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Solid Structures Explained

There are two main categories of solids - crystalline and amorphous. Crystalline solids have a regular repeating atomic structure defined by a unit cell, while amorphous solids lack order and have irregular atomic bonding. Crystalline solids include ionic solids held by electrostatic forces, molecular solids held by dispersion or dipole forces, covalent network solids with covalent bonds, and metallic solids with metallic bonds. Amorphous solids like glass and plastics have a wide range of properties due to their irregular atomic structure.

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136 views3 pages

Solid Structures Explained

There are two main categories of solids - crystalline and amorphous. Crystalline solids have a regular repeating atomic structure defined by a unit cell, while amorphous solids lack order and have irregular atomic bonding. Crystalline solids include ionic solids held by electrostatic forces, molecular solids held by dispersion or dipole forces, covalent network solids with covalent bonds, and metallic solids with metallic bonds. Amorphous solids like glass and plastics have a wide range of properties due to their irregular atomic structure.

Uploaded by

Jashmin Laroza
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Properties of Solids

There are two main categories of solids—crystalline solids and amorphous solids.

Crystalline solids are those in which the atoms, ions, or molecules that make up the solid exist in a regular, well-defined
arrangement. The smallest repeating pattern of crystalline solids is known as the unit cell, and unit cells are like bricks in
a wall—they are all identical and repeating.

There are four types of crystalline solids:

1. Ionic solids—Made up of positive and negative ions and held together by electrostatic attractions. They’re
characterized by very high melting points and brittleness and are poor conductors in the solid state. An example of
an ionic solid is table salt, NaCl.

As we discussed at great length in Ionic Compounds, ionic solids consist of cations and anions held together by the
strength of their opposite charges. The force that holds oppositely charged particles together is called an
"electrostatic force."

In Ionic Compounds, we treated all ionic solids as if they consisted of crystals in which all the ions had identical sizes.
As you can probably guess, ions come in a wide variety of sizes. For example, in sodium chloride, the negatively
charged chloride ions are much larger than the positively charged sodium ions. As a result, the structure of sodium
chloride is a little different than you may have been led to expect. This is illustrated in the following figure:

The type of crystal structure of a particular ionic compound frequently depends on the ratio of the sizes of the anion
and cation.

2. Molecular solids—Made up of atoms or molecules held together by London dispersion forces, dipole-dipole forces,
or hydrogen bonds. Characterized by low melting points and flexibility and are poor conductors. An example of a
molecular solid is sucrose.
3. Covalent-network (also called atomic) solids/Network atomic solids—Made up of atoms connected by covalent
bonds; the intermolecular forces are covalent bonds as well. Characterized as being very hard with very high melting
points and being poor conductors. Examples of this type of solid are diamond and graphite, and the fullerenes. As
you can see below, graphite has only 2-D hexagonal structure and therefore is not hard like diamond. The sheets of
graphite are held together by only weak London forces!
4. Metallic solids—Made up of metal atoms that are held together by metallic bonds. Characterized by high melting
points, can range from soft and malleable to very hard, and are good conductors of electricity.

Amorphous solids do not have much order in their structures. Though their molecules are close together and have little
freedom to move, they are not arranged in a regular order as are those in crystalline solids. Common examples of this
type of solid are glass and plastics.

Instead of being arranged into a regular crystal lattice, the atoms bond in irregular and nonrepeating patterns. These
materials are referred to as amorphous solids.

As a result of this unusual bonding, amorphous solids have a very wide range of properties. Some amorphous solids,
such as window glass, are hard, brittle, and have a high melting point, while other amorphous solids, such as rubber or
plastic, are soft and have very low melting points.

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