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bipyramidal pipysane— @, , VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY the simplest th, Valence shell electron pair repulsion theory (in short VSEPR a cone betwrenth a. *Y for, predicting the geometries of molecules. It is based on i seh a Se tdiGand Wes at, na : by Si an al, pairs in the valence shell. This theory was proposed by Sids theory, the shapes of molecules can fe, , developed by Gillespie and Nyholm (1957). According to this, determined a the diber of alecrton pairs in the valence shell of the central atoms. These electron pairs, , experience electrostatic repulsions from one another and the bonded atoms ina ee adopt that, particular arrangement in space around the central atom which keeps them on the average as far, , apart as possible., Postulates of VSEPR Theory, The main postulates of VSEPR theory are :, , 1. The geometry of a molecule depends upon the nu, , or not) around the central atom., In the formation of a bond, the central atom shares its valence electrons with the surrounding, , atoms. However, in certain cases, all the valence shell electrons may not take part in the bond, formation. The electrons left in the valence shell without forming bonds exist as lone pairs. For, example, in methane, CH, carbon uses all the four valence electrons in forming four bond pairs. On, the other hand, in water, H,O, the central oxygen atom has (1s?, 2s? 2p*) six valence electrons. The |, hydrogen atom shares two of these six valence electrons in bond formation leaving four electrons as, , umber of valence shell electron pairs (whether bonded, , en O°, (Four valence electrons) (Six valence electrons), H, HICH :O1H, H H, (Four bond pairs) (Two bond pairs and two, , lone pairs)
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Ce Ue ED, , , , GENICAL BONDING-1 : COVALENT BOND, , airs. Th, pwolone palrs “INMethane, there ar, a, oe pond pairs and two lone” there are fourbond pairs around carbon, while oxygen in water has, , e pairs a ; :, gectron pairs (bonded as wall as i around it. According to VSEPR theory, it is the total number of, S lone pairs) that determines the geometry of molecule., , 2. Electron pairs tend t, 4 Orep op ., sedlectron pairs try to stay fre ee r ‘because their electron clouds are negatively charged. Asa result, ¥ part as possible to acquire a state of minimum energy or maximum stability,, , 3. Repulsion between th, e pa é DONG pi, s e lo i i the bond pai, ; j : Pairand lone pair of electrons is different than that between the bo irs, gr one lone pair and one bond pair. The repulsive interactions decrease in the order :, , Lone pair— ii, p Lone pair > Lone pair—Bond pair > Bond pair—Bond pair, , The presence of lone pairs j i, ‘Ss i A geomiehy obanch oe in addition to bond pairs will result in certain distortions in the, , 4. Repulsive forces decrease sharply with i i, beara ead mane e angle between the electron pairs. They are strong at, , Regular and Irregular Geometries, , The molecules in which the central atom is surrounded only by similar bonded electron pairs will have, regular geometries while those in which the central atom is surrounded by bond pairs as well as lone pairs will, , , , , , | have irregular geometries., , For example, the molecules such as CH, BF,, PF;, SF, etc., have regular geometries because, the central atoms in these molecules are bonded to similar atoms and there are no lone pairs in these, , molecules., , On the other hand, the molecules such as CH,Cl,, CHCI, etc., in which the central atom is, bonded to different atoms and the molecules such as NH, and H,O which have lone pairs as wellas, bond pairs, have irregular geometries. ., , i ‘tron pairs,, , Depending, upon the number of elec! f, given inT able t TeHHay be noted that the presence of lone pairs, , discussed in case of pa malecules containing different electron pairs, , the molecules have different geometries as, disturbs the geometry as already
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172 MODERN APPROACH TO INORGANIC CHEMISTRY (B.Sc. 1, Sem.|, K.U.K/M.D.UIC.D.L Gy, , , , , , , , , , , , No. of electron pairs Geometry Bond angles Example <, 5 90°, 120° PE;, Trigonal bipyramidal, j, 6 90° SF, |, Octahedral |, 7 72°, 90° IF,, , , , Pentagonal bipyramidal
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Pentagonal bipyramidal, , , , , , , , APPLICATIONS OF VSEPR THEORY, VSEPR theory providesa simpleapproach to predict the shapes of molecules. The results obtainail_, on the basis of this theory are quite in agreement with those obtained from hybridisation concept :, This theory is very simple to use. In this theory no distinction is made between s and p-electr,, We take into account only the number of electron pairs present in the valence shell of the central ato,, The basic Point is that the electron pairs (bonded or lone pairs) try to remain as far apart as possible to daa, a state of stability. g, Let us illustrate this theory by some simple molecules and ions., 1. Ammonia (NH,) molecule, Inammonia molecule, thecentral nitrogenatom (Z=7: 1s2s?2p*) has five valenceelectrons., , the nitrogen atom. The remaining two electrons are present asa lone pair. Thus, in ammonia, nitrogen, , issurrounded by fourelectron pairs, threebond pairsand one lone pair. These four electron pairsadopty, , pair repulsion. Asa result, the lone pair of electrons will repel the bond pairs strongly and bond angles, decreases to 107°. The geometry of ammonia molecule is also regarded as pyramidal (Fig. 43)., , , , Fig, 43. Shape of NH, molecule according to VSEPR theory., , _ 2, Water (HO) molecule inl, In the case of water, the central oxygen atom (Z = 8: 1s? 2s? 2p!) has six valence electrons. 'n""
