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Honors Chemistry Chap 13. Molecular Structure. 13.1 Electron Distribution. Consider 2 models of molec structure which account for their shape 1 st model takes into account the repulsive forces of e- pairs - PowerPoint PPT Presentation
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Honors Chemistry Chap 13Molecular Structure
13.1 Electron DistributionConsider 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
13.1 Electron DistributionIt’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
13.2 Electron Pair RepulsionAlso called VESPR Theory
◦Valence Electron Shared Pair Repulsion Theory
13.2 Electron Pair RepulsionOne 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
13.2 Electron Pair RepulsionRepulsion betw chg clouds
determine arrangement of orbitals & \ the shape of the molec
Electron prs spead 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)
13.2 Electron Pair Repulsion3 e- prs – axes of chg clouds will
be 120o apart◦Trigonal Planar E- prs lie in the same plane as the
nucleus4 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
13.2 Electron Pair RepulsionUnshared 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
13.2 Electron Pair RepulsionCH4, NH3, H2O, & HF – all have 4
clouds around them◦\ expect all 4 clouds to pt to corners
of tetrahedronCH4 – all clouds are shared prs –
size & repulsions are =◦\ bond angle is 109.5o – perfect
tetrahedron◦Shape of molecule is tetrahedral
13.2 Electron Pair RepulsionNH3 – 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
13.2 Electron Pair RepulsionH2O – 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)
13.3 Hybrid OrbitalsThe 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
13.3 Hybrid OrbitalsHowever, C undergoes hybridization
during bondingThe 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 shapeEa can bond to another atom
If ea bonds to an identical atom, the 4 bonds are equivalent
13.4Geometry of Carbon CompoundsMethane – CH4 – 1 C atom & 4 H
atoms◦Bonds involve s orbital of ea H atom
w/ 1 sp3 hybrid orbital of C 109.5o betw ea 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
13.5 Sigma & Pi BondsA 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”
13.5 Sigma & Pi BondsDifferent ways to form a sigma
bond:1. 2 s orbitals2. An s & a p orbital3. 2 p orbitals (overlapping end-to-
end)4. 2 hybrid orbitals ex) sp3’s5. A hybrid orbital & an s orbital
13.5 Sigma & Pi BondsSince p orbitals are not spherical,
when 2 half-filled p orbitals overlap, they can form 1 of 2 types of bonds1. Overlap end-to-end & form a s
bond2. Overlap sideways (parallel) & form
a Pi Bond (p)
13.5 Sigma & Pi BondsEthylene (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
13.5 Sigma & Pi BondsThe 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
13.5 Sigma & Pi BondsAcetylene (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
13.5 Sigma & Pi BondsDouble & triple bonds are less flexible,
shorter, & stronger than a single bondp 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
13.6 Organic NamesNames 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
13.6 Organic NamesPrefixes show # of C atoms in chain
or ring◦H2C CH2 - ethene
◦H C C H - ethyneMolecs whose C atoms form a ring
begin w/ cyclo◦Simplified diagrams can be used to
represent cyclic comps C atoms are @ the vertices
13.6 Organic NamesC 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
13.7 Multiple Bond Molecular ShapesFormaldehyde contains a double
bond betw C & O
13.7 Multiple Bond Molecular ShapesN2 contains a triple bond
13.7 Multiple Bond Molecular ShapesUsing 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
13.7 Multiple Bond Molecular ShapesIn 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
13.7 Multiple Bond Molecular ShapesWhen C has 2 double bonds, the
molec will be linearCO2 -
13.7 Multiple Bond Molecular ShapesKetene:
◦ 2 dbl bonds on 1 C atom – that part is linear
◦ Other C atom has 2 single & 2 dbl bond like formaldehyde
13.7 Multiple Bond Molecular ShapesWhen C is triple bonded to
another atom, molec is linear
13.7 Multiple Bond Molecular ShapesIn 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
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
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
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
13.8 Benzene C6H6 Many ways to represent benzene:
Represent delocalized e-s from p bonds
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
13.9 IsomersIsomerism – the existence of 2 or
more subst w/ the same molecular formula, but diff stuctures◦These structures are isomers Very common in organic chem
13.9 IsomersC4H10 – butane – 2 structures
can be drawn for this formulaButane methyl propane
(isobutane)
◦These are structural isomers or skeleton isomers – C chain is altered
13.9 IsomersGeometric 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
13.9 Isomers◦Cis 2 butene
◦Trans 2 butene
13.9 IsomersCis – 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)
13.9 IsomersPositional Isomers – occurs w/ a
3rd elem or mult bond where the 3rd elem or mult bond can occupy 2 or more diff positions
Functional Isomers – Formed when a 3rd elem 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
13.10 Inorganic CompoundsHybridize like organic comps
◦Be ends in 2s2 – hybridizes 2 orbitals 2 sp orbitals
Linear molec
13.10 Inorganic CompoundsB ends in 2s2 2p1 - 3 orbitals
hybridize◦3 sp2 orbitals
Trigonal planar
13.11 Bond summaryBCl3 – trigonal planar
◦Used to produce high-purity metalsHigher 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.