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Lewis (electron dot) structures show the electron domains in the valence shell and are used to predict
molecular shape.
Chapter 4.3: Covalent structures
Lewis structureCoordinate
bond/dative bondElectron deficientVSEPR theoryElectron domainElectron domain
geometry LinearTrigonal planarBentTetrahedral
Important terms for this section
Molecular geometryLinearTetrahedralBentTrigonal pyramidalTrigonal planar
Dipole momentDelocalized electronsResonanceGiant molecularNanotechnology
Lewis structures are used for covalent molecules only
Used to represent valence electronsSteps:1. calculate total number of val. e- in molecule2. draw skeletal structure of molecule3. use crosses, dots or a line to show e- pairs4. add e- pairs to complete the octets (H only has
2)5. use double or triple bonds if necessary6. check that the total number of e- is equal to #1
Lewis structures
Notes: must add an e- for each negative charge (anion) or subtract one e- for each positive charge (cation)
Square brackets are needed for ions
The bond is formed by one atom donating both e- to be shared
An arrow is used to show the originating e- pointing to the atom that is being shared with
Coordinate bond/dative bond
Two primary exceptions:Be and B
These are called electron deficient and are often the receivers of coordinate bonds
Incomplete octets (expanded in 14.1)
Valence Shell Electron Pair Repulsion theoryElectron pairs repel each other and will orient
themselves as far away from each other as possibleThis is lower energy and more stable
This is how the geometry is determinedElectron pairs and bonded pairs are referred
to as electron domains:Electron pairSingle bondDouble bondTriple bond
VSEPR theory
Total number electron domains around central atom will determine geometric arrangement
Shape of molecule determined by angles between bonded atoms
Lone pairs have higher concentration of charge and therefore cause stronger repulsion than bonded pairs
Lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair
Molecules with lone pairs have some distortions due to higher charge repulsion
Structures that do not have lone pairs ≠ Lewis structures
VSEPR theory
Electron domain geometry: linearMolecular geometry: linear
Two electron domains
Electron domain geometry: trigonal/triangular planar
Molecular geometry no lone pairs: trigonal/triangular planar
Molecular geometry 1 lone pair: bent
Three electron domains
Electron domain geometry: tetrahedralFour electron domains
1. draw Lewis structure2. count number of electron domains around
central atom3. determine e- domain geometry:
2 e- domains = linear3 e- domains = triangular planar4 e- domains = tetrahedral
4. determine molecular geometry from the number of bonding e- domains
5. consider extra repulsion caused by lone pairs and make adjustments
Steps to determine shape of molecule
Bonds can be polar while the molecule is still not considered polar… what??
Whether a molecule is polar depends onType of polar bondsOrientation of bonds (shape)
If dipoles are equal in polarity AND symmetrically arranged:Charge separation cancels out polar bonds =
non-polar
Polar bonds vs Polar molecules
If bonds are different in polarity OR not symmetrically arranged:Dipoles will not cancel = net dipole moment
(overall dipole in molecule)There is directionality in the bonds AND in the
molecule!
Polar bonds vs Polar molecules
Delocalized e- : electrons that are shared between more than 1 bonding positionSpread out e- increases stability of moleculeOccurs with molecules that have multiple
bonds and the bond can be moved to another atom
Example: Ozone (O3)
Delocalized electrons & Resonance
O3: how many bonds are on each oxygen?Are they equal or different strength?
So the correct Lewis structure is:
Resonance: more than 1 valid Lewis structure can be drawn for the moleculeResonance hybrid = liger It is both a lion and tiger at the same time! Not
switching!
Delocalized e- & Resonance
Draw a structure for each species showing the resonance hybrid.
How many bonds are actually on each bond?
Delocalized e- & Resonance
Benzene is special!Draw the resonance
hybrid
Resonance makes benzene super stableRelatively unreactive
Delocalized e- & Resonance
Giant molecular = network covalent = macromolecular structure
These are molecules of covalently bonded atoms that form giant structures (unlike the itty bitty methane CH4, for example)
These have different properties than the smaller molecules
Giant covalent crystalline solids
Allotropes: different forms of an element in the same state of matter
Carbon solids can be in different formsGraphiteDiamondFullerene C60Graphene
All are made of only carbon atoms, but their structures are different and so have different properties
Carbon
Carbon allotropes
Graphene is a relatively new form of carbon but expensive!New conductive material: mix graphene with plasticsTransistors can be faster and smaller than Si transistorsTouch screens printed on plastic – light, flexible, almost
unbreakableSupercapacitors (store charge) replacing batteries, use in
cars and mobile devicesSolar panels – graphene can be wrapped around surfaces
such as clothing, furniture or vehiclesNanotechnology: atomic scale manipulation of matter
Graphone: add H to graphene = variable magnetic properties
Flourographene: add F to graphene = rippled structure can make for an insulator
Carbon allotropes – graphene and nanotechnology
Silicon is right under carbon and forms 4 bondsSecond most abundant element in Earth’s crust
(after O)This can form a diamond-like structure for Si in
elemental stateBut in nature, tends to form silicon dioxide
tetrahedral structures (sand/quartz/glass)Here, SiO2 is actually only empiricalProperties:
StrongInsoluble in H2OHigh melting pointNon-conductor of electricity
Silicon and Silicon dioxide