There are two types of solids i.e. amorphous and crystalline. These solids have structural differences in the arrangement of their atoms. Amorphous solids are pseudo solids, and they have ions, atoms, or molecules that are irregularly organized. However, crystalline solids are true solids, and they have particles that are arranged orderly in a crystal lattice.
Amorphous solids are isotropic. They show the same value of properties in all directions. Those properties include thermal conductivity, refractive index, coefficient of thermal expansion, and electrical conductivity which do not depend on the direction in which they are measured.
Crystalline solids are anisotropic. The value of their physical properties changes with the direction of the measurement. For example, the coefficient of thermal expansion is negative in one direction and positive in another direction in a crystal of silver iodide.
Amorphous vs Crystalline solids
|Amorphous Solids||Crystalline Solids|
|Amorphous solids do not contain repeating units||Crystalline solids have regularly repeating units|
|They have no fixed geometrical shapes||They have perfectly ordered geometrical shapes|
|These solids have no definite melting points||These solids have definite melting points|
|Amorphous solids are isotropic||Crystalline solids are anisotropic|
|Intermolecular and metallic forces are not equally shared between their molecules||Crystalline structures have a good order of intermolecular and metallic forces|
|Amorphous solids are pseudo solids (NOT True Solids)||Crystalline solids are true solids|
|They have no symmetry among their molecules||Crystalline structures are perfectly symmetrical|
|They are less rigid solids||They are highly rigid solids|
|These solids have short range of ordered molecules||These solids have long range of ordered molecules|
|Examples are glass, waxes, rubber, and plastic, etc||Examples are diamond, graphite, metals, table salts, etc|
The word amorphous comes from Greek origin; “a” means ‘not’ and “morphē” means ‘form’. So, the word amorphous means, ‘without form’. The amorphous solid has particles (atoms, ions, and molecules) that are arranged randomly in the so-called crystal lattice. They lack all types of orderly crystal shapes.
The intermolecular forces between the atoms or molecules of the amorphous solids are not equal. The distance between the particles also varies and they do not have a geometrical shape.
Amorphous solids have disordered structures, which is why they closely resemble liquids. For example, glass is considered a highly viscous liquid (sometimes called, supercooled liquid). The liquid nature of glass can be seen in old glass windows when they become thicker at the bottom side of the glass due to downward flow.
Examples of Amorphous solids
- Fused silica
- Some lubricants
- Pitch tar, etc
Crystalline solids are composed of tiny crystals. These tiny crystals are known as unit cells. Unit cells are the building block of crystal lattice which in turn make the crystalline solids.
In a crystal lattice of crystalline solid, particles are arranged in a fine manner. They are organized in three-dimensional structures and their particles are bonded with each other in such a way that intermolecular forces are at their maximum. The forces responsible for the stability of crystalline solids are metallic bonds, molecular bonds, ionic bonds, and van der waals forces.
Crystalline solids have four main types i.e. ionic, molecular, network, and metallic crystalline solids. In ionic solids, ions are responsible for making the crystal. Molecular solids are composed of molecules in their crystal lattice. Network solids have large chains of atoms that are connected with each other through strong bonds and metallic solids have positive metal ions that are attracted by a sea of electrons, known as free electrons.
Examples of Crystalline solids
Ionic crystalline solids:
- Table salt, sodium chloride (NaCl)
- Calcium fluoride (CaF2)
- Silver chloride (AgCl)
- Copper sulfate (CuSO4)
- Magnesium oxide (MgO), etc
Molecular crystalline solids:
- Hydrogen (H2)
- Iodine (I2)
- Dry ice (CO2)
- Silicon tetrachloride (SiCl4)
- Phosphorus (P4), etc
Network covalent solids:
- Silicon dioxide (SiO2)
- Silicon carbide
- Transition elements, etc
Metallic crystalline solids:
- Platinum (Pt)
- Gold (Au)
- Copper (C)
- Tungsten (W)
- Iron (Fe), etc
You already know that ideal things do not exist. Yes, that is true here as well. There is not a single solid in this universe that is entirely crystalline or amorphous. All solids are a mixture of these two forms. It is just a matter of relative percentages why these are called what they are called.
Amorphous solids have crystalline regions where all properties resemble crystalline solids. Similarly, crystalline solids have crystal defects which make them amorphous somewhere.
How are crystalline and amorphous solids similar?
Crystalline and amorphous solids are types of solid. Both have crystal lattices in which particles are arranged at lattice sites. Both have intermolecular forces between their molecules, so, they are similar except for their many differences.
Are amorphous solids elastic or plastic?
Amorphous solids are mostly plastic because the deformation once made cannot be retrieved back. This is due to the disordered arrangement of atoms in amorphous solids.
Why are amorphous solids isotropic in nature?
Amorphous solids are isotropic in nature because the values of their properties are always the same, doesn’t matter from which direction.
Why are crystalline solids called true solids?
Crystalline solids are called true solids because they have atoms that are orderly arranged in the crystal lattices.
How is glass an amorphous solid?
Glass is an amorphous solid because it has particles that are not orderly organized in a crystal lattice. It is a supercooled liquid.
Is ice crystalline or amorphous?
Ice is a crystalline solid. It contains a crystal network, unlike its liquid form, due to which water expands upon freezing.
- Fundamentals of Amorphous Solids: Structure and Properties By Zbigniew H. Stachurski (Australian National University)
- Essential of Physical Chemistry: 2nd edition By B.S Bahl and Arun Bahl and G.D. Tuli