Chemical Bonding and Molecular StructureChemical Bond: It is the force which keeps atoms together in a molecule. The causes of formation of a chemical bond are:Tendency of atoms to complete their octets or duplets by rearrangement of their valence electrons (octed theory).The system acquires minimum energy when atoms are at some equilibrium distance where attractive forces dominate over repulsive forces.Types of Chemical Bonds:Ionic bondCovalent bondMetallic bondCo-ordinate bondIonic Bond:a) Favourable conditions to form Ionic Bond:Formed between electro positive element (group 1,2,13) and electro negative element (group 15,16,17)E.N >=2Lower I.E of one atom and high E.A of second atomHigher lattice energyLarger cations, smaller anionsb) Properties: Crystaline, stronger force of attraction, thermally stable, low volatality, high density, high melting point and boiling point, highly soluble in polar solvent, good conductor of electricity in molten or solution state, nondirectional bond.Covalent Bond:a) Favourable conditions to form Covalent Bond:Formed between two electronegative elements (group 14,15,16,17)E.N < 1.9Small cations, larger anions (Fajan's Rule)High charges on cation and anion (Fajan's Rule)Covalent bond is formed by sharing of electrons by two atoms to complete their octet or dupletb) Properties: Compounds containing covalent bonds under normal condition of pressure and temperature exist as gases or liquids of low boiling point due to weak Vander Wall's forceRelatively low melting point and boiling point, generally non-conductor, and soluble in non-polar solventAs the bond is rigid and directional, compounds with covalent bond show isomerismCovalency of the atom is equal to the number of covalent bonds formed by the atomBond Length: It is the average distance between the nuclei of two bonded atoms. It depends upon:Size of the atom: Bond length increases with increase in the size of atom (HI > HBr > HCl > HF)Multiplicity of bond: Bond length decreases with multiplicity of bond (C-C > C=C )Type of hybridisation: More s-character, shorter is the bond length, greater is the acidity with comparable compoundBond Energy: It is the energy required to break one mole of bonds of a particular type of substance in gaseous state. Bond Angle: It is the internal angle between the orbitals containing electron pairs in the valence shell of the central atom in a molecule:More the lone pairs on the central atom, smaller is the bond angle due to bp repulsionMore electronegative the central atom, more is the bond angleMore electronegative the surrounding atoms, lesser is the bond angleVBT: Proposed by Heitler and London and extended by Pauling and State. The postulates are:Overlapping of atomic orbitals of valence shells of two atoms leads to the formation of a covalent bondHalf filled orbital and opposite spin electrons are used- bond is formed by head on overlapping, - bond is formed by lateral overlappingGreater the overlapping, stronger is the bond- bond is stronger than - bond and - bond is directional while - bond is non-directionalThe directon of the bond is the same as the direction of overlapping of orbitalsThe strength of - bond follows the order s -s < s - p < p - pPaired electrons are shifted to higher energy levels while forming a bondVBT cannot explain paramagnetic behaviour of O2 Hybridization: Number of hybrid orbitals = number of atomic orbitals intermixedHybrid orbitals form -bond on overlappingIt does not take place in and isolated atom. It occurs only during bond formationHybrid orbitals tend to remain far apart therefore repulsion order is lp-lp>lp-bp>bp-bpTotal number of hybrid orbitals of central atom = Number of its - bond pairs + Number of lone pair of electrons around the central atomDifferent types of hybridization are sp, sp2 , sp3 , sp3d ,sp3 d2 ,sp3 d3MOT: The theory was developed by Hund & Mulliken. The basic postulates are:All atomic orbitals are mixed up to form molecular orbitalsTotal M.O = Total A.OTwo A.O. combine to form tow M.Os. One is anti bonding M.O. and the other is bonding M.O.The filling up of electrons must follow Aufbau's principle., Pauli exclusion principle and Hund's rule.Bond order = (Nb - Na )/2 where Nb = Number of electrons in bonding M.O and Nb = Number of electrons in antibonding M.OBond order Bond energy 1/Bond lengthDipole Moment(ยต):Common in covalent compounds when formed between two dissimilar atomsUsed to predict the extent of polarity in a moleculeA vector quantity so follows vector addition or subtraction rule in predicting net dipole moment of a moleculeCan predict the shape of molecules, H2 O - bent, BeF2 - linearMetallic Bond: a) Favourable conditions to form Metallic Bond:Formed between Electro positive ions packed in one of the 3 arrangements (1.CCP(or FCC), 2.HCP, 3 BCC)b) Properties:Negatively charged electrons hold the ion togetherHighly conducting becauls the mobility of these electrons through the latticeHigh m.p and b.pIf soluble, then soluble only in polar solvents of high dielectric constantCo-ordinate Bond: a) Favourable conditions to form Metallic Bond:One of the group or or atom must have a lone pair of electrons whereas the other must have the incomplete octet or duplet.Also known as dative bondb) Properties:Intermediate between ionic and covalent compoundsSparingly soluble in water, not forming ions, largely soluble in non-polar solventsMelting and boiling points are higher than purely covalent compounds but less than purely ionic compoundsStable as covalent compounds, addition compounds are very stableNon conductorBond is rigid and directionalHigh value of dielectric constantOther Bonds:a) H-bonding: When H is attached to N, O, FIt's strength is about one tenth of a covalent bondSmall size and high electronegativity of atoms forms a strong H-bondThey are of twor types; intermolecular (between two molecules) and intramolecular (within a molecule)Used to predict boiling point and density of water, solubility of alcohols and carboxylic acids in water.b) Dipole-dipole interactions:These exist between molecules having permanent dipoles e.g. H-Cl, I-F etcc) Ion-dipole interactions:Between and ion and a polar molecule eg hydration of ions like Na+ , Mg2+ etcSmaller the ion, more the dipole moment, stronger will be these interactionsd) Ion-induced dipole interactions:Between an Ion and a dipole induced by the ion in a nonpolar moleculee) Dipole-induced dipole interactions:Between a dipole and and induced dipole in a non polar moleculef) Dispersion forces:Between two nonpolar substances. The forces are between an instantaneous dipole and and induced dipole eg in O2 , N2 , He, Ne, Ar etcNote: Dipole-dipole, dipole-induced dipole and dispersion forces are called Van der Wall's forces