Crystalline and Amorphous solids Chemistry Notes
These are explained as follows:
→ Crystalline Solids “Those solid substances, in which constituent particles (atoms, molecules or ions) finite geometic pattern in three dimensional network in entire crystal, are called crystalline solids. For example sodium chloride (salt), sucrose, diamond and quartz etc. Generally, crystalline solids are made up of more number of small crystals.
→ In these crystals, constituent particles are properly arranged Here, the pattern of arrangement of particles is regular which repeats at a certain interval in entire crystal. Hence, the arrangement of particles is fixed and periodic repeation pattern upto a large distance. These are true solids. The structure of crystalline quartz is represented in figure 1.5
→ Amorphous solids “Those solid substances, in which constituent particles (atoms, molecules or ions) are not arranged in definite geometric pattern in entire crystal, are called amorphous solids.” These solids are made up of particles having disimilar shape. In these solids, the arrangement of constituent particles is in short range order only. Here arrangement and periodic repeating pattern are seen in short distances only. The structure of such type of solid is similar to that of liquids. For example glass, rubber, plastic etc.
The two dimensional structure of amorphous quartz glass is represented in Fig. 1.6
→ These are following main uses of amorphous solids:
- Amorphous solid rubber is used in the manu facture of tyre-tube, shoe soles etc.
- Plastic is used in the manufacture of domestic items, toys, buckets etc
- Inorganic glass is used in the manufacture of laboratory and domestic apparatus.
- Amorphous silica is used in the preparation of photo voltaic cell because it converts sun light into electricity.
→ Note : Amorphous solids soften at a certain range of temperature. These can be moulded by melting. They are being made crystalline on heating at a certain temperature. Due to this crystallisation, milkiness appeared in the visibility of some objects of glass of ancient edge. These have flowing nature like that of liquids but very less in comparison to them.
→ So, these are called pseudo solids or super-cooled liquids. Due to this property, the lower ends of windows and doors of old buildings are reletively thicker. Its reason is that glass flows towards lower end with very slow motion and make the lower end thick.
Anistorpic and Isotropic nature of Solids Or Anistropy and Isotropy :
Crystalline solids are anisotropic in nature. So, their physical properties like refractive index, electrical or thermal conductivity and electrical resistance etc, represent different values on measuring them in different directions, because the arrangement of particles is different in different directions due to which physical properties are different. It is represented in Fig.
The amorphous solids are isotropic in nature because these do not have long range order and have a symmetric configuration in all directions. So the value of physical property is similar in all directions.
Table 1.2 : Difference between Crystalline and Amorphous Solids
Some Important Points :
→ Isomorphism: The crystalline form of two different substances can be same. Those different substances whose crystalline form or structure is same are called isomorphous and this phenomenon of existing different compounds in same crystalline form is called isomorphism. For example ferrous sulphate (FeSO4. 7H2O) and zinc sulphate (ZnSO4.7H2O) are isomorphous.
→ Similarly, cuprous sulphide (Cu2S) and silver sulphide (Ag2S) are isomorphous. Mitscherlich proposed law of isomorphism on these isomorphous compounds. According to this law, “If the chemical properties of isomorphous substances are similar, their chemical formulas are also similar”.
→ Other Examples: Alum is isomorphous. Its general formula is M2SO4R2 (SO4)3 24H2O. [M = Monovalent basic radical like NH4+, K+ Na+ etc, R = Trivalent basic radical like Al3+, Fe3+. Cr3+ etc)
Potash Alum – K2SO4 . Al2 (SO4)3 24H2O
Chrome Alum – K2SO4.Cr2 (SO4). 24H2O
→ Polymorphism: A substance can have two or more than two crystalline forms. So, two or more than two crystalline forms of a substance are called polymorphic forms or polymorphs. The property by which a substance exists in more than one crystalline forms, is called polymorphism. Example (A) Calcium carbonate (CaCO3) has three polymorphic forms
- Calcite (trigonal)
- Aragonite (orthorhombic)
- Vaterite (hexagonal)
→ Zinc sulphide (ZnS) has two polymorphic forms
- Zinc blende (sphalerite)
→ Allotropy : When an element exists in two or more physical forms having different properties, and different structures it is called allotropes. Allotropes have different crystalline structure as well as different physical and chemical properties.
→ For example Carbon has three allotropes :