Chemistry 20 Lesson 11 Electronegativity, Polarity and Shapes

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1 Chemistry 20 Lessn 11 Electrnegativity, Plarity and Shapes In ur previus wrk we learned why atms frm cvalent bnds and hw t draw the resulting rganizatin f atms. In this lessn we will learn (a) hw the cmbinatin f bnded electrns and lne pairs f electrns result in different mlecular shapes and (b) hw unequal sharing f electrns within bnds alng with the shape f a mlecule result in plar and nnplar mlecules. I. Sterechemistry All mlecules have a definite three-dimensinal shape and the study f the shape f chemical cmpunds is called sterechemistry. T help us predict and understand the shapes f mlecules we use Valence-Shell-Electrn-Pair-Repulsin Thery (VSEPR Thery) which was develped by Rnald Nyhlm and Rn Gillespie in The name implies smething very ugly and cmplicated, but the thery is actually quite simple t understand and use. First f all, since electrns all have the same charge, they repel each ther. The VSEPR Thery prpses that valence electrn pairs, bth shared (bnding) and lne pairs, arrange themselves arund the central atm in a mlecule in such a way as t minimize repulsin between electrn pairs Thus, the bnding and lne pairs f electrns take-up psitins arund the central atm as far away frm ne anther as pssible. (Fr an alternate explanatin f sterechemistry refer t Nelsn Chemistry pages 91 t 103.) VSEPR thery results in five basic shapes that we will deal with: tetrahedral, pyramidal, angular, trignal planar, and linear. (Again, there are mre shapes that ccur in nature, but we will limit ur discussin t the five shapes that arise frm mlecules that frm based n the ctet rule.) Tetrahedral Fr mlecules like CH 4 r SiH 4 r CX 4 r SiX 4 (X represents a halgen) there are fur bnding pairs and zer lne pairs arund a central atm. The repulsin between the fur bnded pairs results is a tetrahedral shape. The shape is three-dimensinal. Therefre, in the shape diagram, dtted lines indicate that the bnd is directed int the plane f the paper and the wedge indicates the bnd is directed ut f the plane f the paper. The slid lines represent bnds that are in the plane f the paper. Dr. Rn Licht

2 Trignal pyramidal Mlecules with three bnding pairs and ne lne pair arund a central atm assume a trignal pyramidal shape arund that central atm. Examples are: NH 3, PH 3, NX 3, PX 3. The pyramidal shape results frm the repulsin f the lne pair n the three bnding pairs. Angular Mlecules with tw bnding pairs and tw lne pairs arund a central atm assume an angular shape arund that central atm. Once again, the repulsin between lne pairs and bnded pairs causes the atms t have an angular shape. Examples include: H 2 O, H 2 S, OX 2, and SX 2 Trignal Planar The shape arund a central atm with three bnding pairs and zer lne pairs is trignal planar. T minimize repulsin, the three electrn pairs arund the central atm are directed t the crners f an equilateral triangle and the shape arund each carbn is described as being trignal planar. Tw imprtant ntes: 1) When deciding n the situatin arund a central atm, duble and triple bnds cunt as ne bnded pair. 2) It is very imprtant t understand that VSEPR thery predicts the shape arund a central atm nly. If there is mre than ne central atm in a mlecule, each central atm can have a different shape arund it. The mlecule mdel picture t the right shws tw central atms bnded tgether by a duble bnd with tw ther atms bnded t each central atm. VSEPR thery tells us that since each central atm has three bnded pairs (a duble bnd cunts as ne bnded pair) and n lne pairs, the shape arund each central atm is trignal planar central atm central atm Dr. Rn Licht

3 Linear The shape arund atms with tw bnding pairs and zer lne pairs is linear. Examples include any carbn atm with a triple bnd in a hydrcarbn. In additin, diatmic mlecules (i.e. mlecules like H 2, O 2, HCl, etc.) are necessarily linear since there are nly tw atms invlved. Frm the descriptins f the shapes abve, we can see that if we knw the situatin f the electrn pairs surrunding a central atm we can predict the shape f the atms arund the central atm. The electrn pairs repel each ther and take up psitins as far frm ne anther as pssible. One may use the fllwing prcedure t predict the shape f a mlecule. 1. Draw the Lewis diagram r the structural diagram. 2. Cunt the number f lne pairs and bnding pairs arund the central atm. Remember that, fr the purpses f sterechemistry, single, duble and triple bnds all cunt as ne bnded pair. 3. Use the fllwing t find the shape: situatin arund central atm shape name diagram # LP # BP 0 4 tetrahedral 1 3 trignal pyramidal 2 2 angular 0 3 trignal planar 0 2 linear r Nw that we understand the shape f a mlecule we can start t understand hw different electrn distributins can lead t an imprtant phenmenn called plarity. But befre we can Dr. Rn Licht

