"CHEMICAL BONDING INTRODUCTIONA molecule is formed if it is more stable and has lower energy than the individual atoms. Normallyonly electrons in the outermost shell of an atom are involved in bond formation and in thisprocess each atom attains a stable electronic configuration of inert gas. Atoms may attain stableelectronic configuration in three different ways by loosing or gaining electrons by sharingelectrons. The attractive forces which hold various constituents (atoms, ions etc) together indifferent chemical species are called chemical bonds. Elements may be divided into threeclasses.? Electropositive elements, whose atoms give up one or more electrons easily, they havelow ionization potentials.? Electronegative elements, which can gain electrons. They have higher value ofelectronegativity.? Elements which have little tendency to loose or gain electrons.Three different types of bond may be formed depending on electropositive or electronegativecharacter of atoms involved.Electropositive element + Electronegative element = Ionic bond (electrovalent bond)Electronegative element + Electronegative element = Covalent bondor less electro positive + Electronegative element = Covalent bond Electropositive + Electropositive element = Metallic bond.ELECTROVALENCYThis type of valency involves transfer of electrons from one atom to another, whereby each atommay attain octet in their outermost shell. The resulting ions that are formed by gain or loss ofelectrons are held together by electrostatic force of attraction due to opposite nature of theircharges. The reaction between potassium and chlorine to form potassium chloride is an exampleof this type of valency.x x x x x x ?? K x K or Cl Cl K Cl x x x x x x x Here potassium has one electron excess of it‘s octet and chlorine has one deficit of octet. Sopotassium donates it‘s electron to chlorine forming an ionic bond. O x Ca x ++ 2–Ca O (Ionic bond) Here the oxygen accepts two electrons from calcium atom. It may be noted that ionic bond is nota true bond as there is no proper overlap of orbitals.Criteria for Ionic Bond: One of the species must have electrons in excess of octet while the other should be deficit ofoctet. Does this mean that all substance having surplus electron and species having deficientelectron would form ionic bond? The answer is obviously no. Now you should ask why? Thereasoning is that in an ionic bond one of the species is cation and the other is anion. To form acation from a neutral atom energy must be supplied to remove the electron and that energy iscalled ionization energy. Now it is obvious that lower the ionization energy of the element the111 easier it is to remove the electron. To form the anion, an electron adds up to a neutral atom andin this process energy is released. This process is called electron affinity.So for an ionic bond one of the species must have low ionization energy and the other shouldhave high electron affinity. Low ionization energy is mainly exhibited by the alkali and alkalineearth metals and high electron affinity by the halogen and chalcogens. Therefore this group ofelements are predominant in the field of ionic bonding.Energy Change During the Formation of Ionic Bond The formation of ionic bond can be consider to proceed in three steps(a) Formation of gaseous cations +-A(g)+I.E.??? A (g)+eThe energy required for this step is called ionization energy (I.E)(b) Formation of gaseous anions --X(g)+e ??? X (g)+E.AThe energy released from this step is called electron affinity (E.A.)(c) Packing of ions of opposite charges to form ionic solid +-A (g)+ X (g) ??? AX(s)+energy The energy released in this step is called lattice energy.Now for stable ionic bonding the total energy released should be more than the energy required.From the above discussion we can develop the factors which favour formation of ionic bond andalso determine its strength. These factors have been discussed below :(a) Ionization energy: In the formation of ionic bond a metal atom loses electron to form cation.This process required energy equal to the ionization energy. Lesser the value of ionizationenergy, greater is the tendency of the atom to form cation. For example, alkali metals formcations quite easily because of the low values of ionization energies.