MOLECULAR SOLIDS

                       Molecular solids 

The crystalline solids in which constituent particles are molecules of the same compound are called molecular solids.

Molecular solids are again classified into three types depending on types of molecules and the nature of intermolecular forces of attraction between the neighbouring molecules.

a) Polar molecular solids
b) Non-polar molecular solids.
c) Hydrogen bonded molecular solids,

a) Polar molecular solids :

i) In these crystalline solidsfhie constituent particles are polar molecules like HCL SOZ, NH3 etc. ii) Polar molecules have dipoles and posseses dipole moment.

iii) These molecfes in solid state are held together by strong dipole-dipole interaction.

iv) They arrange in such a way that opposite end of the neighbouring molecules are brought closer. v) These solids are soft, bad conductor of electricity.

vi) Meltingpoints of these solids are relatively low

b) Non-polar molecular solids :

i) In these solids, the constituent particles are non-polar molecules like H2, C12, CH‘, (:02 or wwkly polar molecules like C0.

ii) In these solids the atoms of non-polar molecules are held by weak dispersion forces or London fom, iii) These solids are soft and do not conduct electricity.

iv) They have very low m.p. which are lower than polar molecular solids

c) Hydrogen bonded molecular solids: 

i) In these crystalline solids. the constituent particles are the molecul to highly electronegative atom like F, O and N.

ii) In these. molecules are held by hydrogen bonds in which H atom of one molec electronegative atom like 0, N and F of another molecule.

iii) As hydrogen bonding ts weaker these solids have very low m p and generally at room temperature they exist in liquid or gaseous state.

iv) Hydrogen bonded solids are non-conductors of electricity.
                                                                                                                                                                   


Hydrogen bonding in Ice and Water:

i) Ice has hemgonal three dimensinal cry stal structure formed due to hydrogen bonding.

ii) In  H20 two hydrogen atoms are covalently bonded to oxygen H atom.

iii) The structure of ice and water are almost identical. iv) H20 molecule is polar molecule.

v) Number of H20 molecules are linked to each other by dipole-dipole interation called hydrogen bonding.

vi) in ice, H2o molecules are linked forming tetrahedral arrangement .

vii) In this arrangement, two covalently bonded H-atoms and two N H-atoms bonded by hydrogen bonds are at four corners of the tetrahedral.

Tetrahedral geometry of water molecules formed by  covalent and hydrogen bond -

                  The volume of single water molecule is 15 x 102‘ cm’, hence total volume of 1 mole of water i.e. 18g of water may be 15 x 102‘ x 6.022 x 1023 = 9 cm}. However volume of 1 mole of water is found to be 18 cm’. Hence in water 9 cm3 space is empty. Due to more vacant space, water can accomodate more solute molecules Hence water is very good solvent.

Ice is lighter than water : 

i) Due to inter molecular‘hydrogen bonding, ice has hexagonal three dimensional crystal structure.

it) Structure of water arid ice are almost same.

iii) on melting of 10;, some hydrogen bonds are broken and vacant space is occupied by water
molecules aria they comes closely.

iv) In ice vacant space is more than tn water. Hence density of ice is less than water.

v) Due to which ice is lighter than water and ice floats on the surface of water.