Class 12 Chemistry Classification Of Crystalline Solid
In this article, we will discuss the ‘Classification Of Crystalline Solid‘, we have already discussed Solid State in the last post.
A crystal is defined as a homogenous, anisotropic substance, having a definite geometrical shape with surfaces that are usually planar and have sharp edges. Several types of binding forces are involved in holding crystals together; hence the physical properties of a crystal depend on the type of bonding.
Depending upon the type of constituent particles and the nature of intermolecular forces between them, the crystalline solids are classified into following categories:
- Ionic solid
- Molecular Solids
- Covalent or Network Solids
- Metallic Solids
Ionic Solids: In the ionic solids, the constituent particles are anions and cations. Each participating ion is surrounded by a typical number of opposite charges. The number of ions that surround a particular ion of the opposite charge is called the coordination number of the ion.
For example, in NaCl crystal, each Na+ ion is surrounded by 6 Cl- ions and each Cl- ion is surrounded by 6 Na+ ions. So, the coordination number of Na+ and Cl- is 6. Similar in CaF2 crystal, the coordination number of Ca+ and F- ions are 8 and 4 respectively. Some other examples are CaCl, ZnS, etc. These ions are held together with the strong electrostatic force of attraction.
Some common characteristics of ionic solids are given below:
- They are hard and brittle.
- The heat of evaporation is very high.
- The melting points and boiling points are very high.
- They are insulators in the solid state as the ions are fixed at one position, but in molten state or solution form, they are good conductors of electricity due to the mobility of ions.
- The vapor pressures of such crystals at ordinary temperatures are very low
They are soluble in polar solvents as well as water, but insoluble in non-polar solvents.
Crystal of NaCl
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Molecular Solids: In these solids the particles that are involved are molecules. Depending upon the nature of the molecules they are further divided into three types.
- Non-Polar Molecular Solids -They possess the particles either atoms like those of noble gases or non-polar molecules like H2, Cl2, I2 etc. The attractive force between them is weak van der Waal force or dispersion force. Due to this weak force, they have low melting and boiling points, are soft in nature and non-conductors of electricity (no ions are present).
- 2. Polar Molecular Solids – These are those solids in which constituent particles are polar molecules like HCl, SO2, etc., the force which held them together is dipole-dipole force of attraction. This force is stronger than the dispersion force of non-polar molecular solids. They are generally gaseous or liquid in nature at room temperature and pressure. These solids are soft, non-conductors of electricity and exist in gaseous or liquid form at room temperature and pressure. Their melting and boiling point is higher than non-polar molecular solids but not so much.
Hydrogen-bonded molecular solids– In these types of solids, the constituent particles are bonded with a hydrogen bond. For example H2O, NH3, HF, etc. So they possess the strong hydrogen bond as an inter-molecular force. Due to this, they have a high melting and boiling point which are higher than the other two types of molecular solids. They exist as a volatile liquid or soft solids at room temperature and pressure.
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Covalent or Network Solids: In this type of solids, the constituent particles are atoms of the same or different elements connected to each other by a covalent bond. So, a network of covalent bonds is formed throughout the crystal and they form a giant molecule.
The most common example is diamond, silicon carbide (SiC) and graphite, etc.
Due to strong covalent bonds, they are hard and brittle and have high melting and boiling points. They are insulators but graphite is an exception in this case because of the free fourth electron of each carbon atom where each carbon atom is linked to three neighboring carbon atoms. So, graphite is a good conductor of electricity. The distance between the adjacent layers is greater than carbon-carbon bond length. These layers are not bonded to each other and can easily slip over each other. This is the reason for the softness and good solid lubricant behavior of graphite.
Some common characteristics of covalent solids are given below:
- They have an open structure, i.e. the directional character of the bonding does not lead to maximum filling of space.
- They are bad conductors of electricity because the bonding orbitals are fully occupied. Graphite is an exception.
- The melting points and boiling points are very high.
- Their refractive indices are very high.
- They absorb light of different frequency in the solid, liquid or gaseous form.
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Metallic Crystals: Metallic crystals consist of metal cations surrounded by a “sea” of mobile valence electrons, these electrons, also referred to as delocalized electrons, do not belong to anyone atom, but are capable of moving through the entire crystal. As a result, metals are good conductors of electricity. The melting points of metallic crystals span a wide range. The attractive force between positively charged ions and a sea of electrons is the cause of metallic bond formation. This is the force holding the metal ions together. Metallic solids have a regular structure with high melting and boiling points. Because of the sea of electrons, they have high thermal and electrical conductivity. The free electrons allow metal atoms to slide over each other and can be easily deformed, so metals are malleable and ductile. They possess bright luster and color due to free electrons. All the metals and alloys are metallic crystals.
Crystal of metallic solids
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