CRYSTALLINE SOLIDS: STRUCTURE, PROPERTIES, TYPES, EXAMPLES - CHEMISTRY - 2025

The crystalline solids are those whose microscopic structures are sorted and obey a distinctive pattern for a particular lattice; for example: cubic, hexagonal, triclinic, rhombohedral, among others.

These solids are said to exist as crystals, displaying facets and geometric designs that reflect how orderly they are inside. Other examples of crystalline solids are diamond, quartz, anthracene, dry ice, potassium chloride, or magnesium oxide.

A crystal clear bar of brown sugar. Source: Pixabay.

A well-known pair of crystalline solids is that of sugar and salt (NaCl). At first glance, both show white crystals; but their properties differ enormously. Sugar is a molecular crystalline solid, while salt is an ionic crystalline solid. The first is made up of sucrose molecules; and the second, of Na ⁺ and Cl ^- ions.

The image above gives a glimpse of how bright sugar crystals can be. However, salt crystals are not far behind. Although sugar and salt seem like brothers, their structures are different: sugar, which is sucrose, has a monoclinic structure; while salt, a cubic structure.

Crystal structure of sodium chloride, NaCl

Powdered sugar and salt (icing) remain crystal clear; its crystals have only gotten so much smaller for our eyes. The crystallinity of a solid, therefore, is defined more by its internal structure, than by its external appearance or its brightness.

Structure of crystalline solids

Orderly structures of crystalline solids. Source: Gabriel Bolívar.

Crystalline solids have ordered structures. Their geometric characteristics will depend on the type of crystalline lattice to which they belong, which in turn is projected externally in the forms of the crystal (crystalline system). The top image illustrates two important concepts of such structures: periodicity and crystalline grains.

The spatial ordering of the particles of a crystalline solid is periodic; that is, it repeats itself over and over in all directions. This creates its own structural pattern for each solid and crystal lattice; for example, this is where salt and sugar begin to differentiate beyond their chemical nature.

In A the rhombuses are arranged to give rise to a larger rhombus. Each purple rhombus represents a particle or a set of particles (atoms, ions or molecules). Thus, a macroscopic crystal of A might be expected to have a rhombohedral appearance.

Meanwhile, in B the rhombuses are arranged in such a way that they originate crystalline grains; these are very small crystals (crystallites). B is then said to be a polycrystalline solid; that is, it is formed by the agglomeration of multiple crystallites.

In conclusion, a solid can be simply crystalline (A), or polycrystalline (B); A forms crystals, while B polycrystals.

Properties

The properties of crystalline solids depend on their type of crystal. It is already known that their structures are orderly, and that they also tend to present brilliant features that mineral lovers fall in love with. However, it was mentioned that a powdery solid, even "off", can also be classified as crystalline.

The way its particles are spatially oriented allows them to have some important properties for characterization. For example, crystalline solids are capable of diffraction of X-rays, creating diffraction spectra from which the microscopic structure of a crystal can be determined.

Also, because the structure is periodic, the heat diffuses in the same way throughout the solid; as long as there are no impurities involved. Thus, the melting points for a crystalline solid are constant, and do not vary no matter how they are measured.

Types of crystalline solids

The types of crystalline solids are based on what type of particles they are made of and what their interactions or bonds are. There are essentially four types: ionic, metallic, molecular and covalent networks.

Even when they present a certain degree of impurities, they continue to be crystalline, even though their properties are affected and they do not show the same values ​​expected for a pure solid.

Ionics

Salt is an example of an ionic crystalline solid, as it is composed of Na ⁺ and Cl ^- ions. Therefore, in this type of solids the ionic bond governs: it is the electrostatic forces that govern the structural ordering.

Metallic

All metallic atoms form metallic crystals. This means that, for example, a silver fork is a conglomerate of fused silver crystals. The internal or microscopic structure is the same in every inch of the object, and remains unchanged from the fork handle to the tip of its teeth.

Molecular

Sugar is an example of a molecular crystalline solid, as it is made up of sucrose molecules. Therefore, this type of solid is composed of molecules, which through their intermolecular interactions (and not covalent bonds) manage to establish an ordered structure.

Covalent networks

Finally, we have the crystalline solids of covalent networks. Covalent bonds predominate in them, as they are responsible for establishing an order and keeping the atoms strongly fixed in their respective spatial positions. We do not speak of ions, atoms or molecules, but of three-dimensional networks.

Examples

Next, and finally, some examples will be cited for each one of the types of crystalline solids.

Ionics

All salts are ionic solids. Likewise, there are sulfides, hydroxides, oxides, halides, and other compounds that are also made up of ions, or their interactions are essentially ionic. So we have:

-KCl

-Case ₄

-Ba (OH) ₂

-CASE ₄

-FeCl ₃

-Na ₂ S

-MgO

-CaF ₂

-NaHCO ₃

- (NH ₄) ₂ CrO ₄

In addition to these examples, the vast majority of minerals are considered ionic crystalline solids.

Metallic

Any metallic element occurs naturally as metallic crystals. Some of them are:

-Copper

-Iron

-Aluminum

-Chrome

-Metal hydrogen (under unimaginable pressures)

-Tungsten

-Zirconium

-Titanium

-Magnesium

-Sodium

Molecular

There are a great variety of molecular crystalline solids. Virtually any organic compound that solidifies will establish crystals if its purity is high, or if its structure is not too intricate. So we have:

-Ice (H ₂ O)

-Dry ice (CO ₂)

-I ₂

-P ₄

-S ₈ (and its polymorphs)

-Anthracene

-Solid oxygen

-Solid ammonia

-Phenolphthalein

-Benzoic acid

Covalent networks

And finally, among some crystalline solids of covalent networks we have:

-Diamond

-Graphite

-Carbon nanotubes

-Fullerenos

-Quartz

-Silicon

-Germanium

-Boron nitride

From this list, carbon nanotubes and fullerenes could also be considered as molecular crystalline solids. This is because, although they consist of covalently bonded carbon atoms, they define units that can be visualized as macromolecules (soccer balls and tubes).

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
3. Wikipedia. (2020). Crystal. Recovered from: en.wikipedia.org
4. Chemistry LibreTexts. (June 16, 2019). Crystalline and Amorphous Solids. Recovered from: chem.libretexts.org
5. Rachel Bernstein & Anthony Carpi. (2020). Properties of Solids. Recovered from: visionlearning.com