Types of Solids

The solids can be of the following two types based on the arrangement of constituent particles.

• Crystalline Solids
• Amorphous Solids

1) Crystalline Solids

Crystalline solids or solids in the crystalline state have regular or definite arrangements of constituent particles (atoms, ions, molecules) throughout the three-dimensional structure of the solid. For example, sodium chloride, quartz, diamond, and more. They are true solids.

Properties of crystalline solids

• They have definite melting points and start melting when the temperature reaches their melting points.
• Their constituent particles follow a specific arrangement.
• They show cleavage property which means when they are cut with a sharp tool they split into two parts each of which has a plain and smooth surface.
• Their heat of fusion or the energy required to melt a given mass at its melting point is definite.
• They are anisotropic which means they have different physical properties in different directions in the crystalline solid.

Types of crystalline solids

Crystalline solids further have the following types based on the bonding between their constituent particles.

1. Molecular Solids

As the name suggests, their constituting particles are molecules, which are held together by weak van der Waal's force of attraction. So, they are soft in nature. They are bad conductors of electricity as they don't have free electrons for the movement of current. They also have low melting and boiling point so tend to vaporize easily such as ice, solid CO2 (dry ice), and more.

Molecular solids are further divided into the following types;

i) Polar molecular solids

As the name suggests, the constituting molecules of these solids are bonded with each other through covalent bonds. The polar nature is due to the difference in the electronegativity of the atoms that are bonded with each other. So, due to the difference in electronegativity partial charges develop on atoms that generates a dipole-dipole interaction force that holds the constituting molecules together. For example, sulphur dioxide (SO₂).

ii) Non-polar molecular solids

In these molecular solids, the constituting particles (molecules) are joined or bonded together through non-polar bonds. These solids are soft as their molecules are held together by weak Van Der Waals forces. The bonds formed between molecules are non-polar as they are formed between the same kind of atoms or molecules such as Cl₂ (a molecule of chlorine gas in which one chlorine atom is bonded to another chlorine atom through a single non-polar covalent bond).

iii) Hydrogen-bonded molecular solids

In this type of molecular solids, a hydrogen bond is present in which hydrogen is bonded to fluorine or oxygen or nitrogen. For example, Hydrogen fluoride (HF), Water (H₂O), etc., are hydrogen-bonded molecules. Their constituent particles have polar covalent bonds due to the difference in the electronegativity of hydrogen and the other element (fluorine, oxygen or nitrogen). So, they are comparatively strong.

2. Ionic Solids

As the name suggests, these solids are made up of ions or their constituting particles are ions that are held together by strong electrostatic forces of attraction. They are of two types positively charged ions (cations) and negatively charged ions (anions).

Ionic solids are hard and brittle, so, their boiling point is high. In solid states, they do not conduct electricity, however, in the molten or aqueous state, they are good conductors of electricity. Some common examples of ionic solids are NaCl, MgO, ZnS, CaF₂, LiF, etc.

3. Covalent Solids or Network Solids

These solids have a network of covalent bonds between their constituting atoms or elements. These atoms or molecules are neutral and can be of the same type as in diamond (carbon atoms joined together by covalent bonds) or different types of atoms as found in silicon carbide. Besides this, a covalent solid is a bad conductor of electricity as it lacks free electrons. Diamond is the hardest known covalent solid, which is used for cutting glass and other hard substances.

4. Metallic Solids

They are made of fixed positive ions, which are surrounded by free electrons. The presence of free electrons makes them good conductors of electricity and heat. Their melting and boiling points range from medium to high. Some examples of metallic solids include copper, nickel, manganese, sodium and potassium, etc.

2) Amorphous Solids

This type of solids does not have a regular geometrical form or three-dimensional arrangement which means their constituting particles do not follow a regular pattern. These solids are also rigid or incompressible but only to a certain extent.

They are represented as super cooled liquids as their particles are organized randomly like particles of liquids. They don't have a sharp melting point so they melt or their transformation from solid to liquid occurs over a range of temperatures.

Properties of Amorphous Solids

Absence of long-range order: Their constituting particles do not have a long-range order of course of action, we can say that they do not have definite geometry of arrangements. They may have an orderly plane in some regions. Their translucent pieces are called crystallites.

Variable Melting Point: They do not have a definite melting point. They tend to melt over a scope of temperature. For example, when glass is heated, it mellows in the beginning and then gradually melts over a temperature range. In this way, the glass can be given different shapes.

Conversion into the crystalline form: Amorphous solid when cooled by annealing, it becomes crystalline at some temperature. This is the reason that the glass objects of ancient times appear milky due to some crystallization.

Flow like liquids: They are pseudo solids or super cooled liquids. So, they tend to flow slowly. For example, the glasses in the windows of old houses are slightly thicker at the bottom than at the top.

Irregular surface: Whey they are cut with a sharp edge tool they spilt into pieces with an irregular surface.

Difference between Crystalline and Amorphous Solid