Crystalline Solids Definition
A crystalline solid or a crystal is a solid substance consisting of atoms, molecules, and ions that are arranged in a definite pattern.
- Crystals can be identified based on their geometrical shape with specific and characteristic orientations; however, it is not possible in all crystals as some might be microscopic.
- The smallest unit of a crystal is called a unit cell, and the crystal is composed of identical and repeating cells.
- Crystals exhibit translational periodicity where positions repeat in space in a regular pattern.
- Crystalline solids can be further divided into various groups; ionic solids, molecular solids, covalent solids, and metallic solids.
- The repeating units in the structure of ionic crystals are cations and anions. Ionic crystals are usually brittle and have high melting points due to the electrostatic attraction. These solids might conduct electricity when present in the liquid state.
- Covalent crystals consist of covalent bonds between atoms, as covalent crystals are atomic crystals and cannot be made out of molecules.
- Metallic crystals are also atomic solids consisting of metal atoms held together by metallic bonds. Metallic crystals are malleable and ductile as the metal atoms can roll and slide past each other without breaking.
- Molecular crystals are composed of molecules held together by weaker bonds like Van der Walls forces. These have lower melting points and do not conduct electricity.
- Crystals are composed of atoms and molecules, which are held together by different bonds depending on the components of the crystals.
- The positions of the atoms and molecules in a crystal are termed lattice points, and these are usually present at the edge of the faces of the crystal.
- Crystalline solids are anisotropic in nature as the physical properties of these solids are different in different directions of the crystals.
Amorphous Solids Definition
Amorphous solids are noncrystalline solids where the atoms or molecules are not organized in a definite lattice pattern.
- Amorphous solids do not have translational periodicity, thus atoms and molecules are not arranged in a definite pattern but also are not distributed as randomly as in gases.
- Amorphous solids only have covalent bonds, which are not as strong as other bonds like ionic bonds and metallic bonds.
- Even though these substances exist in the solid-state, these do not have defined crystalline structure like crystalline solids.
- Because they do not have a defined shape or structure, these substances also do not have a fixed melting point like crystalline solids. These substances instead melt over a range of temperatures.
- Some of the noncrystalline solids do not exhibit all the properties of solids and, thus, are also termed pseudo solids.
- Amorphous solids, when cleaved or broken, produce fragments with irregular and curved surfaces. They have poorly defined patterns when observed in X-rays, and the atoms and molecules are not arranged in a regular array.
- Almost all solids can be converted into amorphous solids, but some solids are intrinsically amorphous because their components either do not fit together or contain some impurities that affect the lattice.
- The structure of amorphous solids is defined by the varying distances of the neighboring units and the number of molecules present in the solid.
- Amorphous substances are isotropic as the physical properties of the substances are the same in all directions.
12 Major Differences (Crystalline vs Amorphous Solids)
|Characteristics||Crystalline Solids||Amorphous Solids|
|Definition||A crystalline solid or a crystal is a solid substance consisting of atoms, molecules, and ions that are arranged in a definite pattern.||Amorphous solids are noncrystalline solids where the atoms or molecules are not organized in a definite lattice pattern.|
|Geometry||Crystalline solids have a well-defined geometrical shape as a result of the regular arrangement of unit cells.||Amorphous substances do not have a well-defined geometrical shape.|
|Also called||Crystalline solids are also termed true solids.||Amorphous solids are also termed pseudo solids or supercooled liquid.|
|Translational periodicity||Crystalline solids exhibit translational periodicity.||Amorphous solids do not exhibit translational periodicity.|
|Range of order||Crystalline solids have a long-range order.||Amorphous solids have short-range order.|
|Melting point||Crystalline solids have a fixed melting point.||Amorphous solids melt over a range of temperatures instead of at a certain temperature.|
|Heat of fusion||Crystalline solids have a characteristic heat of fusion.||Amorphous solids do not have definite heat of fusion.|
|Bonding Networks||Crystalline solids are composed of covalent, van der Waal’s, ionic and metallic bonds.||Amorphous solids only have covalent networks.|
|Unit cell/Repeating units||Crystalline solids are composed of repeating units called unit cells.||Amorphous solids do not have repeating units or unit cells.|
|Chemical Nature||Crystalline solids are isotropic.||Amorphous solids are anisotropic.|
|Rigidity||Crystalline solids are more rigid.||Amorphous solids are less rigid.|
|Examples||Examples of crystalline solids include copper sulfate, table salt, sugar, etc.||Examples of amorphous solids include glass, rubber, cellophane, etc.|
Examples of crystalline solids
- Table salt is an ionic compound composed of sodium and chlorine atoms in equal amounts, arranged to form a cubic crystal lattice.
- The lattice of table salt or sodium chloride is arranged in such a way that a single ion is surrounded by six ions with opposite electrical charges.
- The lattice is arranged to form a regular octahedron. The chlorine ions are much larger than sodium ions.
- The chloride ions are arranged to form cubes, while the smaller atoms of sodium are present in between to fill the gaps between the chloride anions.
- Even though table salt is considered to be composed of sodium chloride, there are other impurities present, usually in the form of nutrient iodine.
Examples of amorphous solids
- Glass is a noncrystalline or amorphous solid that is hardened and made rigid without crystallization.
- Like all amorphous solids, glass lacks a long-range periodicity. However, the extensive chemical bonding results in a high degree of short-range order.
- Glass is mostly composed of silica, SiO2, but other oxides like calcium oxide and sodium oxides can also be present.
- The ability of an oxide to form glass depends on the structural relationship between the oxygen atoms and the cations of the oxide.
- In a glass structure, different oxides bind with oxygen atoms to form a tetrahedral network.
- The atoms and molecules in glass are randomly arranged, with most of the space occupied by oxygen ions with few silicon, boron, and phosphorus atoms.
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