Solids have a definite shape, mass and volume.
Solids are almost incompressible, rigid and have mechanical strength.
Solids have close packed arrangement of atoms.
Solids have high density but very slow diffusion rate.
Solids can have only vibrational motion as the constituents have fixed positions.
In solids, constituents have strong force of attraction as intermolecular distances are short.

Type of Solids
Solids are mainly of the following two types:

Crystalline Solids
In such solids, the constituents are arranged in a definite or orderly manner which repeats itself over long distances.

They have a definite geometry with flat faces and sharp edges.
Such solids have sharp melting points and undergo clean cleavage.
They are considered as true solids.
These show anisotropy that is, different physical properties in different directions.
They show clean cleavage.
They are normally incompressible. Example, Diamond, Quartz.
All elements and compounds are of this kind.

Amorphous Solids
In such solids, the constituents are arranged in a regular or orderly manner over the long range.

Such solids do not have sharp melting points and clean cleavage that is, have an irregular cut.
These are considered as pseudo solids.
These show isotropy that is, same physical properties in all directions.
They do not show clean cleavage.

Note: Due to short-range order. amorphous solids may even have small parts in crystalline and the rest in non-crystalline form, Crystalline parts of an otherwise amorphous substance are called crystallites.

Property	Crystalline solids	Amorphous solids
Shape	Definite characteristic geometrical shape	Irregular shape
Melting point	Melt at a sharp and characteristic temperature	Gradually soften over a range of temperature
Cleavage property	When cutting with a sharp-edged tool, they split into two pieces and the newly generated surfaces are plain and smooth	When cutting with a sharp-edged tool, they cut into two pieces with irregular surfaces
Heat of fusion	They have a definite and characteristic enthalpy of fusion	They do not have a definite enthalpy of fusion
Anisotropy	Anisotropic in nature	Isotropic in nature
Nature	True solids	Pseudo solids or super cooled liquids
Order in the arrangement of constituent particles	Long-range order	Only short-range order

Isotropy and Anisotropy

Isotropic :
Amorphous solids are isotropic in nature. Their properties such as mechanical strength, refractive index and electrical conductivity, etc.are the same in all directions. It is because there is no long-range order in them and arrangement of particles is not definite along with all the directions. Hence, the overall arrangement becomes equivalent in all directions. Therefore, the value of any physical property would be the same in any direction.

Anisotropic :
Crystalline solids are anisotropic in nature, that is, some of their physical properties like electrical resistance or refractive index show different values when measured along with different directions in the same crystals. This arises from the different arrangement of particles in different directions. This is illustrated in Fig. 1.2. This figure shows a simple two - dimensional pattern of arrangement of two kinds of atoms. Mechanical properties such as resistance to shearing stress might be quite different in two directions indicated in the figure. Deformation in CD direction displaces row which has two different types of atoms while in AB direction rows made of one type of atoms are displaced.

Classification of Crystalline Solids

Molecular Solids
Their molecules are held together by dispersion forces, London forces, dipole-dipole forces or hydrogen bonds. On the basis of the type of interactive forces these solids are studied under the following sub-headings.

Non-Polar Molecular Solids: Either atoms (e.g., He, Ne, Ar) or molecules (e.g., H₂ , I₂ , Cl₂) are bonded together by weak dispersion forces or London forces. These are non-conductor soft solids with m.p. and low enthalpies of vaporisation. They are volatile in nature hence, at room temperature and pressure they are available in liquid or gaseous state.
Example: Iodine, Solid H₂ and CO₂ (dry ice). Naphthalene, Camphor etc.
Polar Molecular Solids: Polar Covalent molecules are held together by strong dipole-dipole forces. These are soft non-conducting solids with low Melting point and Boiling Points, which are still higher than non-polar molecular solids. They have high enthalpy of vaporisation.
Example: Solid HCI, NH₃ and SO₂ etc.
Hydrogen-Bonded Molecular Solids: Polar covalent molecules containing 'H' atom as positive pole and N, O or F atom as negative pole are held together by intermolecular H-bonding. Under the room temperature and pressure conditions, they are volatile liquids or soft solids and non-conductors of electricity.
Example: Ice

