Difference between Crystalline and Amorphous Solids

A solid state is simply one of the states of matter. One of the many different states of matter is solid. Solids have a distinct volume, mass, and shape. Solids differ from liquids and gases in that they exhibit unique characteristics.  These solid states or shapes depending on how the particles are arranged in a specific or indeterminate geometry. There are, however, a few exceptions when it comes to the particles that make up the solid material. These particles are held together by powerful forces between them, irrespective of whether they are molecular, ionic, metallic, or covalent. Crystalline solids are made up of an array of particles that are uniformly arranged and kept together by intermolecular forces. On the other hand, the particles are not arranged in regular arrays in amorphous solids.

Solids are classified into two types based on the arrangement of constituent particles: 

1. Crystalline solid
2. Amorphous Solid

Crystalline Solids

A crystalline solid has a well-arranged large small crystal. A crystal is an ordered arrangement of constituent particles (atoms, molecules, or ions). 

Crystalline solid has a long-range order which means that there is a consistent pattern of particle arrangement that repeats itself on a regular basis across the entire crystal. Typical crystalline solids examples are sodium chloride and quartz.

Properties of Crystalline Solids

1. Crystalline solids have a sharp melting point and begin to melt at a specific temperature.
2. The shapes are well defined and also particle arrangements of crystalline solids are well-defined.
3. Crystalline solid has cleavage property, which means that when cut with the edge of a sharp tool, they split into two pieces and the newly formed surfaces are smooth and plain.
4. They have a distinct heat of fusion (amount of energy needed to melt a given mass of solid at its melting point).
5. Crystalline solids are anisotropic. Anisotropic solids have physical properties, such as electrical resistance or refractive index, that differ when measured in different directions within the same crystal.
6. True solids are crystalline solids.

Types of Crystalline Solids

Crystalline solids are classified into four types based on the nature of their intermolecular forces: molecular, ionic, metallic, and covalent solids. Let us now learn more about these classifications. 

• Molecular Solids: In molecular solids, molecules are constituent particles. Molecular solids are further divided into three categories-
  1. Non-polar Molecular Solids – These are made up of either atoms, such as argon and helium, or molecules formed by non-polar covalent bonds, such as H₂, Cl₂, and I₂. The atoms or molecules in these solids are held together by weak dispersion forces or London forces. 
  2. Polar Molecular Solids – Polar covalent bonds form the molecules of substances such as HCl, SO₂, and others. The molecules of these solids are held together by relatively stronger dipole-dipole interactions. These soft solids are electrically inactive. Examples of such solids include solid SO₂ and solid NH₃.
  3. Hydrogen-Bonded Molecular Solids – These solids’ molecules have polar covalent bonds between H and F, O, or N atoms. Strong hydrogen bonding holds molecules of solids like H₂O together (Ice). 
• Ionic Solids: Ions are the particles that make up ionic solids. Ionic solids are three-dimensional arrangements of cations and anions held together by strong electrostatic forces. These solids are naturally hard and brittle. Their melting and boiling points are both very high. Because the ions cannot move freely, they are electrical insulators in the solid state. When the ionic solid is molten or dissolved in water, the ions become free to move and conduct electricity.
• Metallic Solids: Metals are a well-organized collection of positive ions that are surrounded and held together by a sea of free electrons. These electrons are mobile and are distributed evenly throughout the crystal. Each metal atom adds one or more electrons to the sea of mobile electrons. Metals’ high electrical and thermal conductivity is due to these free and mobile electrons. These electrons flow through the network of positive ions when an electric field is applied.
• Covalent or Network Solids: The formation of covalent bonds between adjacent atoms throughout the crystal results in a wide range of nonmetal crystalline solids. They are also known as giant molecules. Because covalent bonds are strong and directional in nature, atoms are held very tightly in their positions. These solids are extremely hard and brittle. They have extremely high melting points and may decompose prior to melting. They are electrical insulators that do not conduct electricity. Diamond and silicon carbide are two well-known examples of such solids.

Amorphous Solids

Amorphous solids (Greek amorphous = no form) are made up of irregularly shaped particles. Short-range order exists in the arrangement of constituent particles (atoms, molecules, or ions) in such a solid. Only over short distances is a regular and periodically repeating pattern observed in such an arrangement. 

Amorphous solids include gels, plastics, various polymers, wax, and thin films.

Properties of Amorphous solid

1. Amorphous solids soften gradually over a temperature range and can be shaped into various shapes when heated.
2. Amorphous solids are pseudo-solids or supercooled liquids, which means they move very slowly. If you look at the glass panes that are fixed to the windows of old buildings, you will notice that they are slightly thicker at the bottom than at the top.
3. Amorphous solids have an irregular shape, indicating that the constituent particles do not have a definite geometry of arrangement.
4. When amorphous solids are cut with a sharp edge tool, irregular surfaces are formed.
5. Because of the irregular arrangement of the particles, amorphous solids do not have definite heat of fusion.
6. Because of the irregular arrangement of particles, amorphous solids are isotropic in nature, which means that the value of any physical property would be the same along any direction.

Difference Between Crystalline Solids and Amorphous Solids

To help you see how crystalline and amorphous solids are not the same, here are some big differences between crystalline and amorphous:

Difference between Crystalline and Amorphous
Crystalline Solids	Amorphous Solids
A crystalline solid has well-arranged constituent particles.	Constituent particles of amorphous solids are not well arranged.
Crystalline solids are true solids.	Amorphous solids are pseudo-solids.
Crystalline solids are anisotropic.	Amorphous solids are isotropic.
Crystalline solids have a sharp melting point and begin to melt at a specific temperature.	Amorphous solids soften gradually over a temperature range and can be shaped into various shapes when heated.
The shapes are well defined and also particle arrangements of crystalline solids are well-defined.	The shape of amorphous solids is irregular and also particle arrangement is not well defined.
Crystalline solids when cut with the edge of a sharp tool, they split into two pieces and the newly formed surfaces are smooth and plain.	When amorphous solids are cut with a sharp edge tool, irregular surfaces are formed.

This are some significant distinctions noted differences between amorphous and crystalline solids.

Crystalline and Amorphous Solids – FAQs

What causes solids to be rigid?

In solids all the constituent particles are strongly connected also the bonds between the atoms are very strong that’s why solids are rigid.

Why do solids have a specific volume?

Because of the rigidity of their structure, solids retain their volume. Interparticle forces are extremely strong. Furthermore, interparticle spaces are scarce and small. As a result, applying pressure to them will not change their volumes.

Ionic solids conduct electricity when molten but not when solid. Explain.

An ionic solids conduct electricity when molten because electrons are free and they can move from one point to another but in solid-state all the constituent particles are strongly connected so that electrons are not able to move to conduct electricity.

What are electrical conductors, malleable and ductile solids?

Metallic solids are electrical conductors, malleable and ductile. In metallic solids, there is a metallic bond. 

Why is glass classified as a supercooled liquid?

Glass is considered a supercooled liquid because it exhibits some of the properties of liquids despite being an amorphous solid. It is, for example, slightly thicker at the bottom. This is only possible if it has flown like liquid, albeit very slowly.