steel = iron + carbon
Some covalently bonded substances DO NOT form discrete molecules.
Diamond, a network of covalently bonded carbon atoms
Graphite, a network of covalently bonded carbon atoms
Graphene can be described as a one-atom thick layer of graphite.
High-quality graphene is strong, light, nearly transparent and an excellent conductor of heat and electricity.
Andre Geim and Konstantin Novoselov at the University of Manchester won the Nobel Prize in Physics in 2010 "for groundbreaking experiments regarding the two-dimensional material graphene"
Pure silicon is structurally the same as diamond, but is a semiconductor rather than an insulator.
The conductivity increases at higher temperature.
Conductivity of silicon can be improved by doping with other elements.
Substances whose conductivity is increased by doping with atoms having more valence electrons than the host crystal. Here, silicon (4 valence e-’s) is doped with phosphorus (5 valence e-’s).
Substances whose conductivity is increased by doping with atoms having fewer valence electrons than the host crystal. Here, silicon (4 valence e-’s) is doped with aluminum (3 valence e-’s).
Strong covalent forces within molecules
Weak covalent forces between molecules
Ionic compounds at room conditions are generally crystal lattices of alternating cations and anions.
Sodium chloride and sodium fluoride form simple cubic crystals.
NaCl Unit Cell
Lithium niobate, LiNbO3
Ionic compounds are represented by empirical formulas, because they do not form discrete molecular structures.