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Understanding Molecular Structure in Honors Chemistry

Explore the models of molecular structure, electron distribution, and pair repulsion in this guide. Learn about the VESPR theory, electron pair arrangements, bond angles, and hybrid orbitals in carbon compounds.

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Understanding Molecular Structure in Honors Chemistry

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  1. Honors Chemistry Chap 13 Molecular Structure

  2. 13.1 Electron Distribution • Consider 2 models of molec structure which account for their shape • 1st model takes into account the repulsive forces of e- pairs • 2nd model considers ways in which orbitals can overlap to form orbitals around more than 1 nucleus • E-’s in these orbitals bind the atoms together

  3. 13.1 Electron Distribution • It’s useful to use Lewis e- dot diagrams to describe the shape of molecs. or polyatomic ions • Shaired Pairs – prs of e-’s involved in bonding • Unshared Pairs – prs. of e-’s not involved in bonding • Lone pairs

  4. 13.2 Electron Pair Repulsion • Also called VESPR Theory • Valence Electron Shared Pair Repulsion Theory

  5. 13.2 Electron Pair Repulsion • One way to account for molec shape is to look @ e- repulsion • Ea bond & lone pair in outer level for a charge cloud that repels other chg clouds – due to like charges • Also due to Pauli Exclusion Principle – e- of like spins may not occupy the same vole of space • Repulsion due to like spins is much greater than repulsion due to like charges

  6. 13.2 Electron Pair Repulsion • Repulsion betwchg clouds determine arrangement of orbitals & \ the shape of the molec • Electron prsspead as far apart as possible to minimize repulsive forces. • If there are 2 e- prs, they will be on opp sides of the nucleus • Linear (180o apart)

  7. 13.2 Electron Pair Repulsion • 3 e- prs – axes of chg clouds will be 120o apart • Trigonal Planar • E- prs lie in the same plane as the nucleus • 4 prs – axes of chg clouds will be as far apart as possible – 109.5o • Tetrahedral • Will not lie in the same plane • 4 faces; ea is an equilateral triangle w/ the nucleus @ the center

  8. 13.2 Electron Pair Repulsion • Unshared pr is pear-shaped w/ stem end @ nucleus • Acted upon by 1 nucleus • Shared pr is more slender bec it’s atracted by 2 nuclei • Less repulsion bec it takes up less space • lone prs – most repulsion – take up the most space • Repulsion betw unshared & shared pr is intermediate

  9. 13.2 Electron Pair Repulsion • CH4, NH3, H2O, & HF – all have 4 clouds around them • \ expect all 4 clouds to pt to corners of tetrahedron • CH4 – all clouds are shared prs – size & repulsions are = • \ bond angle is 109.5o – perfect tetrahedron • Shape of molecule is tetrahedral

  10. 13.2 Electron Pair Repulsion • NH3 – 1 lone pr & 3 shared prs – since lone pr occupies more space, shared pairs are pushed together • \ bond < is 107o • E- clouds form tetrahedron; but atoms of the molec for trigonal pyramid • Shape of molecule is trigonal pyramidal

  11. 13.2 Electron Pair Repulsion • H2O – 2 unshared prs & 2 shared prs • Add’l cloud size of unshared prs causes even greater reduction in bond < - 104.5o • E- clouds are tetrahedral, but molec is bent • HF – only 1 bond axis, \ no bond angle • 180o – molec is linear • The diff in molec shape results from unequal space occupied by unshared prs & bonds (shared prs)

  12. 13.3 Hybrid Orbitals • The 2nd model of molec shape considers the diff ways 2 & p orbitals ma overlap when e-’s are shared • C has 4 outer e-’s • Expect 2 half-filled p orbitals avail for bonding

  13. 13.3 Hybrid Orbitals • However, C undergoes hybridization during bonding • The 1s orbital & 3 p orbitals combine into 4 equivalent hybrid orbitals. • Called sp3 hybrids or hybrid orbitals • The 4 orbitals are degenerate – same energy • Ea contains 1 e- • The sp3 hybrids are arranged in tetrahedral shape • Ea can bond to another atom • If ea bonds to an identical atom, the 4 bonds are equivalent

