Crystalline and Amorphous Solids: Definition, Diagram, Examples

Solids, the first of the states of matter, have a fixed shape and volume. Crystalline and amorphous solids are the two kinds of solids. The kind where atoms or molecules are arranged, for example, table salt, quartz, and diamond, is in a repeating pattern of high order. As a result of the ordered structure, crystalline solids show sharp melting points and well-defined crystals. On the other hand, amorphous solids lack this long-range order. Their atoms or molecules are more randomly arranged in some manner, just like in liquid. When we talk about examples, we can say that solids like glass, plastic, and gels fall into this category. These materials lack sharp melting points; they tend to soften over a range of temperatures. Every day we encounter both of these kinds of solids. The crystalline solids find a great deal of use in technology and jewelry, while the amorphous kinds are found almost everywhere in household items and packaging materials. Understanding how these two types of solids vary from each other helps us appreciate them better

General Characteristics of Solid-State

Solid is the state of any matter in which constituents are firmly attached due to strong forces.

• Solids have a definite shape, mass, and volume.
• Solids are almost incompressible, rigid, and have mechanical strength.
• Solids have the close close-packed arrangement of atoms.
• Solids have high density but a very slow diffusion rate.
• Solids can have only vibrational motion as the constituents have fixed positions.
• In solids, constituents have a strong force of attraction as intermolecular distances are short.

Type of Solids
Solids are mainly of the following two types:

Crystalline Solids
In such solids, the constituents are arranged in a definite or orderly manner which repeats itself over long distances.

• They have a definite geometry with flat faces and sharp edges.
• Such solids have sharp melting points and undergo clean cleavage.
• They are considered as true solids.
• These show anisotropy that is, different physical properties in different directions.
• They show clean cleavage.
• They are normally incompressible. For example, diamonds and quartz.

Amorphous Solids
In such solids, the constituents are arranged in an irregular or disorderly manner over the long range.

• Such solids do not have sharp melting points and clean cleavage that is, have an irregular cut.
• These are considered as pseudo-solids.
• These show isotropy that is, the same physical properties in all directions.
• They do not show clean cleavage.

Note: Due to short-range order. amorphous solids may even have small parts in crystalline and the rest in non-crystalline form, crystalline parts of an otherwise amorphous substance are called crystallites.

Property

Crystalline solids

Amorphous solids

Shape

Definite characteristic geometrical shape

Irregular shape

Melting point

Melt at a sharp and characteristic

temperature

Gradually soften over a range of temperature

Cleavage property

When cutting with a sharp-edged tool, they split into two pieces and the newly generated surfaces are plain and smooth

When cutting with a sharp-edged tool, they cut into two pieces with irregular surfaces

Heat of fusion

They have a definite and characteristic enthalpy of fusion

They do not have a definite enthalpy of fusion

Anisotropy

Anisotropic in nature

Isotropic in nature

Nature

True solids

Pseudo-solids or supercooled liquids

Order in the arrangement of constituent particles

Long-range order

Only short-range order

Isotropy and Anisotropy

Isotropic :
Amorphous solids are isotropic in nature. Their properties such as mechanical strength, refractive index, and electrical conductivity, etc. are the same in all directions. It is because there is no long-range order in them and the arrangement of particles is not definite along with all the directions. Hence, the overall arrangement becomes equivalent in all directions. Therefore, the value of any physical property would be the same in any direction.

Anisotropic :
Crystalline solids are anisotropic in nature, that is, some of their physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystals. This arises from the different arrangements of particles in different directions. This is illustrated in Fig. 1.2. This figure shows a simple two - dimensional pattern of the arrangement of two kinds of atoms. Mechanical properties such as resistance to shearing stress might be quite different in the two directions indicated in the figure. Deformation in the CD direction displaces a row that has two different types of atoms while in the AB direction rows made of one type of atoms are displaced.

Molecular Solids
Their molecules are held together by dispersion forces, London forces, dipole-dipole forces, or hydrogen bonds. On the basis of the type of interactive forces, these solids are studied under the following sub-headings.

• Non-Polar Molecular Solids: Either atoms (e.g., He, Ne, Ar) or molecules (e.g., H₂, I₂, Cl₂) are bonded together by weak dispersion forces or London forces. These are non-conductor soft solids with m.p. and low enthalpies of vaporization. They are volatile in nature hence, at room temperature and pressure they are available in liquid or gaseous state.
  Examples: Iodine, Solid H_2, and CO₂ (dry ice). Naphthalene, Camphor, etc.
• Polar Molecular Solids: Polar Covalent molecules are held together by strong dipole-dipole forces. These are soft non-conducting solids with low Melting points and Boiling Points, which are still higher than non-polar molecular solids. They have a high enthalpy of vaporization.
  Example: Solid HCI, NH_3, and SO_2, etc.
• Hydrogen-Bonded Molecular Solids: Polar covalent molecules containing the 'H' atom as the positive pole and the N, O or F atom as the negative pole are held together by intermolecular H-bonding. Under room temperature and pressure conditions, they are volatile liquids or soft solids and non-conductors of electricity.
  Example: Ice

Ionic Solids
There is a regular arrangement of positively and negatively charged ions throughout the solid Where ions are held together by strong coulombic or electrostatic forces. These solids are very hard and brittle and have very high melting points. In a solid state, ions are not free to move, hence they are insulators but in a molten state or in an aqueous state, it's ions become free to move and it become a conductor. Ionic solids have high enthalpies of vaporization. They are soluble in polar solvents like H₂O but insoluble in non-polar solvents such as C₆H₆, CS₂, CCl₄ etc.
Examples: LiF, NaCl, KNO₃, Na₂SO_4, etc.