(b) Electron affinity: Electron affinity is the energy released when gaseous atom acceptselectron to form a negative ion. Thus, the value of electron affinity gives the tendency of anatom to form anion. Now greater the value of electron affinity more is the tendency of an atomto form anion. For example, halogens having highest electron affinities within their respectiveperiods to form ionic compounds with metals very easily.(c) Lattice energy: Once the gaseous ions are formed, the ions of opposite charges come closetogether and pack up three dimensionally in a definite geometric pattern to form ionic crystal.Since the packing of ions of opposite charges takes place as a result of attractive forcebetween them, the process is accompanied with the release of energy referred to as latticeenergy. Lattice energy may be defined as the amount of energy released when one mole ofionic solid is formed by the close packing of gaseous ion.In short, the conditions for the stable ionic bonding are: (a) I.E. of cation forming atom should be low: (b) E.A. of anion – forming atom should be high; (c) Lattice energy should be high. Born Haber Cycle Determination of lattice energy The direct calculation of lattice enthalpy is quite difficult because the required data is often notavailable. Therefore lattice enthalpy is determined indirectly by the use of the Born – Haber cycle. 2 The cycle uses ionization enthalpies, electron gain enthalpies and other data for the calculation oflattice enthalpies. The procedure is based on the Hess‘s law, which states that the enthalpy of areaction is the same, whether it takes place in a single step or in more than one step. In order tounderstand it let us consider the energy changes during the formation of sodium chloride frommetallic sodium and chlorine gas. The net energy change during the process is represented by?H . f 1 Na(s) ? Cl (g) ? ? ? NaCl(s) ;Energychange ? ( ?H ) 2f 2 1 S D 2 S ? Sublimation of sodium D ? Dissociation energyof Cl 2 Na(g) Cl(g) U IE ? IE of sodium(IE) EA EA ? EA of chlorine(EA) ? IE ?e U ? Lattice energy of NaCl ? ? Na (g) Cl (g) Example1.Calculate the lattice enthalpy of MgBr . Given that 2 -1 mol Enthalpy of formation of MgBr = -524 kJ 2 1 ? Some of first & second ionization enthalpy (IE + IE ) = 148 kJ mol 1 2-1 mol Sublimation energy of Mg = +2187 kJ-1 mol Vaporization energy of Br (I) = +31kJ 2 -1 mol Dissociation energy of Br (g) = +193kJ2 -1 mol Electron gain enthalpy of Br(g) = -331 kJ ? Solution:?H ? S ?I.E ? ?H ?D ? 2 ?E.A. ?Uf vap ? U ? ?H ? S ? I.E ? ?H ? D ? 2 ?E.A. or ? ? f vap or U = ?524? [2187 ? 148 + 31 + 193 + 2? ( ?331)]?1 ?? 524 1897 = = ?2421kJ molCharacteristics of ionic compounds: The following are some of the general properties shown by these compounds (i) Crystalline nature: These compounds are usually crystalline in nature with constituent units asions. Force of attraction between the ions is non-directional and extends in all directions. Eachion is surrounded by a number of oppositely charged ions and this number is called co-ordinationnumber. Hence they form three dimensional solid aggregates. Since electrostatic forces ofattraction act in all directions, therefore, the ionic compounds do not posses directionalcharacteristic and hence do not show stereoisomerism.(ii) Due to strong electrostatic attraction between these ions, the ionic compounds have high meltingand boiling points.(iii) In solid state the ions are strongly attracted and hence are not free to move. Therefore, in solidstate, ionic compounds do not conduct electricity. However, in fused state or in aqueous solution,the ions are free to move and hence conduct electricity.(iv) Solubility: Ionic compounds are fairly soluble in polar solvents and insoluble in non-polarsolvents. This is because the polar solvents have high values of dielectric constant which definedas the capacity of the solvent to weaken the force of attraction between the electrical charges 3 immersed in that solvent. This is why water, having high value of dielectric constant, is one of thebest solvents. The solubility in polar solvents like water can also be explained by the dipole nature of waterwhere the oxygen of water is the negative and hydrogen being positive, water molecules pull theions of the ionic compound from the crystal lattice. These ions are then surrounded by waterdipoles with the oppositely charged ends directed towards them. These solvated ions lead anindependent existence and are thus dissolved in water. The electrovalent compound dissolves inthe solvent if the value of the salvation energy is higher than the lattice energy of thatcompounds.??AB ?Lattice energy ? A ?BThese ions are surrounded by solvent molecules. This process is exothermic and is calledsolvation.? ? ??A ? x solv. ? A solv. ? energy? ? ? ? ?? x ? ? ?? B ? y solv. ? B solv. ? energy? ? ? ?y ??The value of solvation energy depend on the relative size of the ions. Smaller the ions more isthe solvation. The non-polar solvents do not solvate ions and thus do not release energy due towhich they do not dissolve ionic compounds.