Lewis structures and VSEPR

CO2

NH3

H2OCH4CH4

Lewis structures

Please draw the valence shell picture for NH3 and CO2.

NH3

N needs three covalent bonds to fill its valence shell.

NH H

H

CO2

C needs four covalent bonds to fill its valence shell.

O requires two covalent bonds to fill its valence shell

C OO

maximum number of pairs of electrons two main group atoms can share is 3 pairs.

Lewis structures show the bonding.

Lewis structures do not show the molecular shape.

Valence Shell Electron PairRepulsion or VSEPR

VSEPR model gives molecular shape.

Bonds formation, not electron pair repulsion,is responsible for molecular geometries.

L. S. Bartell J. Chem. Educ., 1968, 45 (12), p 754

VSEPR:

(1) Determine steric number (SN)

(2) Use SN to predict molecular shape

SN

2

3

4

SN

5

6

axial site

equitorial site

more electronegatve atomalways chooses axial site.

two methods to determine SN for VSEPR

1) central atom method

2) Lewis structure method

Central atom method of determining SN(easiest method when there is one atomin the center of the molecule/ion bondedto all the other atoms)

Central atom method is based on the periodic table.

N takes 1 e- awayfrom central atom count.

H contributes 1 e-to central atomelectron count.

Br contributes 1 e-.

O contributes 0 e-.

Z = Zentralatom

Use central atom method to determine SN and shape of:

Determine SN and shape of:

General rule: (99+% true in stable molecules/ions) The least electronegative main groupatom (excluding H) is at the center.

Determine SN and shape of:

XeF5+ XeO3 IOF4

-

SOCl2

c(F) > c(O) > c(N) = c(Cl)

The remarkable molecule OFCl

two methods to determine SN for VSEPR

1) central atom method

2) Lewis structure method

As the molecules get less central-atom-likeit is easiest to use the Lewis-structure methodfor determining the steric number, SN

sulfuricacid

pyro-sulfuricacid

Use the Lewis-structuremethod to determinea plausible molecularshape for pyro-sulfuricacid.*

*method requires one to determineLewis structure before determining molecular shape

or

compound XnYm

Is XnYm a molecule or extended solid?

Ionic metal atoms often 6 coordinate

the shape of XnYm

Locate X and Y onthe periodic table.

Have you seen XnYm before?

Are X and Y fromopposite sides ofthe periodic table?

Is Dc small or are Xor Y 1st and 2nd rowelements?

Are X and Y both p-blockelements?

Use ionic bonding model

Use covalent bonding model

central atommethod

determineLewis structure

determineLewis structure

applyVSEPR

Determine a plausible molecular/extended solid shape. If a portion of the compound might be covalent use covalent model for that portion.

Is XnYm a molecule or extended solid?

yes

no yes

no

yes

Are X and Yboth metals

Use metallic bonding

yes

no

atoms have 10-16bonds each, oftenin closest packing

yes no no

Determine shapes of:

I3- CO2

NH3 TeF6

formal charge

Two standard methods for electron counting

a) formal charge method: Divide equally the shared electrons in bonds between the bondedatoms. Lone pairs ( ) are assigned to host atom.

b) oxidation state method

oxygen startswith six outer shell electrons

carbon startswith four outer shell electrons

O

C

Two standard methods for electron counting

a) formal charge method: Divide equally the shared electrons in bonds between the bondedatoms. Lone pairs ( ) are assigned to host atom.

oxygen startsand ends with six outer shell electrons. O formalcharge is zero.

O

carbon startsand ends with four outer shell electrons. C formalcharge is zero.

C

Please draw a Lewis structure for each of the the above three species. Where ever an atom has non-zero formal chargeplease note the non-zero charge next to the atom.

Please draw a Lewis structure for each of the the above three species. Where ever an atom has non-zero formal chargeplease note the non-zero charge next to the atom.

Correct Lewis structures require placement of all non-zero formal charges on the Lewis diagram.

octet rule

If formal charge is zero :

If formal charge is +1 then atom behaves as if it is an atom one column to the left.

If formal charge is -1 then atom behaves as if it is an atom one column to the right.

and so forth.

assuming atom has eight valence shell electrons.

assuming atom has eight outer shell electrons.

modern and traditional Lewis structures

deducing modern Lewis structures

1. if at all possible, all main group atoms (besides H) have an outer (valence) shell with eight electrons. Hydrogen just needs two. This is the octet rule. Outer shell electrons are called valence electrons.

2. having done step 1, make formal charges as small as possible.

3. no more than 3 bonds (multiple bonds) allowed between two main group atoms. One of the two atoms in this multiple bond istypically a second row element.

it's a puzzle and needs to be solved like a puzzle

draw Lewis structures for all the ions below:

carbonate has 3 resonancestructures

how many resonancestructures does bicarbonatehave?

draw Lewis structures for all the ions to theleft:

deducing traditional Lewis structures

1. make formal charges as small as possible.

2. second row main group elements almost always obey the octetrule.

3. other rows obey octet rule if it doesn't interfere with rules 1 and 2.

4. multiple bonds: allowed to make up to 3 bonds between a pair ofmain group atoms. One of the two atoms in multiple bond is typically a second row element.

it's a puzzle and needs to be solved like a puzzle

please deduce both modern and traditional Lewis structures for the molecules and ions belowPlease also deduce SN and geometry:

SO42-, the sulfate ion

XeO4

PF3Cl2

the modern Lewis structure of PF3Cl2

is a resonant ''detached'' diagram whereeach resonant structure looks like the molecule is separated in two. But the tworesonant structures taken together show a single cohesive whole molecule.

Detached diagrams like this always require the three atoms to be in a straight row, a syzygy. The bonding is called the 3-center 4-electron bond.

comparative studies of modern andtraditional Lewis structures show themodern method is more correct.

comparative studies of modern andtraditional Lewis structures show themodern method is more correct.