Name Hnrs Chemistry / / Beynd Lewis Structures Exceptins t the Octet Rule Mdel Hydrgen is an exceptin t the ctet rule because it fills its uter energy level with nly 2 electrns. The secnd rw elements B and Be ften have fewer than eight electrns arund them in their cmpunds. Beryllium will nly have 4 electrns and brn will nly have 6 electrns in their structural frmulas. These electrn deficient cmpunds are very reactive. The secnd-rw elements carbn, nitrgen, xygen and flurine always bey the ctet rule and must have 8 electrns in their structural frmulas. Secnd rw elements never exceed the ctet rule, since their valence rbitals (2s and 2p) can accmmdate nly 8 electrns. Third rw and heavier elements ften satisfy the ctet rule but can exceed the ctet rule by using their empty valence d rbitals. (ex: PCl 5, SF 6 ) When writing the Lewis structure fr a mlecule, satisfy the ctet rule fr the atms first. If electrns remain after the ctet rule has been satisfied, then place them n the central atm. Paramagnetism & Diamagnetism Mst materials have n magnetism until they are placed in a magnetic field. Hwever, in the presence f such a field, magnetism f tw types can be induced. Paramagnetism causes a substance t be attracted t the inducing magnetic field due t having unpaired electrns. Diamagnetism causes the substance t be repelled frm the inducing magnetic field due t having all paired electrns. T determine whether r nt an element is paramagnetic r diamagnetic, yu need t understand the element s electrn cnfiguratin and rbital diagram. Example 1: H: 1s 1 The s rbital can hld 2 electrns, with nly 1 electrn it has unpaired electrns and is paramagnetic. Orbital diagram: Example 2: He: 1s 2 The s rbital can hld 2 electrns, with 2 electrns the rbital is full and is diamagnetic. Orbital diagram: Example 3: O: 1s 2 2s 2 2p 4 Fllwing the Aufbau and Hund rules, xygen has a full 1s rbital a full 2s rbital. The 2p rbitals has ne full rbital and 2 half filled rbitals s it is paramagnetic. Orbital diagram: A quick way t understand: Elements in grup 2, grup 12 and grup 18 have all paired electrns and are diamagnetic. All ther elements are paramagnetic. Mst mlecules have paired electrns and are diamagnetic. NO & NO 2 have an dd numbers f electrns and are paramagnetic. Thrugh experimentatin, scientists have discvered that the liquid O 2 mlecule is paramagnetic. This fact vilates the ctet rule and prvides evidence that ther mdels f bnding and structure may wrk better. Frmal Charge The frmal charge f an atm in a mlecule is the difference between the number f valence electrns n the free atm and the number f valence electrns assigned t the atm in the mlecule. Frmal Charge = valence e - n free atm(frm peridic table) dts tuching the atm When dealing with bnds, cunt nly the dt (end) next t the atm. Fr example: Calculate the frmal charge fr each element in the structural frmula shwn t the right. Oxygen: val. e - #dts Frmal Charge = (6) (7) = -1 Sulfur: Frmal Charge = (6) (4) = +2 Any negative frmal charges are expected t reside n the mst electrnegative atms. Oxygen has a higher electrnegativity s it makes sense that it has the negative frmal charge. 1
But, atms in mlecules are best suited when they have frmal charges as clse t zer as pssible. Cnsider an alternative mdel fr sulfate which brings frmal charge lwer: The frmal charge fr each element in the structural frmula shwn t the right. Oxygen (single) : val. e - - dts Frmal Charge = (6) (7) = -1 Oxygen (duble) : Frmal Charge = (6) (6) = 0 Sulfur: Frmal Charge = (6) (6) = 0 Overall the frmal charge is lwered s this mdel prves that the secnd diagram is preferred accrding t the frmal charge mdel. The ctet rule mdel wrks best fr elements in perid 2. The frmal charge mdel wrks best fr elements in perids 3 and beynd. Neither mdel is perfect but bth apprach a generalizatin (mdel) f all mlecules t best describe what is bserved in the real wrld. Hybridizatin sp 3 Hybridizatin Cnsider methane (CH 4 ). The electrn cnfiguratin f carbn is [He] 2s 2 2p 2. The 2p and 2s atmic rbitals f carbn will lead t tw different type f C H bnds: thse frm the verlap f the carbn 2p rbital with the 1s rbital f hydrgen and thse frm the verlap f a carbn 2s rbital with the 1s rbital f hydrgen. This is nt the case. The methane mlecule is tetrahedral with bnd angles f 109.5 Carbn adpts a set f rbitals ther than its native 2s and 2p rbitals t bnd t the hydrgen atms in frming the methane mlecule. The 2s and 2p rbitals present n an islated carbn atm may nt be the best set f rbitals fr bnding. It makes sense t assume that the carbn atm has fur equivalent atmic rbitals arranged tetrahedrally. Such a set f rbitals can be btained by cmbining the carbn 2s and 2p rbitals as shwn belw. This mixing f the native atmic rbitals t frm special rbitals fr bnding is called hybridizatin. The fur new rbitals are called sp 3 rbitals because they are frmed frm ne s rbital and three p rbitals(s 1 p 3 ). The carbn atm underges sp 3 hybridizatin r is sp 3 hybridized. There are 5 types f hybridizatin; they are listed belw. Hybridizatin can best be understd by cunting the number f effective pair r bnding sites. A bnding site is any bnd (single, duble and triple bnds are called cunted as ne bnding site) r unshared pair f electrns n the central atm in a mlecule. 2