  14. 13.4Geometry of Carbon Compounds • Methane – CH4 – 1 C atom & 4 H atoms • Bonds involve s orbital of ea H atom w/ 1 sp3 hybrid orbital of C • 109.5obetwea C – H bond axis • C exhibits catenation • Occurs when 2 C atoms bond w/ ea other by overlap of an sp3 orbital from ea C atom • Other sp3 orbitals may bond w/ s orbital of H

  15. 13.5 Sigma & Pi Bonds • A covalent bond is formed when 2 orbitals from diff atoms overlap & share an e- pair • Sigma Bond (s)– formed when the 2 orbitals that overlap lie directly on the bond axis • Overlap end-to-end or “head-on”

  16. 13.5 Sigma & Pi Bonds • Different ways to form a sigma bond: • 2 s orbitals • An s & a p orbital • 2 p orbitals (overlapping end-to-end) • 2 hybrid orbitals ex) sp3’s • A hybrid orbital & an s orbital

  17. 13.5 Sigma & Pi Bonds • Since p orbitals are not spherical, when 2 half-filled p orbitals overlap, they can form 1 of 2 types of bonds • Overlap end-to-end & form a s bond • Overlap sideways (parallel) & form a PiBond (p)

  18. 13.5 Sigma & Pi Bonds • Ethylene (ethene, C2H4) shows both types of bonding • In both C atoms, 3 orbitals hybridize • 1 s & 2 p form 3 sp2 orbitals • Lie in the same plane ~ 120o bond angle • The 3rd p orbital does not hybridize • Perpendicular to plane of sp2 orbital • An sp2 orbital from ea C atom overlaps end-to-end - s bond

  19. 13.5 Sigma & Pi Bonds • The 2 remaining sp2 orbitals from ea C atom bond w/ 2 separate H atoms • sp2 to s s bond • The unhybridized p orbitals overlap sadeways - p bond • C atoms have a s bond & a p bond betw them • Double bond – 2 prs of e-’s are shared

  20. 13.5 Sigma & Pi Bonds • Acetylene (ethyne) • 1 s & 1 p orbital hybridize to form and sp hybrid orbital in ea C atom • Leaves 2 p orbitals perpendicular to ea other & perpendicular to the sp hybrids • An sp from ea C overlap to for a s bond • 2 p orbitals from ea C ovrlap to form 2 p bonds • \ acetylene has 1 s & 2 p bonds betw C atoms • Triple bond – 3 shared prs of e-’s

  21. 13.5 Sigma & Pi Bonds • Double & triple bonds are less flexible, shorter, & stronger than a single bond • p bonds – easier to break bec e-’s forming bond are farther from nuclei • \molecs containing multiple bonds are usually more reactive than similar molecs w/ only a single bond • Unsaturated Comps – comps which contain double or triple bonds betw C atoms

  22. 13.6 Organic Names • Names for organic comps have a suffix which describes how the atoms are bonded • Comps ending in a n e have all single bonds betw C atoms • Saturated Comps - CnH2n+2 • Comps ending in e n e have a double bond betw C atoms • Comps ending in y n e have a triple bond betw C atoms

  23. 13.6 Organic Names • Prefixes show # of C atoms in chain or ring • H2C CH2 - ethene • H C C H - ethyne • Molecs whose C atoms form a ring begin w/ cyclo • Simplified diagrams can be used to represent cyclic comps • C atoms are @ the vertices

  24. 13.6 Organic Names • C forms 4 bonds • 4 single bonds • 1 double & 2 single bonds • 1 triple & 1 single bond • 2 double bonds • \ assume a C atom has enough H atoms bonded to it to give it 4 bonds

  25. 13.7 Multiple Bond Molecular Shapes • Formaldehyde contains a double bond betw C & O

  26. 13.7 Multiple Bond Molecular Shapes • N2 contains a triple bond

  27. 13.7 Multiple Bond Molecular Shapes • Using VESPR Theory we can still predict the shapes of molecs containing multiple bonds • A double bond occupies more space than a single bond • 4 e-’s betw bonded atoms instead of 2 • Triple bond occupies even more space • 6 e-’s

