Types of Crystalline Solids: The Seven Crystal System

Crystalline solids are composed of small crystals having a specific geometrical shape. They have a regular arrangement of atoms, ions, and molecules in a three-dimensional pattern called a crystal lattice. For example, the crystals of sugar and table salt, etc. There are several types of crystalline solids i.e. ionic, molecular, network, and metallic solids.

The external shape of a crystal is known as the habit of a crystal. The smooth or plane surfaces of the crystal are called faces, which have angles between them called, interfacial angles. These angles are always the same for a given crystalline solid, or they can be taken as a reference or standard for a certain crystal system.

Crystalline solids are classified on the basis of their bonds. These bonds hold the ions, molecules, and atoms together in the crystal lattice. There are four main types of crystalline solids. These are ionic solids, molecular solids, network covalent solids, and metallic solids.

Structures of Crystalline Solids

The particles of crystalline solids are organized in a crystal lattice. The building blocks of the crystal lattice are called unit cells.

The crystal lattice of a crystalline substance shows the position of particles ( atoms, ions, or molecules) in three-dimensional space. These positions or points are represented by dots or circles in the above illustration which are called lattice points or lattice sites.

A unit cell is composed of one atom or ion at each corner of the face. Sometimes, an atom or ion is also present in the interior of a cell. There are two main categories for unit cells i.e. body-centered and primitive cells.

A unit cell having an interior point is called a body-centered cell.
A unit cell that does not have any interior points is called a primitive cell. It contains atoms or ions at the corner only.

Types of unit cells (crystals)

The overall structure and shape of a crystalline solid depend on the type of unit cell. A French mathematician named August Bravais introduced a system of the seven-unit cells. These unit cells are also known as Bravais unit cells. These cells differ in two vast possible variations to become seven types of crystals.

The relative lengths of the edges along the three axes (a, b, c).
The three angles between the edges (𝛼, 𝛽, 𝛾).

properties and parameters of unit cell crystal

The seven types of unit cells (crystals) are:

Cubic unit cell
Tetragonal unit cell
Orthorhombic unit cell
Rhombohedral unit cell
Hexagonal unit cell
Monoclinic unit cell
Triclinic unit cell

1. Cubic (isometric) unit cell

Relative axial lengths: a = b = c
Angles: 𝛼 = 𝛽 = 𝛾 = 90^o

2. Tetragonal unit cell

Relative axial lengths: a = b ≠ c
Angles: 𝛼 = 𝛽 = 𝛾 = 90^o

3. Orthorhombic unit cell

Relative axial lengths: a ≠ b ≠ c
Angles: 𝛼 = 𝛽 = 𝛾 = 90^o

4. Rhombohedral (trigonal) unit cell

Relative axial lengths: a = b = c
Angles: 𝛼 = 𝛽 = 𝛾 ≠ 90^o

5. Hexagonal unit cell

Relative axial lengths: a = b ≠ c
Angles: 𝛼 = 𝛽 = 90^o 𝛾 = 120^o

6. Monoclinic unit cell

Relative axial lengths: a ≠ b ≠ c
Angles are 𝛼 = 𝛽 = 90^o 𝛾 ≠ 90^o

7. Triclinic unit cell

Relative axial lengths: a ≠ b ≠ c
Angles: 𝛼 ≠ 𝛽 ≠ 𝛾 ≠ 90^o

Unit Cell (Crystals)	Relative axial length	Angles	Examples
Cubic	a = b = c	𝛼 = 𝛽 = 𝛾 = 90^o	Na⁺Cl⁻, Cs⁺Cl⁻, Ca₂⁺(F⁻)₂, Ca₂⁺O₂⁻
Tetragonal	a = b ≠ c	𝛼 = 𝛽 = 𝛾 = 90^o	(K⁺)₂PtCl₆²⁻, Pb₂⁺WO₄²⁻, NH₄⁺ Br⁻
Orthorhombic	a ≠ b ≠ c	𝛼 = 𝛽 = 𝛾 = 90^o	(K⁺)₂SO₄²⁻, K⁺NO₃⁻,Ba²⁺SO₄²⁻, Ca²⁺CO₃²⁻
Rhombohedral	a = b = c	𝛼 = 𝛽 = 𝛾 ≠ 90^o	Ca²⁺CO₃²⁻, Na⁺NO₃⁻
Hexagonal	a = b ≠ c	𝛼 = 𝛽 = 90^o 𝛾 = 120^o	Agl, SiC, HgS
Monoclinic	a ≠ b ≠ c	𝛼 = 𝛽 = 90^o 𝛾 ≠ 90^o	Ca²⁺SO₄²⁻, 2H₂O, K⁺ClO₃⁻, (K⁺)₄Fe(CN)₆⁴⁻
Triclinic	a ≠ b ≠ c	𝛼 ≠ 𝛽 ≠ 𝛾 ≠ 90^o	Cu²⁺SO₄²⁻, 5H₂O, (K⁺)₂Cr₂O₇²⁻

Types of Crystalline Solids

There are the following types of crystalline solids:

Ionic solids
Molecular solids
Network covalent solids
Metallic solids

Ionic Crystalline Solids

Ionic solids are made up of positive and negative ions, which is the reason they are called ionic. They have a strong electrostatic force of attraction (ionic bonds) between oppositely charged ions. These forces are maximized when cations and anions come close to each other and get packed together in a crystal lattice.