The table belw is a guide t determining hybridizatin f a mlecule. Specific details n each type f hybridizatin will fllw. # f Bnding Sites Hybridizatin 2 sp 3 sp 2 4 sp 3 5 dsp 3 6 d 2 sp 3 sp 2 hybridizatin Ethylene (C 2 H 4 ) has a duble bnd act as ne bnding site, and tw single bnds cunting as ne bnding site each; s each carbn is surrunded by three bnding sites. Since ne 2s and tw 2p rbitals are used t frm these hybrid rbitals, this is called sp 2 hybridizatin. In frming the sp 2 rbitals, ne 2p rbital n carbn has nt been used. This remaining p rbital (p z ) is riented perpendicular t the plane f the sp 2 rbitals. The three sp 2 rbitals n each carbn can be used t share electrns, as shwn belw. In each f these bnds, the electrn pair is shared in an area centered n a line running between the atms. This type f cvalent bnd is called a sigma(σ) bnd. In the ethylene mlecule, the σ bnds are frmed using sp 2 rbitals n each carbn atm and the 1s rbital n each hydrgen atm. The secnd bnd must therefre result frm sharing an electrn pair in the space abve and belw the σ bnd. The parallel p rbitals can share an electrn pair, t frm a pi(π) bnd. σ bnds are frmed frm rbitals whse lbes pint tward each ther, but π bnds result frm parallel rbitals. A duble bnd always cnsists f ne σ bnd where the electrns are lcated between the atms and ne π bnd which ccupies the space abve and belw the σ bnd. Type f bnd # f sigma(σ) bnd # f pi(π) bnds single bnd 1 0 duble bnd 1 1 triple bnd 1 2 3
sp hybridizatin In the CO 2 mlecule the carbn has tw bnding sites (each duble bnd is ne bnding site.). Tw bnding sites arund an atm will always require sp hybridizatin f that atm. dsp 3 hybridizatin* Since five bnding sites are needed fr atms such as phsphrus in PF 5, a dsp 3 hybridizatin is necessary. The dsp 3 hybridized phsphrus atm in PF 5 mlecule uses its 5 dsp 3 rbitals t share electrns with five chlrine atms. d 2 sp 3 hybridizatin* Since six bnding sites are needed fr atms such as sulfur in SF 6, a d 2 sp 3 hybridizatin is necessary. A set f six effective pairs arund a given atm always requires an ctahedral arrangement which in turn requires d 2 sp 3 hybridizatin f that atm. * Current evidence suggests that hybridizatin invlving d rbitals des nt exist, and there is cntrversy abut the need t teach any hybridizatin. Until this cntrversy is settled hybridizatin is a mdel that will be used t better understand mlecular gemetry. Hmewrk: 1. Name fur elements that fllw the ctet rule. 2. Name tw elements that have fewer than eight electrns arund them in their cmpunds. 3. Why can elements in the third rw and greater can exceed their ctet? 4. When writing Lewis structures and the ctet must be exceeded, where shuld the additinal electrns be placed? 5. What term is used t describe substances in which all the electrns are paired? 6. What effect des an external magnet have n a diamagnetic substance? 7. What term is used t describe substances in which nt all the electrns are paired? 8. What effect des an external magnet have n a paramagnetic substance? 9. Identify each f the fllwing elements as being either paramagnetic r diamagnetic. a. Magnesium - b. Argn - c. Cbalt - d. Phsphrus - e. Mercury - f. Ptassium - 4
10. Experimental evidence has shwn that xygen mlecules are paramagnetic. With this in mind, draw the irregular Lewis structure fr the xygen (O 2 ) mlecule. 11. Draw structural frmulas fr the fllwing cvalent cmpunds that d nt fllw the ctet rule and write hw they d nt fllw the ctet rule. Determine the frmal charge fr each element in the mlecule. BF 3 PCl 5 SF 6 BeCl 2 ClF 3 RnCl 2 BrF 5 XeF 4 12. Nitrgen mnxide (NO) and nitrgen dixide (NO 2 ) have an dd number f ttal electrns. Use yur knwledge f frmal charge t determine the crrect Lewis structures fr each f the mlecules. Yu must shw a frmal charge calculatin fr each atm in each pssible Lewis structure. 5
Name Frmula Lewis Structure Hybridizatin f central element # sigma bnds # pi bnds phsphate in brn trifluride phsphrus pentachlride carbn dixide ammnia methane diatmic nitrgen sulfur dixide 6
Name Frmula Lewis Structure Hybridizatin f central element # sigma bnd # pi bnds chlrine trifluride nitrate in carbnate in hydrcyanic acid HCN dihydrgen mnxide silicn tetrafluride beryllium dichlride At the cmpletin f this assignment yu will be prepared t take the fllwing Chapter 5 n-line quizzes: diamagnetic r paramagnetic quiz sigma and pi bnds quiz 1 elemental diamagnetic r paramagnetic quiz sigma and pi bnds quiz 2 frmal charge quiz 1 sigma and pi bnds quiz 3 hybridizatin quiz 1 sigma and pi bnds quiz 4 hybridizatin quiz 2 sigma and pi bnds quiz 5 7