Classes of Crystalline Solids
Ck12 Science
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AUTHOR Ck12 Science
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C ONCEPT
Concept 1. Classes of Crystalline Solids
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Classes of Crystalline Solids
• List the four types of crystalline solids. • Describe the properties of each type.
We often take a lot of things for granted. We just assume that we will get electric power when we connect a plug to an electrical outlet. The wire that comprises that outlet is almost always copper, a material that conducts electricity well. The unique properties of the solid copper allow electrons to flow freely through the wire and into whatever device we connect it to. Then we can enjoy music, television, work on the computer, or whatever other activity we want to undertake. 1
www.ck12.org Classes of Crystalline Solids
Crystalline substances can be described by the types of particles in them and the types of chemical bonding that takes place between the particles. There are four types of crystals: (1) ionic, (2) metallic, (3) covalent network, and (4) molecular. Properties and several examples of each type are listed in the following table and are described below.
TABLE 1.1: Crystalline Solids –Melting and Boiling Points Type of Crystalline Solid
Examples (formulas)
Melting Point (°C)
Ionic
NaCl CaF2 Hg Na Au W B C (diamond) SiO2 H2 I2 NH3 H2 O
801 1418 -39 371 1064 3410 2076 3500 1600 -259 114 -78 0
Metallic
Covalent network
Molecular
Normal Boiling Point (°C) 1413 2533 630 883 2856 5660 3927 3930 2230 -253 184 -33 100
1. Ionic crystals – The ionic crystal structure consists of alternating positively-charged cations and negativelycharged anions. The ions may either be monatomic or polyatomic. Generally, ionic crystals form from a combination of Group 1 or 2 metals and Group 16 or 17 nonmetals or nonmetallic polyatomic ions. Ionic crystals are hard and brittle and have high melting points. Ionic compounds do not conduct electricity as solids, but do conduct when molten or in aqueous solution.
FIGURE 1.1 NaCl crystal.
2. Metallic crystals – Metallic crystals consist of metal cations surrounded by a “sea” of mobile valence electrons. These electrons, also referred to as delocalized electrons, do not belong to any one atom, but are capable of moving through the entire crystal. As a result, metals are good conductors of electricity. As seen in the Table 1.1, the melting points of metallic crystals display a wide range. 3. Covalent network crystals – A covalent network crystal consists of atoms at the lattice points of the crystal, with each atom being covalently bonded to its nearest neighbor atoms. The covalently bonded network is threedimensional and contains a very large number of atoms. Network solids include diamond, quartz, many metalloids, and oxides of transition metals and metalloids. Network solids are hard and brittle, with extremely high melting and 2
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Concept 1. Classes of Crystalline Solids
FIGURE 1.2 Metallic crystal lattice with free electrons able to move among positive metal atoms.
boiling points. Being composed of atoms rather than ions, they do not conduct electricity in any state.
FIGURE 1.3 Diamond is a network solid and consists of carbon atoms covalently bonded to one another in a repeating three-dimensional pattern. Each carbon atom makes four single covalent bonds in a tetrahedral geometry.
4. Molecular crystals – Molecular crystals typically consist of molecules at the lattice points of the crystal, held together by relatively weak intermolecular forces. The intermolecular forces may be dispersion forces in the case of nonpolar crystals, or dipole-dipole forces in the case of polar crystals. Some molecular crystals, such as ice, have molecules held together by hydrogen bonds. When one of the noble gases is cooled and solidified, the lattice points are individual atoms rather than molecules. In all cases, the intermolecular forces holding the particles together are far weaker than either ionic or covalent bonds. As a result, the melting and boiling points of molecular crystals are much lower. Lacking ions or free electrons, molecular crystals are poor electrical conductors.
FIGURE 1.4 Ice crystal structure.
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www.ck12.org Summary
• • • •
Ionic crystals are composed of alternating positive and negative ions. Metallic crystals consist of metal cations surrounded by a “sea” of mobile valence electrons. Covalent crystals are composed of atoms which are covalently bonded to one another. Molecular crystals are held together by weak intermolecular forces.
Practice
Use the link below to answer the following questions: http://www.citycollegiate.com/solid3.htm 1. What kind of three-dimensional structure do crystalline solids have? 2. Do crystalline solids have shapes? 3. Characterize the melting points and boiling points of crystalline solids. Review
1. 2. 3. 4. • • • •
What is an ionic crystal? What type of crystal is a diamond? What forces hold molecular crystals together? Which type of crystal is a good conductor of electricity? covalent: Are composed of atoms which are covalently bonded to one another. ionic: Composed of alternating positive and negative ions. metallic: Consist of metal cations surrounded by a “sea” of mobile valence electrons. molecular: Held together by weak intermolecular forces.
References 1. Image copyright Matt Valentine, 2013. http://www.shutterstock.com. Used under license from Shutterstock.com 2. User:H Padleckas/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Sodium_chloride_crystal.p ng. Public Domain 3. CK-12 Foundation - Joy Sheng. . CC-BY-NC-SA 3.0 4. User:YassineMrabet/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Carbon_lattice_diamon d.png. Public Domain 5. User:Yadevol/Wikipedia. http://commons.wikimedia.org/wiki/File:Iceviistructure-ru.gif. Public Domain
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