Classification of crystalline solids

A crystalline solid is a type of solid-state substance that exists in the solid state. At atmospheric pressure, the melting point of a solid is higher than the temperature of the surrounding environment. Solids are classed as amorphous or crystalline based on how their constituents are organised within the solid. Solids that include subatomic particles (atoms, molecules, or ions) are known as crystalline solids because their subatomic particles are arranged in a regular and three-dimensional structure. Crystallisation occurs in certain compounds, such as sodium chloride and quartz.

Classification of crystalline solids

The type of bonding between the constituent particles of the crystalline solids determines four categories in which classification of crystalline solids based on different binding forces are done. They are as follows:.

• Molecular solids 

These solids are made up of molecules, which are the smallest particles. Weak Van der Waal forces of attraction hold these molecules together. They are soft because of the presence of weak forces. Because there are no free electrons in molecular solids, they are poor conductors of electricity. As a result of their low melting and boiling temperatures, they are readily combustible. Some examples of molecular solids are ice, solid CO2, etc. The three classes of molecular solids are as follows.

• Polar molecular solids

The molecules of these molecular solids are linked together by a polar covalent connection. The electronegativity differences among the atoms involved in the bonding process create the polarity of their bond. Partially charged atoms create a dipole-dipole interaction force responsible for holding a solid like SO2in place.

• Non-polar molecular solid

This type of molecular solid is formed by atoms/elements forming the molecule, then connected by a nonpolar bond. The Van der Waals forces of these solids are weakly pliable. The same atoms or molecules are linked together; hence there is no polarity in the bonds (one chlorine atom is bonded to another by a single nonpolar bond).

• Hydrogen bonded molecular solids

As an example, hydrogen bonds with fluorine, oxygen, and nitrogen. Compared to other covalent connections, these polar covalent bonds are extremely strong. Since N, O, or F have different electronegativity than hydrogen, hydrogen-hydrogen bonds develop polarity. Hydrogen-bonded molecular solids, such as water (H2O) and hydrogen fluoride (HF), are examples of these solids.

• Ionic solids

Solids that are produced by ions are included in this category. The strong electrostatic forces of attraction within the material bring these ions together. Cations and anions are two types of ions that are both positively and negatively charged (negatively charged). The electrostatic force of attraction is the name given to the attraction between cations and anions. High melting temperatures, hardness, and brittleness result from these powerful forces. Ionic solids transmit electricity in a molten or liquid state. Because only in these states are the ions free to travel, unlike solid-state where they are fixed, this is why. NaCl, LiF, and other solids are examples of this type of material.

• Covalent solids

These solids are also called network solids because they are composed of a dense network of covalent bonds between the atoms that make up the solid. As in diamond, all of its constituent atoms are carbon atoms linked together by covalent bonds; however, in silicon carbide (also known as carborundum), the atoms are different. As the hardest substance on Earth, diamond is a covalent solid. The hardness makes it useful in the glass-cutting sector. In addition to the lack of free electrons, the formation of covalent bonds makes covalent solids poor electrical conductors.

• Metallic solids

Free electrons surround the permanent positive ions in the structure of these solids. Metallic solids are excellent conductors of electricity and heat because of the availability of free electrons. There are positive ions in the pool of electrons in metallic materials. Metallic solids can have melting and boiling points that range from moderate to high. These solids might be firm or flimsy (like sodium and potassium). Copper, Nickel, and Manganese are examples of metals.

Properties of crystalline solids: crystalline solid classification

Following are the properties of crystalline solids

• These are quite hard and inflexible. 
• Because of their high melting point, they’re quite durable. 
• They make up a crystalline system of atoms and molecules.

Important points regarding Classification of Solids

• Positive and negative ions combine to form ionic crystals. 
• There is an ocean of mobile electrons around the metallic cations. 
• Crystals made of covalent atoms are called covalent crystals. 
• In molecular crystals, the intermolecular forces are quite weak.

Conclusion

When the atoms, ions, or molecules that make up a solid are organised in an ordered pattern, the substance is crystallised. The vast majority of solids are crystalline. Because of the crystal structure, the intermolecular force of attraction is at its strongest in crystalline solids. Ionic bonds, covalent bonds, hydrogen bonds, and van der Waals forces all play a role in the crystal’s stability. Small, individual crystals form the crystal aggregates that make up a crystalline solid. This chapter is very important to be studied to know about solid-state and it has huge importance in Chemistry in higher studies.