Crystalline solids are classified into four main types
1) Molecular solids
2) Ionic solids
3) Metallic solids
4) Covalent solids
1) 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.
Ionic solids :
i) In these crystalline solids constituent particles are cation or anion.
ii) All salts are crystalline ionic solids.
iii) In ionic solids ions are held together by strong elestrostatic force of attraction.
iv) The arrangment of ions depends on
a) Size of mtion and anion
b) The charge on the ions
c) The polarisability of anions
v) lonic solids are hard and brittle and have high mp. and b.p.
vi) in solid state they are non conductor but in molten state or in aqueous solution they conduct electricity due to free ions.
vii) They undergo distorion and fracture in crystal structure on applying shearing force.
i) In these solids, the constituent particles are positively charged metal ions.
ii) The bonding electrons are delocalised in a metal.
iii) In metallic solids metal cation and free ele3) Metallic solids :
ctrons are held together by electrostatic force of attraction called metallic bond.
iv) Metallic bonds are stronger than ionic and covalent bonds. Metallic solids have high m.p. and b.p.
v) In metallic solid several layers are . arranged one over the other.
vi) By applying shearing force to metals, the layers can slide and hence show the properties like malleability and ductility.
4) Covalent solids :
i) These are crystalline solids in which the constituent particles are non-metal atoms linked to neighbouring atoms by covalent bonds through out the crystal.
ii) Covalent solids are called giant solids.
iii) They are hard or brittle depending upon type of covalent bonding.
iv) They have high metling points.
v) They are good conductors of electricity or insulators depending upon the availability of free electrons. e.g. diamond, graphite, carborundum, fullerence.
Diamond :
i) It is covalent crystalline solid
ii) It has giant network structure containing very large number of carbon atoms bonded to each other by covalent bond.
iii) Each carbon tom sp3-hybridised and bonding continues in all directions of four corners of regular tetrahedron .
iv) It is hardest substance with m. p. 3550°C .
Graphite :
i) Graphite is a covalent crystalline solid, which is an allotrope of carbon.
ii) all the carbon atoms in graphite are sp2-hybridised.
iii) Each carbon atom is covalently bonded to other three sp2-hybridised carbon atoms forming beta - bonds and fourth electron in 2Pz orbital of each carbon atom is used in the formation of piy-bond.
iv) In graphite layers consisting of hexagonal carbon network and held together by weak vander Waal’s forces.
v) Distance between layers is 340 pm while C-C bond length is 141 .5 pm
vi) n-bond electrons in the ring are delocalised and responsible for electrical conductivity.
vi) As layers of carbon atoms in graphite can slide over another layer, hence it is used as a lubricant
Fullerence :
i) Fullerence is a new allotrope of carbon discoyered in 1985.
ii) Fullerences are carbon cluster componds of formula C32. C 50‘ C 60, C70 and C84
iii) Fullerences are obtained by irradiating carbon to high power lasers.
iv) C60 fullerence is called buckminster fullerence it looks like soccer ball.
v) On sphere 60 sp2-hybridised carbon atoms are placed at equidistant and joined by covalent bonds.
vi) Fulletence consists of fused system of live and six membered ring.
vii) Fullerence react with potassium to form compound Ksscoo which act as a super conductor at l8K.
viii) Tubes made from fullerence and graphite are called nanombes. They are used as electric conductor, molecular sensors and semiconductors.
xi) Fullerences are observed in carbon soot.
No comments:
Post a Comment