A crystalline solid is a type of solid whose fundamental three-dimensional structure consists of a highly regular pattern of atoms or molecules, forming a crystal lattice. The majority of solids are crystalline solids, and the different arrangements of atoms and molecules within them can change their properties and appearance.
What Is a Solid?
A solid is a state of matter in which the substance keeps its shape and maintains a consistent volume. This makes a solid distinct from liquids or gases; liquids maintain a consistent volume but take the shape of their container, and gases take the shape and volume of their container.
The atoms and molecules in a solid can either be arranged in a regular pattern, making it a crystalline solid, or be arranged without a pattern, making it an amorphous solid.
Atoms or molecules in a crystal form a periodic, or repeating, pattern in all three dimensions. This makes the internal structure of a crystal highly organized. The crystal's constituent atoms or molecules are held together through bonds. The type of bond holding them together, ionic, covalent, molecular or metallic, depends on what the crystal is made of.
The smallest unit of the structural pattern is called a unit cell. A crystal is made up of these identical unit cells repeated over and over in all three dimensions. This cell is the most fundamental component of the crystal's structure, and determines some of its properties. It also determines the pattern a scientist sees when they look at the crystal using X-ray diffraction, which can help them identify the crystal's structure and composition.
The positions of the atoms or molecules that make up the unit cell are called lattice points.
Crystallization and Phase Changes
When a liquid cools to its freezing point, it becomes a solid in a process called precipitation. When a substance precipitates into a regular crystalline structure, it is called crystallization.
Crystallization begins with a process called nucleation: Atoms or molecules cluster together. When those clusters are stable enough and large enough, crystal growth begins. Nucleation can sometimes be more easily jump-started by using seed crystals (pre-made clumps) or a rough surface, which encourages the formation of clusters.
A given atomic or molecular material may be able to form multiple crystal structures. The structure that the material crystallizes into will depend on certain parameters during the crystallization process, including temperature, pressure and the presence of impurities.
Types of Crystalline Solids
There are four main types of crystalline solids: ionic, covalent network, metallic and molecular. They are distinguished from each other based on what atoms or molecules they are made of, and how those atoms or molecules are bonded to each other.
The repeating pattern in the structure of ionic crystals is made up of alternating positively-charged cations with negatively-charged anions. These ions can be atoms or molecules. Ionic crystals are usually brittle, with high melting points.
As solids, they do not conduct electricity, but they can conduct electricity as liquids. They can be made up of either atoms or molecules, as long as they are charged. A common example of an ionic solid would be sodium chloride (NaCl), known as table salt.
Covalent network crystals, sometimes simply called network crystals, are held together by covalent bonds between their constituent atoms. (Note that covalent network crystals are atomic solids, meaning they cannot be made out of molecules.) They are very hard solids, have high melting points and do not conduct electricity well. Common examples of covalent network solids are diamond and quartz.
Metallic crystals are also atomic solids, made of metal atoms held together by metallic bonds. These metallic bonds are what give metals their malleability and ductility, as they allow the metal atoms to roll and slide past each other without breaking the material. The metallic bonds also allow valence electrons to move freely throughout the metal in an "electron sea," which makes them great conductors of electricity. Their hardness and melting points vary widely.
Molecular crystals are made up of bonded molecules, unlike metallic and network crystals, which are made up of bonded atoms. Molecular bonds are relatively weak compared to atomic bonds and can be caused by a variety of intermolecular forces including dispersion forces and dipole-dipole forces.
Weak hydrogen bonds hold some molecular crystals, such as ice, together. Because molecular crystals are held together by such weak bonds, their melting points tend to be much lower, they are worse conductors of heat and electricity, and they are softer. Common examples of molecular crystals include ice, dry ice and caffeine.
The solids formed by the noble gases are also considered molecular crystals despite being made of singular atoms; the noble gas atoms are bonded by similar forces as the ones weakly binding molecules together in a molecular crystal, which gives them very similar properties.
A polycrystal is a solid that is composed of multiple types of crystal structures, that are themselves combined in a non-periodic pattern. Water ice is an example of a polycrystal, as are most metals, many ceramics and rocks. The larger unit consisting of a singular pattern is called a grain, and a grain may contain many unit cells.
Conductivity in Crystalline Solids
An electron in a crystalline solid is limited in how much energy it can have. The possible values of energy it can have make up a pseudo-continuous "band" of energy, called an energy band. An electron can take any value of energy within the band, as long as the band is unfilled (there is a limit to how many electrons a given band can contain).
These bands, while considered continuous, are technically discrete; they just contain too many energy levels that are too close together to resolve separately.
The most important bands are called the conduction band and valence band: The valence band is the range of highest energy levels of the material in which electrons are present at absolute zero temperature, while the conduction band is the lowest range of levels that contain unfilled states.
In semiconductors and insulators these bands are separated by an energy gap, called the band gap. In semimetals, they overlap. In metals, there is essentially no distinction between them.
When an electron is in the conduction band, it has enough energy to move about the material freely. This is how these materials conduct electricity: through the movement of electrons in their conduction bands. Since the valence band and conduction band have no gap between them in metals, it's easy for metals to conduct electricity. Materials with a larger band gap tend to be insulators; it's difficult to get an electron enough energy to jump the gap and go into the conduction band.
Another type of solid is an amorphous solid, which does not have a periodic pattern. The atoms and molecules within amorphous solids are largely disorganized. Because of this, they share many similarities to liquids, and in fact have no set melting point.
Instead, because the distances between neighboring atoms or molecules in the structure vary, thermal energy passes through the material unevenly. The material melts slowly over a large range of temperatures.
Examples of amorphous solids include rubber, glass and plastic. Obsidian and cotton candy are also examples of amorphous solids.
About the Author
Meredith is a science writer and physicist based in Seattle. She received her Bachelor of Science degree in physics from the University of Illinois at Urbana-Champaign and her Master of Science degree in physics from the University of Washington. She has written for Live Science, Physics, Symmetry, and WIRED, and was an AAAS Mass Media Fellow in 2019.