Ionic Solids
There is a regular arrangement of positively and negatively charged ions throughout the solid Where ions are held together by strong coulombic or electrostatic forces. These solids are very hard and brittle and have very high melting points. In solid state, as ions are not free to move, hence they are insulators but in molten state or in aqueous state, it's ions become free to move and it becomes a conductor. Ionic solids have high enthalpies of vaporisation. They are soluble in polar solvents like H₂O but insoluble in non-polar solvents such as C₆H₆, CS₂, CCl₄ etc.
Examples : LiF, NaCl, KNO₃, Na₂SO₄ etc.

Metallic Solids
Metal cores (ie., kernels) and a sea of mobile electrons are the constituents of metallic solids. Each metal atom contributes one or more electrons towards the sea of electrons. These electrons are evenly spread out throughout the crystals and weak forces of attraction or metallic bond binds together kernels and sea of electrons.
Metallic crystals may be hard as well as soft having moderate enthalpies Of fusion. Mobile sea Of electrons is responsible for many properties of metals such as malleability (can be beaten into thin sheets), ductility (can be drawn into wires), metallic lustre, thermal conductivity and electrical conductivity etc.
Example: Copper, Iron. Nickel. Metal alloys etc.

Covalent or Network Solids
In these, atoms are bonded together by covalent bond formation throughout the crystal It means there is a continuous network of covalent bonds a giant three-dimensional structure or giant molecule. Covalent bonds are strong and directional in nature. These solids are very hard. brittle and very high melting. Due to the absence of any free electrons or ions, they are insulators. Their enthalpies of fusion are very high.
Example: Diamond, Graphite, Boron Nitride (BN). Silicon Carbide (SiC) etc. are common examples of these solids.

Diamond: It has a three-dimensional network of a large number of sp³ hybridised carbon atoms each bonded tetrahedrally to four more carbon atoms by single covalent bonds. It makes diamond extremely hard crystal with very high mp ≃ 3843 K. Diamond does not conduct electricity at all.
Graphite: Each carbon atom is sp² hybridised and covalently bonded to three other carbon atoms of the same layer by single bonds. forming a layer of hexagonal rings. At each carbon atom, the fourth valence electron is available free, which moves among different layers and provides good electrical and thermal conducting nature to graphite. Different layers connect by van der Waals forces. As the forces are quite weak, the layers can slide over each other and make graphite a soft, lubricating solid.

Different Types of Solid

Type of Solid	Constituent Particles	Bonding/ Attractive Forces	Examples	Physical Nature	Electrical Conductivity	Melting Point (in K)	Bond Energy in KJ/mol
(1)Molecular solids (i) Non polar	Molecules	Dispersion or London forces	Ar, CCl₄, H₂, I₂, CO₂	Soft	Insulator	Very low < 273	Low<40
(ii) Polar		Dipole-dipole interactions	HCl, SO₂	Soft	Insulators	Low < 273	Low < 40
(iii) Hydrogen Bonded		Hydrogen Bonding	H₂O(ice)	Hard	Insulators	Low < 273	Low < 40
(2) Ionic solids	Ions	Coulombic or electrostatic	NaCl, MgO, ZnS, CaF₂	Hard But Brittle	Insulators in solid state but conductors in molten state and aqueous solutions	High > 1300K	400 - 4000
(3) Metallic solids	Positive ions in a sea of delocalised electrons	Metallic Bonding	Fe, Cu, Ag, Mg	Hard but malleable and ductile	Conductors in solid state as well as in molten state	Fairly High 800 - 1000	80 - 1000
(4) Covalent or network solids	Atoms	Covalent Bonding	SiO₂(quartz), SiC, C(diamond), C(graphite)	Hard Soft	Insulators Conductor (exception)	Very High > 3900	150 - 500