A solid is defined as that form of matter which possesses rigidity and hence possesses a definite shape and a definite volume.
Free movement of particles in the gaseous state and in the liquid state is the reason for their fluidity which is due to weaker intermolecular forces while the particles in the solid state are held together with strong intermolecular forces and don’t possess translator motion.
They can only vibrate about their fixed position. Therefore, solids are rigid and have definite shape.
General Characteristic of Solids
The general characteristics of solids are as follows. As mentioned above, solids have two main properties.
- They have strong intermolecular forces and short internuclear distance due to close packing of constituent particles.
- Their constituent particles don’t possess translator motion but can oscillate only around their mean position.
Due to these two basic properties, solids possess the following characteristic properties.
The solids are classified into two main types.
- They have a definite shape, mass and volume.
- They are rigid and incompressible.
- They have high density.
Crystalline Solids - All solid metals, nonmetals and compounds come in this category.
Amorphous Solids - Rubber, glass, pitch, polymers of high molecular mass, quartz glass, etc., are the examples of it.
These are the substances whose constituent particles possess a regular orderly arrangement. They are considered as true solids. For example, NaCl (rock salt), Sucrose (sugar), Diamond, Quartz, etc.
The main properties of these solids are mentioned below.
1. Arrangement of constituents particles
In a crystalline solid, the particles (ion, molecule or atoms) are arranged in definite geometric pattern in the three dimensional network. This is known as long range order. This arrangement repeats periodically over the entire crystal. Due to this arrangement, they have a short range as well as a long range order.
2. Melting point
The crystalline solids have a sharp melting point, so possess definite heat of fusion.
In a crystalline solid, the properties like electrical conductance, refractive index, thermal expansion, etc., have different values in different directions.
This type of behavior is called Anisotropy and the substances with this property are called Anisotropic.
4. Clean Cleavage with knife
A crystalline solid gives a clean surface after cleaving it with a knife rather than an irregular breakage.
Classification of crystalline solids
They can be classified into different categories depending upon the type of constituent particles and the nature of intermolecular forces between them. Various categories are:
Covalent or Network Solids
- Metallic 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 a strong electrostatic force of attraction.
Some common characteristics of ionic solids are given below.
- They have high melting point and boiling point due to strong electrostatic force of attraction.
- In the solid state, they behave like an electrical insulator as the ions are not free to move, but in an aqueous solution, they are good conductors of electricity because the ions become free.
- They are generally soluble in a polar solvent but not in a non-polar solvent.
- They are hard and brittle because their stability depends upon the preservation of their geometric pattern.
i) Molecular Solids
In these solids the particles are molecules. Depending upon the nature of the molecules they are further divided into three types.
1. 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). They are generally gaseous or liquid in nature at room temperature and pressure.
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.
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.
3. Hydrogen-bonded molecular solids - In these types of solids, the constituent particles are bonded with hydrogen bond. For example, H2O, NH3, HF, etc.
So they possess the strong hydrogen bond as intermolecular 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 volatile liquid or soft solids at room temperature and pressure.
ii) 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 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 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 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.
iii) Metallic Solids
These solids contain metal atoms as constituent particles. As metals have a good tendency to lose their valence electron and change in to positively charged metal ions (kernel). These electrons can easily move throughout the whole crystal and form the sea of free electrons.
So the attractive force between the kernels and mobile valence electrons is termed as metallic bond.
This is the force which holds the metal ions together.
Because of these strong metallic bonds, metals can maintain a regular structure and usually have high melting and boiling points. They possess high electrical and thermal conductivity because the free electrons flow with a charge or heat energy through the metal.
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 come in this category.
These are the substances whose constituent particles don't possess a regular orderly arrangement.
The main properties of these solids are mentioned below:
1. Arrangement of constituents particles
In an amorphous solid, the particles are arranged in a regular manner up to a small region only. This is called short range order. So, in these solids, the particles are not in regular arrangement and possess only short range order and have irregular shape.
2. Melting point
The amorphous solids melt gradually over a temperature range, so do not possess definite heat of fusion.
In amorphous solids, the various physical properties like electrical conductivity, refractive index, thermal expansion, etc., are same in all directions just like liquids and gases. This type of behavior is called isotropy and the substances with this property are called isotropic. So they are isotropic in nature due to random arrangement of particles.
4. Clean Cleavage with knife
Like a crystalline solid they don't give a clean surface after cleavage with knife.
They undergo an irregular breakage.
5. Super cooled liquid
They have a tendency to flow like liquid, although very slowly, so they are also called super cooled liquids. This tendency is illustrated by the fact that glass panes in the window of old historical buildings are found to be thicker at the bottom than at the top.
6. Pseudo solids
They are not considered true solids. Due to short range order, small parts of amorphous solids may be crystalline and rest may be non-crystalline. This part of amorphous solids is called crystallizes.
So they are also known as pseudo solids. This is the reason for the milkiness of window glass of old buildings because due to heating during the day and cooling at nights, glass acquires some crystalline character.
Amorphous solids are very useful materials. Here are a few applications of it:
- Amorphous solids glass and plastics are very useful materials and are widely used in construction, house ware, laboratory ware, etc.
- Amorphous silica is one of the best materials for converting sunlight into electricity (photovoltaic).
- Amorphous solid rubber is used in making tyres, shoe soles etc.
Comparison between Crystalline Solids and Amorphous Solids
|Crystals have definite and regular geometry and have long range as well as short range order of constituent particles.
|The particles in the constituent are arranged irregularly. They do not have any definite geometry and have short range order.
|Crystals possess high melting points.
|They are devoid of sharp melting points.
|The crystals external forms have regularity when these are formed.
|No external regularity in their form when these amorphous solids are formed.
|They give a clean surface after cleavage with knife.
|Usually, the amorphous solids exhibit irregular cut.
|They have definite heat of fusion.
||Amorphous solids do not possess any particular heat of fusion.
| Crystalline solids are very rigid and their molecules cannot be deformed by mild distorting force.
||Amorphous solid do not exhibit rigidity. Deformation could be done by bending or compressing them.
|Crystalline solids are considered as true solid.
||Amorphous solids are considered super cooled liquids or also pseudo solids.
| Crystalline solids display anisotropism.
||Amorphous solids display isotropism.