  28. 13.7 Multiple Bond Molecular Shapes • In formaldehyde – • 3 clouds around C atom • 2 single & 1 double bond • No unshared prs; assume trigonal planar shape; 120o • However, since double bond takes up more space than single bonds, H-C-H bond angle is less than 120o - 116o • The H – C – O bond angle is more than 120o • 122o

  29. 13.7 Multiple Bond Molecular Shapes • When C has 2 double bonds, the molec will be linear • CO2 -

  30. 13.7 Multiple Bond Molecular Shapes • Ketene: • 2 dbl bonds on 1 C atom – that part is linear • Other C atom has 2 single & 2 dbl bond like formaldehyde

  31. 13.7 Multiple Bond Molecular Shapes • When C is triple bonded to another atom, molec is linear

  32. 13.7 Multiple Bond Molecular Shapes • In most comps, outer level is considered full w/ 8 e-s • If outer level is 3rd or higher, atom can contain > 8 e-’s • Mostly nonmetals (usually halogens) form comps w/ outer level containing 10, 12, or 14 e-’s • This is how Noble Gases react

  33. 13.8 Benzene C6H6 • One of the tip 20 industrial chemicals in US • Used in drugs, dyes, solvents • Highly toxic & a carcinogen • Ea C atom in the benzene ring has 3 sp2 hybrids & 1 p orbital • sp2 orbital from ea of the 6 C atoms overlap & form a ring of 6 s bonds

  34. 13.8 Benzene C6H6 • p orbitals overlap sideways & form ring of p bonds • Left over sp2 orbital from ea C overlaps w/ s orbital from H atom

  35. 13.8 Benzene C6H6 • One main characteristic of benzene is the p e-’s can be shared among all C atoms • - delocalized • Delocalization causes greater stability in benzene

  36. 13.8 Benzene C6H6 • Many ways to represent benzene: Represent delocalized e-s from p bonds

  37. 13.8 Benzene C6H6 • Conjugated system – group of atoms which contain multiple p overlap • Multiple p bonds • Multiple double or triple bonds • C C C C • Conjugated systs add special stability to the molecs

  38. 13.9 Isomers • Isomerism – the existence of 2 or more subst w/ the same molecular formula, but diff stuctures • These structures are isomers • Very common in organic chem

  39. 13.9 Isomers • C4H10 – butane – 2 structures can be drawn for this formula • Butane methyl propane (isobutane) • These are structural isomers or skeleton isomers – C chain is altered

  40. 13.9 Isomers • Geometric isomers – coposed of the same atoms bonded in the same order, but w/ diff arrangement of atoms around a double bond • p bond prevents atoms from rotating w/ respect to ea other • A diff arrangement around a dbl bond since rotation is not possible

  41. 13.9 Isomers • Cis 2 butene • Trans 2 butene

  42. 13.9 Isomers • Cis – the CH3 group (or anything other than H) are next to ea other (on same side) • Trans - the CH3 group (or anything other than H) are on opposite side (across)

  43. 13.9 Isomers • Positional Isomers – occurs w/ a 3rdelem or mult bond where the 3rdelem or mult bond can occupy 2 or more diff positions • Functional Isomers – Formed when a 3rdelem can be bonded in 2 diff ways • A mass spectrometer can be used to distinguish betw isomers having similar props. • Uses charge to mass ratios of ion fragments

  44. 13.10 Inorganic Compounds • Hybridize like organic comps • Be ends in 2s2 – hybridizes 2 orbitals • 2 sp orbitals • Linear molec

  45. 13.10 Inorganic Compounds • B ends in 2s2 2p1 - 3 orbitals hybridize • 3 sp2 orbitals • Trigonal planar

  46. 13.11 Bond summary • BCl3 – trigonal planar • Used to produce high-purity metals • Higher atomic mass elems tend to hybridize their bonding orbitals much less than lighter elems do • May be bec heavier atoms can have more bonded atoms around them bec they are larger.

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