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Classes of Crystalline Solids

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Classes of Crystalline Solids

What are common things that we connect to wires?

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.

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.

Crystalline Solids – Melting and Boiling Points
Type of Crystalline Solid Examples (formulas) Melting Point (°C) Normal Boiling Point (°C)
Ionic

NaCl

CaF 2

801

1418

1413

2533

Metallic

Hg

Na

Au

W

-39

371

1064

3410

630

883

2856

5660

Covalent network

B

C (diamond)

SiO 2

2076

3500

1600

3927

3930

2230

Molecular

H 2

I 2

NH 3

H 2 O

-259

114

-78

0

-253

184

-33

 100

1. Ionic crystals -- The ionic crystal structure consists of alternating positively-charged cations and negatively-charged 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.

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 above , the melting points of metallic crystals display a wide range.

Metallic crystal lattice with free electrons able to move among positive metal atoms.

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 three-dimensional 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 boiling points. Being composed of atoms rather than ions, they do not conduct electricity in any state.

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.

Ice crystal structure.

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. What is an ionic crystal?
  2. What type of crystal is a diamond?
  3. What forces hold molecular crystals together?
  4. Which type of crystal is a good conductor of electricity?

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