LSS_Solids Lesson

Solids

Solids can be crystalline (composed of crystals) or non-crystalline. Non-crystalline solids are also called amorphous. Glass and plastic are examples. Amorphous solids are more similar to liquids than solids. Their structure does not repeat, but is often found in long, chain-like structures that are intertwined. In both cases, the motion of the individual particles is limited. Interparticle interactions and the ability to pack the particles together provide the main criteria for the structures of solids.

Types of Crystalline Solids

There are four general types of crystalline solids with different properties, depending on the type of crystals of which they are composed. They are:

1. ionic
2. molecular
3. covalent
4. metallic.

The terms molecular and covalent both refer to covalently bonded molecules. The difference is that the term molecular solid refers to typical covalently bonded molecules. The term covalent solid refers to what are called network solids. We have not yet discussed these.

Properties of Crystalline Structures

Below is a summary of properties of crystalline structures. You should remember the properties of ionic, molecular (we called this non-network covalent), and metallic solids from the bonding unit.

The properties for the covalent network solids will not reveal an answer yet. You will learn these after you review. 

Watch this video on Crystalline Structures.

Below are a few more details about each type of solid.  

Ionic Solids

Many properties of ionic solids are related to their structure. Ionic solids generally have low vapor pressure due to the strong Coulombic interactions of positive and negative ions arranged in a regular three-dimensional array. Ionic solids tend to be brittle due to the repulsion of like charges caused when one layer slides across another layer. When ionic solids are dissolved in water, the separated ions are free to move; therefore, these solutions will conduct electricity. Dissolving a nonconducting solid in water, and observing the solution's ability to conduct electricity, is one way to identify an ionic solid. The attractive force between any two ions is governed by Coulomb's Law:  

Note that Coulomb's Law can be expressed in terms of energy (as done in the previous unit) or in terms of force as done above. Here the force is directly proportional to the charge of each ion and inversely proportional to the square of the distance between the centers of the ions. For ions of a given charge, the smaller the ions, and thus the smaller the distance between ion centers, the stronger the Coulombic force of attraction, and the higher the melting point. Ions with higher charges lead to higher Coulombic forces, and therefore higher melting points.  

Metallic Solids

A metallic solid can be represented as positive kernels (or cores) consisting of the nucleus and inner electrons of each atom surrounded by a sea of mobile valence electrons. Metals are good conductors because the electrons are delocalized and relatively free to move. Metals are malleable and ductile because deforming the solid does not change the environment immediately surrounding each metal core. Metallic solids are often pure substances, but may also be mixtures called alloys.

Pure metals are useful but their applications are often limited to each individual metal's properties.   However, metals can be mixed to form alloys. Alloys allow metal mixtures that have increased resistance to oxidation, increased strength, conductivity, and melting point. Essentially any property can be manipulated by adjusting alloy concentrations.

Brass is a common alloy. Brass is a mixture of pure zine and copper. Brass is stronger and resists corrosion better then pure zinc or copper. The combination also has a low melting point allowing it to be easily cast into many different shapes and sizes.

There are two types of alloys: interstitial and substitutional.  

Interstitial alloys form between atoms of different radius, where the smaller atoms fill the interstitial spaces between the larger atoms. Interstitial simply means the empty space between the matter, in this case the protons and electrons. Steel is an example in which carbon occupies the interstices in iron. The interstitial atoms do not appreciably expand the lattice, so the density is often substantially increased. The interstitial atoms make the lattice more rigid, decreasing malleability and ductility.

Substitutional alloys form between atoms of comparable radius, where one atom substitutes for the other in the lattice. Brass is an example in which some copper atoms are substituted with a different element, usually zinc. The density typically lies between those of the component metals, and the alloy remains malleable and ductile.

Molecular Solids

Molecular solids consist of nonmetals, diatomic elements, or compounds formed from two or more nonmetals. Molecular solids are composed of distinct, individual units of covalently bonded molecules attracted to each other through relatively weak intermolecular forces. Molecular solids are not expected to conduct electricity because their electrons are tightly held within the covalent bonds of each constituent molecule. Molecular solids generally have a low melting point because of the relatively weak intermolecular forces present between the molecules. Molecular solids are sometimes composed of very large molecules, or polymers, with important commercial and biological applications.

Network Covalent Solids

Network covalent solid can be arranged in two-dimensional (2D) or three-dimensional (3D) structures. 2D network solids are generally soft because adjacent layers can slide past each other relatively easily. The major forces of attraction between the layers are London dispersion forces. One is example is graphite. Graphite is an allotrope (different form of an element) of carbon that forms sheets of 2D networks. Graphite has a high melting point because the covalent bonds between the carbon atoms making up each layer are relatively strong. 3D covalent networks tend to be rigid and hard because the covalent bond angles are fixed. Diamonds are an allotrope of carbon that form 3D networks. Silicon forms a 3D structure similar in geometry to a diamond. It is a semiconductor. Silicon's conductivity increases as temperature increases. Generally, covalent network solids form in the carbon group because of their ability to form four covalent bonds.

Remember to work on the module practice problems as you complete each section of content.  

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