Posted on

Chemistry of Solids

adamantine structure of quartz

 

 

Solid State Chemistry

 

 

Solid state chemistry is overlooked in general chemistry.  It would be an under statement to say solid state chemistry is the substance of civilization. Many of the most interesting and practical uses of chemistry fall into the category of solid state chemistry.

 

Periodic Table

 

The quickest method to make a good guess as to what type of solid comes from a  formula, use the periodic table. Elements are divided into two areas: metals and nonmetals, Figure 1.

 

 

Metals on the left, nonmetals on the right
Figure 1: Metals in gray, nonmetals in red, metalloids in blue, inert elements in beige

 

Metals are shown in gray: main group metals, transition metals, rare earth metals, and post transition metals. They donate electrons to form positive ions in ionic compounds. Metals form alloys with other metals.

 

Nonmetals are pale red. When nonmetals interact with metals, they add electrons  and become negative ions. 

Nonmetals also form bonds with other nonmetals through sharing electrons. This forms molecules or other times forms molecular or network solids.

 

Metalloids have the properties of both metals and nonmetals. This means the element behaves as both a metal which forms an ion, a metal that exists in metallic bonds, or form covalent bonds with other nonmetals.

 

 

Types of Solids

 

Ionic Materials

Ionic solids are composed of positive and negative ions arranged in borderless arrays. Positive ions come from metal elements that release electrons to become cations. Negative ions result when nonmetal elements add electrons, which become anions.

Ionic compound showing array
Figure 2: Representation of simple ionic compound with cations in blue and anions in red

The attraction between positive and negative charges is very strong. Ionic compounds act as solids at room temperature.

Ionic solids make up important materials like metal oxides used to extract metals, electrolytes in batteries, and essential nutrients like sodium chloride, (table salt).

 

Molecular Materials

 

Molecular compounds form when nonmetal elements bond to nonmetal elements. They form molecules. The molecules are modular basic units held together with covalent bonds. Covalent bond result from two nonmetal elements that share valence electrons. This allows each element to possess eight electrons in their outer-most electron shell.

Examples of molecules
Figure 3: Examples of simple molecules: water, ammonia, and methane

The solid however, is not held together by covalent bonds. The bulk material forms from interactions between the molecules. The interactions which bind molecules are called intermolecular interactions. 

structure of two molecular solids
Figure 4: Methane and water when frozen, show the structure of two molecular solids

Molecules take the form of solids, liquids, or gases. Among cohesive forces which enforce the structure of solids, intermolecular forces are the weakest.

Figure 4 shows two solid forms of simple molecules.

Ice, which is composed of water molecules, has an enforced higher order structure like diamonds. The dotted lines between water molecules are interactions between a partially positive charge on hydrogen with a partially negative charge on oxygen. 

 

In contrast, methane, CH4, has much weaker intermolecular attractions because methane is nonpolar. When methane does become solid, it solidifies at a much lower temperature than water: 0ºC for water versus -182ºC for methane.

 

The reason these two molecules have such different melting points is because water molecules have much stronger intermolecular interactions than methane.

 

Metallic Materials

 

When metal elements bond to other metal elements, metal-metal bonds result. You can understand many of the physical properties you remember when you think of a ‘metal based on the way metal atoms form the bulk material you know as a metal, Figure 5.

 

Atomic model of metal
Figure 5: Metal composed of atoms (blue) and a medium of freely moving valence electrons (red arrows)

The metal atoms (blue) sit inside a matrix of  shared valence electrons. The electrons are shown by bent red arrows. The arrows show the electrons move constantly and hop from one positive metal nuclei to another.

The background pink inside shows an overall negative charge. This allows the total charge of the metal to be neutral.

 

Network Solid Materials

 

Network solids hold together with covalent bonds between nonmetal atoms, Figure 6.

 

network solid
Figure 6: atoms held together by covalent bonds in an infinite array

⦿ Covalent Molecules and Covalent Networks

It is important not to confuse covalent molecules with a covalent network of atoms. 

  • Molecules come from internal covalent bonds. The molecules hold together with intermolecular forces
  • Covalent networks form from covalent atoms throughout the bulk of the material.

 

Organization of Solid

 

The basic structural unit of the silicate ion, (SiO4-4) is shown in Figure 7. The properties of various materials you know: quartz crystal, glass, and fiberglass have different properties. This happens because the way the individual silicates are arranged in space.

The structure is tetrahedral with a silicon atom in the center while oxygen atoms with a (-1) charge occupy each point of the tetrahedron.

 

molecular silicate structure
Figure 7: Tetrahedral structure of silicate anion

 

Crystalline Materials

 

When solids form a regular repeating structure, they solidify as crystals. If you examine a piece of quartz, you see the rock has a regular geometric shape, Figure 8.

 

quartz crystal
Figure 8: Regular geometric structure of quartz crystal.

Notice the outer walls of the crystal have a hexagonal shape. Its faces also have a clean and regular face. These types of geometric faces are only possible when there is an underlying regular structure.

The tetrahedral silicates connect with each other on the molecular level in a regular six member ring structure fused together. That builds the hexagonal structure you see when you look at a crystal, Figure 9.

 

adamantine structure of quartz
Figure 9: Silica quartz has a rigid crystalline structure

 

 

This arrangement is called “adamantyl”. It is common among other covalent networks like elemental carbon, and phosphorous pentoxide, P2O5. This arrangement is one of the most stable arrangements in nature.

 

 

 

 

Amorphous Materials

 

Heat silicates until quartz (or more often sand) turns into a liquid. As the melt cools, you are rewarded with glass. Though glass also forms from silicate anions, because it cooled too fast to form an orderly structure, the silicates attach to each other any way they happen to find each other, Figure 10.

 

 

glass structure
Figure 10: Silicate glass is composed of silicates with no long range order

 

Composite Materials

 

If you physically combine two materials on a bulk scale, the result is a composite. You may already know about fiberglass, Figure 11. The name “fiberglass” is a bit misleading. There are fibers made of glass. However the familiar structural material is made out of strands of glass fiber embedded in a plastic.

 

 

composite structure
Figure 11: Fiberglass is made of glass fibers embedded in a polymer support.

The blue cylinders represent strands of glass fibers. The gray polymer area can be made of plastics like: polyethylene, (PE), polyvinyl chloride, (PVC), or polypropylene.

The World of Solids Waits

The most important kinds of solids that make your world fall into a few types based on how a solid is organized at the microscopic level. Each category deserves a topic in its own right. However the most basic ideas makes the best place to start.