For example, the lattice of sodium chloride. Each ion is surrounded by another opposite charge ion. These ions are fixed in their lattice. This is the reason, that ionic crystalline solids are hard and rigid with high melting points.

Crystalline solid is broken down easily when an external force is applied to it. The upper layer of ions slides away from their oppositely charged ions. As result, the same charged ions come close to each other. This increases the electrostatic force of repulsion between them which displaces the plane of ionic crystal and breaks it.

Ionic crystalline solids do not conduct electricity. This is because these ions have fixed positions. The fused state of ionic crystalline solid has the ability to conduct electricity because ions are free to move in a fused/molten state.

Examples of ionic crystalline solids

Sodium chloride (NaCl)
Calcium fluoride (CaF₂)
Silver chloride (AgCl)
Copper sulfate (CuSO₄)
Magnesium oxide (MgO), etc

Molecular Crystalline Solids

Molecular crystalline solids consist of molecules at lattice sites in the crystal. They have van der Waals forces of attraction which depend on the type of solid. Polar solids have dipole-dipole interactions while nonpolar solids have instantaneous dipole-induced dipole interactions. When molecular crystalline solids start to melt, it overcomes the van der Waal forces. This is a reason, these crystals have low melting points.

Molecular crystalline solids can be further categorized as polar, non-polar, and hydrogen-bonded molecular crystalline solids.

For example, Dry ice (solid carbon dioxide) is the best example of a molecular crystalline solid. The molecules of carbon dioxide (CO₂) are arranged in lattice sites of a crystal lattice.

molecular crystalline solid and molecular crystals

Examples of molecular crystalline solids

Hydrogen (H₂)
Iodine (I₂)
Dry ice (CO₂)
Silicon tetrachloride (SiCl₄)
Phosphorus (P₄), etc

Related topics

Network Covalent Crystalline Solids

The network crystalline solids have atoms at their lattice points or sites. Each atom is covalently bonded with the nearest atom. This makes a network of covalently bonded atoms that are considered a single giant molecule. This type of solids is called network covalent solids because of the network formed between a large number of atoms. However, the atoms are bonded with each other by a strong covalent bond. This is the reason, why these crystals have high melting points.

Network covalent solids include a variety of giant network crystals. Such as graphite, diamond, quartz, metalloids, and some transition elements.

network covalent solid: structure of diamond

Examples of network covalent solids

Graphite
Diamond
Silicon dioxide (SiO₂)
Silicon carbide
Boron carbide
Transition elements, etc

Metallic Crystalline solid

Metallic solids are composed of atoms joined together to form large sheets. These atoms are placed in the form of layers, one above the other joined by a metallic bond. The valence electrons of these atoms are delocalized and leave behind positive ions. These positive ions are present at the lattice sites surrounded by free-moving electrons (an electronic sea) throughout the whole crystal. These electrons move through empty spaces between the ions. This establishes a strong electrostatic force of attraction between the electrons and positive ions.

Metallic crystalline solids are good conductors of heat and electricity. They can be deformed upon the application of external force.

Examples of metallic crystalline solids

All metals are examples of metallic crystalline solid. The most important metals are:

Platinum (Pt)
Gold (Au)
Copper (C)
Tungsten (W)
Iron (Fe)
Magnesium (Mg), etc

Key takeaway(s)

Concepts Berg

What are the 7 types of crystal or unit cells?

There are 7 types of unit cells/crystal system is:

Cubic
Tetragonal
Orthorhombic
Rhombohedral
Hexagonal
Monoclinic
Triclinic

What are the types of crystalline solids?

There are four main types of crystalline solids that are ionic, molecular, network covalent, and metallic crystalline solids.

What is the importance of crystalline solids?

Crystalline solids can be used in different places such as diamonds used in beautiful jewelry, quartz is used in the manufacturing of watches and clocks. They are also used in many industries.

Why are crystalline solids true solids?

They are called true solids because atoms or ions are arranged in a crystal lattice, in a perfect way.

Why does a crystalline solid have a fixed melting point?

The bonds between the atoms of crystalline solids have equal strength. So when heat is applied to these particles they break at the same time. Therefore they have a fixed melting point.

What is the non-crystalline form of carbon?

Charcoal is a noncrystalline form of carbon.

Is graphite amorphous or crystalline?

Graphite is a crystalline solid.

What are amorphous solids?

Amorphous solids contain atoms or ions that are arranged in irregular patterns.

References

Essential of Physical Chemistry: 2nd edition By B.S Bahl and Arun Bahl and G.D. Tuli
Properties of Matter: Solids (livescience.com)