(v) Ionic reactions: Ionic compound furnish ions in solutions. Chemical reactions are due to thepresence of these ions. For example?? 2Na SO ? ? ?2Na ? SO2 4 4 2??BaCl ? ? ?Ba ? 2Cl2 COVALENCY This type of valency involves sharing of electrons between the concerned atoms to attain theoctet configuration with the sharing pair being contributed by both species equally. The atoms arethen held by this common pair of electrons acting as a bond, known as covalent bond. If twoatoms share more than one pair then multiple bonds are formed. Some examples of covalentbonds are ??x?? x??x xxCl xCl – ClCl N? N? x x N ? Nx x????Sigma and Pi Bonding: When two hydrogen atoms form a bond, their atomic orbitals overlap toproduce a greater density of electron cloud along the line connecting the two nuclei. In thesimplified representations of the formation of H O and NH molecules, the O—H and N—H bonds2 3 are also formed in a similar manner, the bonding electron cloud having its maximum density onthe lines connecting the two nuclei. Such bonds are called sigma bonds ( ?-bond).A covalent bond established between two atoms having the maximum density of the electroncloud along the line connecting the centre of the bonded atoms is called a ?-bond.A ?-bond isthus said to possess a cylindrical symmetry along the internuclear axis.Let us now consider the combination of two nitrogen atoms. Of the three singly occupied p- orbitals in each, only one p-orbital from each nitrogen (say, the p may undergo ?head –on?x overlap to form a ?-bond. The other two p-orbitals on each can no longer enter into a directoverlap. But each p-orbital may undergo lateral overlap with the corresponding p-orbital on the 4 neighbour atom. Thus we have two additional overlaps, one by the two p orbitals, and the othery by the two p orbitals. These overlaps are different from the type of overlap in a ?-bond. For eachz set of p-orbitals, the overlap results in accumulation of charge cloud on two sides of theinternuclear axis. The bonding electron cloud does no more posses an axial symmetry as with the?-bond; instead, it possess a plane of symmetry. For the overlap of the p atomic orbital, the xyz plane provides this plane of symmetry; for the overlap of the p atomic orbitals, the zx planey serves the purpose. Bonds arising out of such orientation of the bonding electron cloud aredesignated as ?-bonds. The bond formed by lateral overlap of two atomic orbitals havingmaximum overlapping on both sides of the line connecting the centres of the atoms is called a ?- bond. A ?-bond possess a plane of symmetry, often referred to as the nodal plane.?-Bond(a) s-s overlappings s (b) s-p+overlappings p (c) p-p+overlappingp p ? - Bond: This type of bond is formed by the sidewise or lateral overlapping of two half filledatomic orbitals.p p CO-ORDINATE COVALENCY A covalent bond results from the sharing of pair of electrons between two atoms where eachatom contributes one electron to the bond. It is also possible to have an electron pair bond whereboth electrons originate from one atom and none from the other. Such bonds are calledcoordinate bond or dative bonds. Since in coordinate bonds two electrons are shared by twoatoms, they differ from normal covalent-bond only in the way they are formed and once formedthey are identical to normal covalent –bond.It is represented as [ ? ?] Atom/ion/molecule donating electron pair is called Donor or Lewis base. Atom / ion / moleculeaccepting electron pair is called Acceptor or Lewis acid [ ? ?] points donor to acceptor + + NH , NH has three (N – H) bond & one lone pair on N – atom. In NH formation this lone pair is4 3 4 + donated to H (having no electron) + + NH + H? ? ?? ? NH3 4 Lewis base Lewis acid 5 "