CHEMICAL BONDING AND MOLECULAR STRUCTURE

1 100 CHEMISTRYS cientists are constantly discovering new compounds, orderlyarranging the facts about them, trying to explain with theexisting knowledge, organising to modify the earlier views orevolve theories for explaining the newly observed 4 After studying this Unit, you will beable to understand K ssel-Lewisapproach to CHEMICAL BONDING ; explain the octet rule and itslimitations, draw Lewisstructures of simple molecules; explain the formation of differenttypes of bonds; describe the VSEPR theory andpredict the geometry of simplemolecules; explain the valence bondapproach for the formation ofcovalent bonds; predict the directional propertiesof covalent bonds; explain the different types ofhybridisation involving s, p andd orbitals and draw shapes ofsimple covalent molecules; describe the MOLECULAR orbitaltheory of homonuclear diatomicmolecules; explain the concept of BONDING ANDMOLECULAR STRUCTUREM atter is made up of one or different type of normal conditions no other element exists as anindependent atom in nature, except noble gases.

2 However,a group of atoms is found to exist together as one specieshaving characteristic properties. Such a group of atoms iscalled a molecule. Obviously there must be some forcewhich holds these constituent atoms together in themolecules. The attractive force which holds variousconstituents (atoms, ions, etc.) together in differentchemical species is called a CHEMICAL bond. Since theformation of CHEMICAL compounds takes place as a resultof combination of atoms of various elements in differentways, it raises many questions. Why do atoms combine?Why are only certain combinations possible? Why do someatoms combine while certain others do not? Why domolecules possess definite shapes? To answer suchquestions different theories and concepts have been putforward from time to time. These are K ssel-Lewisapproach, Valence Shell Electron Pair Repulsion (VSEPR)Theory, Valence Bond (VB) Theory and MOLECULAR Orbital(MO) Theory.

3 The evolution of various theories of valenceand the interpretation of the nature of CHEMICAL bonds haveclosely been related to the developments in theunderstanding of the STRUCTURE of atom, the electronicconfiguration of elements and the periodic table. Everysystem tends to be more stable and BONDING is nature sway of lowering the energy of the system to attain BONDING AND MOLECULAR SSEL-LEWIS APPROACH TOCHEMICAL BONDINGIn order to explain the formation of chemicalbond in terms of electrons, a number ofattempts were made, but it was only in 1916when K ssel and Lewis succeededindependently in giving a satisfactoryexplanation. They were the first to providesome logical explanation of valence which wasbased on the inertness of noble pictured the atom in terms of apositively charged Kernel (the nucleus plusthe inner electrons) and the outer shell thatcould accommodate a maximum of eightelectrons.

4 He, further assumed that theseeight electrons occupy the corners of a cubewhich surround the Kernel . Thus the singleouter shell electron of sodium would occupyone corner of the cube, while in the case of anoble gas all the eight corners would beoccupied. This octet of electrons, representsa particularly stable electronic postulated that atoms achieve thestable octet when they are linked bychemical bonds. In the case of sodium andchlorine, this can happen by the transfer ofan electron from sodium to chlorine therebygiving the Na+ and Cl ions. In the case ofother molecules like Cl2, H2, F2, etc., the bondis formed by the sharing of a pair of electronsbetween the atoms. In the process each atomattains a stable outer octet of Symbols: In the formation of amolecule, only the outer shell electrons takepart in CHEMICAL combination and they areknown as valence electrons. The inner shellelectrons are well protected and are generallynot involved in the combination Lewis, an American chemist introducedsimple notations to represent valenceelectrons in an atom.

5 These notations arecalled Lewis symbols. For example, the Lewissymbols for the elements of second period areas under:Significance of Lewis Symbols : Thenumber of dots around the symbol representsthe number of valence electrons. This numberof valence electrons helps to calculate thecommon or group valence of the element. Thegroup valence of the elements is generallyeither equal to the number of dots in Lewissymbols or 8 minus the number of dots orvalence ssel, in relation to CHEMICAL BONDING ,drew attention to the following facts: In the periodic table, the highlyelectronegative halogens and the highlyelectropositive alkali metals are separatedby the noble gases; The formation of a negative ion from ahalogen atom and a positive ion from analkali metal atom is associated with thegain and loss of an electron by therespective atoms; The negative and positive ions thusformed attain stable noble gas electronicconfigurations.

