Lewis Dot Structure Rules:Lewis Dot Structure Rules:

Treat ions separately (e.g. NH4Cl)

Count only valence electrons Assemble bonding framework Fill up non-bonding electrons on outer atoms Fill up non-bonding electrons on inner atoms Calculate Formal Charge Minimize Formal Charge

To do Lewis Structures:To do Lewis Structures:Must be able to recognize

polyatomic ionsMust be able to identify

valence electrons

Must be able to construct Bond framework

Periodic Table : Column numbers!

More complex: H outside High outside Formula hints

Acidic Hs bond to O atomsAcidic Hs bond to O atoms

Hints on Lewis Dot StructuresHints on Lewis Dot Structures1.1. Octet “rule” is the most useful guideline.Octet “rule” is the most useful guideline.2.2. Carbon forms 4 bonds.Carbon forms 4 bonds.3.3. Hydrogen typically forms Hydrogen typically forms oneone bond to other bond to other

atoms.atoms.4.4. When multiple bonds are forming, they are When multiple bonds are forming, they are

usually between C, N, O or S.usually between C, N, O or S.5.5. Nonmetals can form single, double, and Nonmetals can form single, double, and

triple bonds, but not quadruple bonds.triple bonds, but not quadruple bonds.6.6. Always account for single bonds and lone Always account for single bonds and lone

pairs before forming multiple bonds.pairs before forming multiple bonds.7.7. Look for resonance structures.Look for resonance structures.

PClPCl33

Bonding Pairs

Lone Pairs(a.k.a. nonbonding electrons)

5+(3*7)=26 e-

Try Some Examples:Try Some Examples:

CH3CH2NH2

Cl2CO

Ozone (O3)

NO2 vs. N2O

SPENT LOTS O’ TIME PRACTICING…

Formal ChargeFormal ChargeDifference between the # of valence electrons in the free atom and the Difference between the # of valence electrons in the free atom and the # of electrons assigned to that atom in the Lewis structure.# of electrons assigned to that atom in the Lewis structure.

FC = formal charge; G.N. = Group NumberFC = formal charge; G.N. = Group Number#BE = bonding electrons; #LPE = lone pair electrons#BE = bonding electrons; #LPE = lone pair electrons

If Step 4 leads to a positive formal charge on an inner atom beyond the If Step 4 leads to a positive formal charge on an inner atom beyond the second row, shift electrons to make double or triple bonds to minimize second row, shift electrons to make double or triple bonds to minimize formal charge, even if this gives an inner atom with more than an octet formal charge, even if this gives an inner atom with more than an octet

of electrons.of electrons.

LPEBE ##2

1- G.N. FC

Multiple Bonds• It is possible for more than one pair of electrons to be

shared between two atoms (multiple bonds):• One shared pair of electrons = single bond (e.g. H2);

• Two shared pairs of electrons = double bond (e.g. O2);

• Three shared pairs of electrons = triple bond (e.g. N2).

• Generally, bond distances shorten with multiple bonding.

H H O O N N

Covalent BondingCovalent Bonding

Octet in each case

Odd Number of Electrons…

NO Number of valence electrons = 11

N O N O

O O O OOxygen is a ground state"diradical"

NO2Number of valence electrons = 17

O2

Resonance occurs when more than one valid Lewis structure can be written for a particular molecule (i.e. rearrange electrons)

Molecules and atoms which are neutral (contain no formal charge) and with an

unpaired electron are called Radicals

N OO N OO N OO

Beyond the OctetBeyond the Octet

Elements in the 3Elements in the 3rdrd period or higher can period or higher can have more than an octet if needed.have more than an octet if needed.

Atoms of these elements have valence d Atoms of these elements have valence d orbitals, which allow them to orbitals, which allow them to accommodate more than eight electrons.accommodate more than eight electrons.

More than an Octet…

PCl5

Elements from the 3rd period and beyond, have ns, np and unfilled nd orbitals which can be used in bonding

P : (Ne) 3s2 3p3 3d0

Number of valence electrons = 5 + (5 x 7) = 40 P

Cl

ClCl

ClCl

SF4S : (Ne) 3s2 3p4 3d0

Number of valence electrons = 6 + (4 x 7) = 34

SF

F

F

FThe Larger the central atom, the more atoms you can bond to it – usually small atoms such as F, Cl and O allow central atoms such as P and S to expand their valency.

Less than an Octet…

BCl3Group 3A atom only has six electrons around it

However, Lewis acids “accept” a pair of electrons readily from Lewis bases to establish a stable octet

Cl

AlCl

Cl

N

H

H

H

Cl

AlCl

Cl

N

H

H

H

+

Lewis acid Lewis base salt

+_

B

Cl

Cl Cl

VSEPR DefinitionsVSEPR Definitions

Electron groupElectron group –set of electrons that occupies –set of electrons that occupies a particular region around an atom.a particular region around an atom.

LigandLigand – an atom or a group of atoms bonded – an atom or a group of atoms bonded to an inner atomto an inner atom

Steric numberSteric number – the sum of the number of – the sum of the number of ligands plus the number of lone pairs; in other ligands plus the number of lone pairs; in other words, the total number of groups associated words, the total number of groups associated with that atom.with that atom.

KNOW THESE!KNOW THESE!

Lone Pairs Take up a Bit More Lone Pairs Take up a Bit More Space…Space…

Experiments show Experiments show that sulfur that sulfur tetrafluoride has bond tetrafluoride has bond angles of 86.9angles of 86.9° and ° and 101.5 °. Give an 101.5 °. Give an interpretation of these interpretation of these bond anglesbond angles

NOTE: Sizes and electronegativities of exterior atoms also affect bond angles!!!