids, atomic orbitals merge into energy bands; gaps between bands determine conductor, semiconductor or insulator behavior.
Band Theory is a advanced topic in Bonding & Molecular Structure. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Bonding & Molecular Structure first: Properties of Ionic Compounds, Properties of Covalent Compounds, Properties of Metals.
The next topics in Bonding & Molecular Structure are Semiconductors and Doping.
Yes. The interactive lesson includes a self-check question on Band Theory, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Band Theory is topic 32 of 104 in Bonding & Molecular Structure.