6 The noble gases (with theexception of helium which has a dupletof electrons) have a particularly stableouter shell configuration of eight (octet)electrons, ns2np6. The negative and positive ions arestabilized by electrostatic example, the formation of NaCl fromsodium and chlorine, according to the abovescheme, can be explained as:Na Na+ + e [Ne] 3s1 [Ne]Cl + e Cl [Ne] 3s2 3p5 [Ne] 3s2 3p6 or [Ar]Na+ + Cl NaCl or Na+Cl Similarly the formation of CaF2 may beshown as:Ca Ca2+ + 2e [Ar]4s2[Ar]F + e F [He] 2s2 2p5[He] 2s2 2p6 or [Ne]Ca2+ + 2F CaF2 or Ca2+(F )2 The bond formed, as a result of theelectrostatic attraction between thepositive and negative ions was termed as2021-22102 CHEMISTRYthe electrovalent bond. The electrovalenceis thus equal to the number of unitcharge(s) on the ion.

7 Thus, calcium isassigned a positive electrovalence of two,while chlorine a negative electrovalence ssel s postulations provide the basis forthe modern concepts regarding ion-formationby electron transfer and the formation of ioniccrystalline compounds. His views have provedto be of great value in the understanding andsystematisation of the ionic compounds. Atthe same time he did recognise the fact thata large number of compounds did not fit intothese RuleK ssel and Lewis in 1916 developed animportant theory of CHEMICAL combinationbetween atoms known as electronic theoryof CHEMICAL BONDING . According to this,atoms can combine either by transfer ofvalence electrons from one atom to another(gaining or losing) or by sharing of valenceelectrons in order to have an octet in theirvalence shells. This is known as octet BondLangmuir (1919) refined the Lewispostulations by abandoning the idea of thestationary cubical arrangement of the octet,and by introducing the term covalent Lewis-Langmuir theory can beunderstood by considering the formation ofthe chlorine molecule,Cl2.

8 The Cl atom withelectronic configuration, [Ne]3s2 3p5, is oneelectron short of the argon formation of the Cl2 molecule can beunderstood in terms of the sharing of a pairof electrons between the two chlorine atoms,each chlorine atom contributing one electronto the shared pair. In the process bothchlorine atoms attain the outer shell octet ofthe nearest noble gas ( , argon).The dots represent electrons. Suchstructures are referred to as Lewis Lewis dot structures can be writtenfor other molecules also, in which thecombining atoms may be identical ordifferent. The important conditions being that: Each bond is formed as a result of sharingof an electron pair between the atoms. Each combining atom contributes at leastone electron to the shared pair. The combining atoms attain the outer-shell noble gas configurations as a resultof the sharing of electrons. Thus in water and carbon tetrachloridemolecules, formation of covalent bondscan be represented as: or Cl Cl Covalent bond between two Cl atomsThus, when two atoms share oneelectron pair they are said to be joined bya single covalent bond.

9 In many compoundswe have multiple bonds between atoms. Theformation of multiple bonds envisagessharing of more than one electron pairbetween two atoms. If two atoms share twopairs of electrons, the covalent bondbetween them is called a double bond. Forexample, in the carbon dioxide molecule, wehave two double bonds between the carbonand oxygen atoms. Similarly in ethenemolecule the two carbon atoms are joined bya double bonds in CO2 molecule2021-22103 CHEMICAL BONDING AND MOLECULAR STRUCTUREWhen combining atoms share threeelectron pairs as in the case of twonitrogen atoms in the N2 molecule and thetwo carbon atoms in the ethyne molecule,a triple bond is Representation of SimpleMolecules (the Lewis Structures)The Lewis dot structures provide a pictureof BONDING in molecules and ions in termsof the shared pairs of electrons and theoctet rule. While such a picture may notexplain the BONDING and behaviour of amolecule completely, it does help inunderstanding the formation and propertiesof a molecule to a large extent.

10 Writing ofLewis dot structures of molecules is,therefore, very useful. The Lewis dotstructures can be written by adopting thefollowing steps: The total number of electrons required forwriting the structures are obtained byadding the valence electrons of thecombining atoms. For example, in the CH4molecule there are eight valence electronsavailable for BONDING (4 from carbon and4 from the four hydrogen atoms). For anions, each negative charge wouldmean addition of one electron. Forcations, each positive charge would resultin subtraction of one electron from the totalnumber of valence electrons. For example,for the CO32 ion, the two negative chargesindicate that there are two additionalelectrons than those provided by theneutral atoms. For NH4+ ion, one positivecharge indicates the loss of one electronfrom the group of neutral atoms. Knowing the CHEMICAL symbols of thecombining atoms and having knowledgeof the skeletal STRUCTURE of the compound(known or guessed intelligently), it is easyto distribute the total number of electronsas BONDING shared pairs between theatoms in proportion to the total bonds.