Solids: Conductors, Insulators and Semiconductors Conductors: mostly metals Insulators: mostly nonmetal materials we’ll study allotropes of carbon and study their properties Semiconductors: metalloids 2

Solids: Conductors, Insulators and Semiconductors Conduction Band: white Conductor Band gap No gap Valence Band in red Insulator Semiconductor 2

Bonding in Metals The electron-sea model is a simple depiction of a metal as an array of positive ions surrounded by delocalized valence electrons. Metals are good conductors of electricity because of the mobility of these delocalized valence electrons. A metal also conducts heat well because the mobile electrons can carry additional kinetic energy. 2

Bonding in Metals 2

Bonding in Metals Molecular orbital theory gives a more detailed picture of the bonding in metals. Because the energy levels in a metal crowd together into bands, this picture of metal bonding is called band theory. 2

Bonding in Metals Molecular orbital theory gives a more detailed picture of the bonding in metals. According to band theory, the electrons in a crystal become free to move when they are excited to the unoccupied orbitals of a band. 2

Bonding in Metals Molecular orbital theory gives a more detailed picture of the bonding in metals. In a metal, this requires little energy since the unoccupied orbitals lie just above the occupied orbitals of highest energy. 2

Bonding In Metals: Lithium according to Molecular Orbital Theory

Sodium According to Band Theory Conduction band: empty 3s antibonding No gap Valence band: full 3s bonding

3s bonding and antibonding should be full Magnesium Overlap of 3s and 3p bands means that top of 3s antibonding is not full and bottom of 3p bonding is partially filled. 3s bonding and antibonding should be full

Magnesium Conduction band: empty No gap: conductor Valence band: full

Solids: Conductors, Insulators and Semiconductors Conduction Band: white Conductor Band gap No gap Valence Band in red Insulator Semiconductor 2

Allotropes of Carbon Diamond: high thermal conductivity, extremely strong, insulator Graphite: high thermal conductivity, conductor electrodes for electrolysis and batteries; essentially pencil “lead” Fullerenes: discovered in 1986, amazing possibilities 2

Diamond Diamond has a three-dimensional network structure in which each carbon is singly-bonded to four others with sp3 hybridization. Diamond is a covalent network solid each carbon covalently bonded to 4 others. Diamonds are the hardest substance known. must break carbon-carbon bonds Diamonds have a melting point of 3550°C. 2

Structure of Diamond

Diamond Diamond has a three-dimensional network structure in which each carbon is singly-bonded to four others with sp3 hybridization. Why do diamonds conduct heat? Metals conduct heat because the the mobile electrons can carry additional kinetic energy. Diamonds are insulators and have no mobile electrons. Diamonds conduct heat through high frequency (= high energy) vibrations that transmit over long distances Diamonds conduct heat 4 times better than copper! 2

Graphite Graphite has a layered structure, in which the carbon atoms in each layer bond to three other carbons with sp2 orbitals. Graphite’s primary use is in the manufacture of electrodes for electrolysis and batteries. Of the covalent network solids, only graphite conducts electricity. This is due to the delocalization of the resonant p electrons in graphite’s sp2 hybridization. 2

Structure of Graphite

Fullerenes The fullerenes are a family of molecules with a closed cage of carbon atoms arranged in pentagons and hexagons. Each carbon is sp2 hybridized. The most symmetrical member is buckminsterfullerene, C60. Buckminsterfullerenes show potential for applications in superconductivity and catalytic activity. 2

Buckminsterfullerene Figure 13.25: A frame model of C60. By permission of Dr. Richard Smalley, Rice University

Solids: Conductors, Insulators and Semiconductors Diamond Graphite Conductor Band gap = 5.5 eV ≈ 530 kJ/mol No gap Insulator 2

Solids: Conductors, Insulators and Semiconductors Band Gap for Semiconductors Diamond 5.5 eV Si 1.1 eV Ge 0.67 eV Band gap Semiconductor 2

Semiconductors Metalloids: semiconducting elements low electrical conductivity at room temperature Electrical conductivity increases with temp. Gap between valence and conduction band is intermediate in size 2

Semiconductors Semiconducting elements form the basis of solid state electronic devices. Metalloids (such as silicon or germanium) are semiconducting elements whose electrical conductivity increases as temperature increases. A striking property of these elements is that their conductivities increase markedly when they are doped with small quantities of other elements. 2

Semiconductors Semiconducting elements form the basis of solid state electronic devices. When silicon is doped with phosphorus, it becomes an n-type semiconductor, in which electric current is carried by electrons. 2

Semiconductors Semiconducting elements form the basis of solid state electronic devices. When silicon is doped with boron, it becomes a p-type semiconductor, in which an electrical current is carried by positively charged holes 2

Semiconductors Semiconducting elements form the basis of solid state electronic devices. Joining a p-type semiconductor to an n-type semiconductor produces a p-n junction, which can function as a rectifier. A rectifier is a device that allows current to flow in one direction, but not the other. 2

Figure 13.29: Effect of doping silicon.

Figure 13.30: A p-n junction as a rectifier.