Metallic Solids
Metal cores (ie., kernels) and a sea of mobile electrons are the constituents of metallic solids. Each metal atom contributes one or more electrons toward the sea of electrons. These electrons are evenly spread out throughout the crystals and weak forces of attraction or metallic bonds binds together kernels and a sea of electrons.
Metallic crystals may be hard as well as soft having moderate enthalpies Of fusion. Mobile sea Of electrons is responsible for many properties of metals such as malleability (can be beaten into thin sheets), ductility (can be drawn into wires), metallic luster, thermal conductivity and electrical conductivity etc.
Example: Copper, Iron. Nickel. Metal alloys etc.

Covalent or Network Solids
In these, atoms are bonded together by covalent bond formation throughout the crystal It means there is a continuous network of covalent bonds a giant three-dimensional structure, or a giant molecule. Covalent bonds are strong and directional in nature. These solids are very hard, brittle, and very high melting. Due to the absence of any free electrons or ions, they are insulators. Their enthalpies of fusion are very high.
For example: Diamonds, Graphite, Boron Nitride (BN), Silicon Carbide (SiC), etc. are common examples of these solids.

• Diamond: It has a three-dimensional network of a large number of sp³ hybridized carbon atoms each bonded tetrahedrally to four more carbon atoms by single covalent bonds. It makes diamond extremely hard crystal with very high mp ≃ 3843 K. Diamond does not conduct electricity at all.
• Graphite: Each carbon atom is sp² hybridized and covalently bonded to three other carbon atoms of the same layer by single bonds. forming a layer of hexagonal rings. At each carbon atom, the fourth valence electron is available free, which moves among different layers and provides a good electrical and thermal conducting nature to graphite. Different layers are connected by van der Waals forces. As the forces are quite weak, the layers can slide over each other and make graphite a soft, lubricating solid.

Recommended topic video on (Crystalline and Amorphous Solids )

Some Solved Examples

Example 1
Question: Which of the following is an amorphous solid?
1) Glass
2) NaCl
3) AgCl
4) ZnS

Solution: Glass is an amorphous solid because it has an irregular arrangement of constituent particles. Hence, the answer is option (1).

Example 2
Question: Which of the following are crystalline solids?
1) NaCl, KCl, Diamond
2) NaCl, Rubber, Glass
3) Diamond, Plastic, NaCl
4) KNO3, Glass, Quartz

Solution: Crystalline solids have a regular arrangement of constituent particles. Therefore, NaCl, KCl, and Diamond are crystalline solids. Hence, the answer is option (1).

Example 3
Question: Select the correct statements.
(A) Crystalline solids have long-range order.
(B) Crystalline solids are isotropic.
(C) Amorphous solids are sometimes called pseudo solids.
(D) Amorphous solids soften over a range of temperatures.
(E) Amorphous solids have a definite heat of fusion.
Choose the most appropriate answer from the options given below.
1) (A), (B), (E) only
2) (B), (D) only
3) (C), (D) only
4) (A), (C), (D) only

Solution: Statements (A), (C), and (D) are correct. Hence, the answer is option (4).

Example 4
Question: Some of the physical properties of crystalline solids like refractive index show different values when measuring along different directions in the same crystal. This property is called
1) Isotropy
2) Cleavage property
3) Anisotropy
4) None of these

Solution: Crystalline solids are anisotropic in nature, meaning they have different physical properties in different directions. Hence, the answer is option (3).

Example 5
Question: Which of the following is an amorphous solid?
1) Glass
2) NaCl
3) AgCl
4) ZnS

Solution: Glass is an amorphous solid because its constituent particles are arranged irregularly. Hence, the answer is option (1).

Summary

Structure is the primary difference between the two kinds of solids: crystalline and amorphous solids. Crystalline solids are defined as possessing a highly ordered arrangement of their atoms or molecules, whereby sharp melting points are exhibited and clear crystal structures are formed. For instance, table salt, diamond, or quartz are all members of this group. Because it has the capability to form clear and well-defined crystals, this kind of solid is found to be essential in various uses and applications, may it be in technology, construction, or jewelry. On the other hand, amorphous solids are arranged in a very disorderly manner, as seen in liquids, and include substances such as glass, plastic, and gels. Concerning amorphous solids, there are no sharp melting points; they soften over some temperature range. The materials are highly applied in mundane objects, such as window panes, containers, and many household articles, since they can be easily prepared and engineered to have desired properties. In some cases, the distinction between crystalline and amorphous structures is related to devising either strong, desirable materials or materials that are flexible and conductive. Being able to identify this difference allows us insight into and appropriate use of many of the materials central to both our lives and the furthering of technologies.