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