Solid materials are made up of large collection of atoms where the individual atoms are bonded together. The type of bonding which exists between atoms is different for all material.
An atom with a completely filled outer electron shell has a high degree of stability. The atoms of other elements with incomplete outer shells attempts, by combination with other atoms, to satisfy this condition for stability. The principal types of inter atomic bonds which may be formed are the ionic, covalent, metallic, Van Der Wall’s or molecular bonds.
The solids are also classified as per the bond structure found in them. They are Ionic, Metallic, Covalent and Molecular.
Molecular bond or Van Der Wall’s Bond.Inert gases and molecules like methane, which have no valence electrons available for crystalline binding, obtain a weak attractive force as a result of polarization of electrical charges. Polarization is displacement of the centers of positive and negative charges in an electrically neutral atom or molecule when it is brought close to its neighboring atoms. Its neighbors’ also become polarized. The resulting weak electrical attraction between neighboring atoms or molecules is the molecular or van der wall’s bond. Take water as an example. Research has determined that the hydrogen atoms are bonded to the oxygen atoms at an angle of 104.5°. This angle produces a positive polarity at the hydrogen-rich end of the molecule and a negative polarity at the other end. A result of this charge imbalance is that water molecules are attracted to each other. This is the force that holds the molecules together in a drop of water.
Molecular Solids.Molecular or Van Der Wall’s solids are those solids created by the molecular bond. These solids can have both crystalline and non crystalline structure. Molecules are their structural units.
Properties of Molecular Solid. They have low density as the crystals are formed of elements with low atomic number. These solids are goods insulator as no valence (free) electrons are available in the solids. Molecular solids are soft, have low melting and boiling points and high vapour pressure. These solids are usually transparent to light. Examples of molecular solids are inert gases like helium, neon, argon etc. in solid state.