4 discuss this prperty in terms f sterechemistry, we must first discuss hw different electrn attracting abilities between atms results in altered electrn distributins. II. Electrnegativity In lessns 9 and 10 we learned abut the sharing f electrns t frm cvalent bnds. Hwever, experimental evidence indicates that cvalently bnded atms ften exhibit unequal attractins fr shared electrns. In ther wrds, when tw atms frm a bnd the shared electrns will spend mre time arund ne atm than the ther. In fact, different atms have different electrn attracting abilities. The relative attractin that an atm has fr shared electrns in a cvalent bnd is knwn as its electrnegativity. A scale f electrnegativities was develped by Linus Pauling fr which he wuld win the Nbel prize in The scale is a number in which the mst electrnegative atm, flurine, was assigned a value f 4.0. Examinatin f the electrnegativities f elements given in the peridic table indicates the fllwing trends. 1. Electrnegativities increase frm left t right within a perid. 2. Electrnegativities decrease frm tp t bttm within a grup. 3. Electrnegativities f the nn-metals are high while thse f the metals are lw. This bservatin is cnsistent with the fact that metals tend t lse electrns (lw electrnattracting-ability) and nn-metals gain electrns (high electrn-attracting-ability). Nte that the nly inert gas that has an electrnegativity value is xenn. This reflects the fact that inert elements nrmally d nt frm bnds. Hwever, sme clever chemists came up with a way t artificially induce xenn and flurine t frm xenn hexafluride. Further, electrn attracting ability is always t be understd in the cntext f a bnd r relatinship with ther atms. T speak f the electrnegativity f an individual atm des nt make sense. The electrnegativity scale is based n experimental evidence and reflects the relative reactivity f metals and nn-metals (i.e. the lwest electrnegativity belngs t the mst reactive metals, Cs and Fr, while the highest electrnegativity belngs t the mst reactive nn-metal, F). III. Plar and nn-plar cvalent bnds Hw des a difference in electrnegativity effect a cvalent bnd? The shared electrns within a bnd are mre strngly attracted t the atm with the higher electrn-attracting ability (i.e. higher electrnegativity). When we cmpare the electrnegativities fr hydrgen chlride, fr example, H Cl the shared electrns will spend mre time arund Cl since it has a greater electrn attracting ability. In additin, since the bnding electrns (e ) are arund the Cl atm the majrity f the time, the Cl end f the mlecule becmes partially negative ( ) and the H end partially psitive (+). (+) H Cl ( ) Cvalent bnds in which the bnding electrns are unequally shared are called plar cvalent bnds. Plar cvalent bnds can be said t have a charge separatin r a bnd diple. Such a bnd diple can be represented by an arrw with the arrwhead pinting twards the partially Dr. Rn Licht

5 negative, mre electrnegative, atm. If, n the ther hand, the atms invlved have the same electrn attracting ability the result is a nn-plar cvalent bnd. IV. Assignment part A In the fllwing diagrams draw an arrw t indicate the directin f the plar bnd. N H B F H C N I Br Cl C Cl N H C O H H F F I I S C Explain why cesium and francium are the mst reactive metals. Explain why flurine is the mst reactive nn-metal. V. Plarity f mlecules S far we have learned abut the sterechemistry f mlecules and plar/nn-plar cvalent bnds. In rder t determine the plarity f a mlecule, the sterechemistry f the mlecule must be knwn alng with the presence f bnd diples. The steps t be fllwed in determining the plarity f a mlecule are: 1. Draw a Lewis r structural diagram f the mlecule. When drawing a structural diagram remember that there may be lne pairs arund the central atm(s) that determine the shape f the mlecule. 2. Apply the VSEPR rules t draw r visualise the shape f the mlecule. In general, if a mlecule is trignal pyramidal r angular, the result is a plar mlecule. Fr the ther shapes cntinue t the next step. 3. Use electrnegativities t determine bnd diples and draw these n the shape/structural diagram. 4. Imagine the bnd diple arrws as frce vectrs. If bnd diples cancel r are symmetrical arund the central atms, the mlecule is nn-plar; if bnd diples d nt cancel, the mlecule is plar. In every case, bnd plarity is determined by additin f the bnd diples arrws in cncert with the directins they pint. Dr. Rn Licht

6 The table belw summarises the situatins that give rise t mlecular plarity r nn-plarity when the atms bnded t the central atm are identical. If nn-identical atms are bnded t the central atm, the bnd diples may nt cancel. Fr example, CH 4 is nn-plar, but CH 3 Cl is plar because the C Cl bnd diple is nt cancelled by the C H bnd diples. Nte: It is very imprtant that yu, the student, d nt cnfuse a plar bnd with a plar mlecule. A plar bnd is the result f unequal sharing f electrns between tw atms within a mlecule. A plar mlecule is a mlecule that has an verall plarity due t the shape f the mlecule and/r the presence f bnd diples within the mlecule. Dr. Rn Licht

7 VI. Assignment part B Fr each f the fllwing mlecules: a. draw the Lewis r structural diagram b. determine the shape arund the central atm(s) c. draw arrws t represent bnd diples d. determine whether the mlecule is plar r nt Mlecular Substance Lewis r structural diagram Shape Arund Central Atm(s) Shape Diagram & Bnd Diples Plarity f mlecule O O H 2 O angular plar H H H H HF NH 3 NH 4 + N 2 HBr OCl 2 C 2 H 2 SiCl 4 Dr. Rn Licht

8 Mlecular Substance Lewis r structural diagram Shape Arund Central Atm(s) Shape Diagram & Bnd Diples Plarity f mlecule CO 2 CHI 3 C 2 H 3 Cl CH 4 C 2 H 6 C 2 H 4 CH 3 OH O 2 Dr. Rn Licht

9 Mlecular Substance Lewis r structural diagram Shape Arund Central Atm(s) Shape Diagram & Bnd Diples Plarity f mlecule O 3 H 2 O 2 C 2 H 5 OH Dr. Rn Licht