Writing Lewis Structures of Writing Lewis Structures of Molecules Molecules

1.1. Determine the central atom (atom in the Determine the central atom (atom in the middle)middle)- usually is the “single” atom- usually is the “single” atom- least electronegative element- least electronegative element- H never in the middle; C always in the middle- H never in the middle; C always in the middle

2.2. Count the Count the totaltotal number of valence e number of valence e-- (group (group #)#)- add ion charge for “-”- add ion charge for “-”- subtract ion charge for “+”- subtract ion charge for “+”

3.3. Attach the atoms together with one pair of Attach the atoms together with one pair of electronselectrons

Writing Lewis Structures Writing Lewis Structures Cont.Cont.

4. All remaining e4. All remaining e-- = LONE PAIRS! = LONE PAIRS!

- lone pairs are NOT involved in bonding- lone pairs are NOT involved in bonding

5. Place lone pairs around atoms to5. Place lone pairs around atoms to fulfill the “octet rule” fulfill the “octet rule”

- some elements may violate this octet- some elements may violate this octet

rule – (H=2, Be=4, B=6)rule – (H=2, Be=4, B=6)

6. If more e6. If more e-- are still needed, create double are still needed, create double

or triple bonds around the central atom.or triple bonds around the central atom.

- single = 1 pair of shared electrons (2 e- single = 1 pair of shared electrons (2 e--))

- double = 2 pair of shared electrons (4 e- double = 2 pair of shared electrons (4 e--))

- triple = 3 pair of shared electrons (6 e- triple = 3 pair of shared electrons (6 e--))

ResonanceResonance When there is more than one Lewis structure for a When there is more than one Lewis structure for a

molecule that differ molecule that differ onlyonly in the position of the in the position of the electrons they are called electrons they are called resonance structuresresonance structures– Lone Pairs and Multiple Bonds in different positionsLone Pairs and Multiple Bonds in different positions

Resonance only occurs when there are double bonds Resonance only occurs when there are double bonds and when the same atoms are attached to the and when the same atoms are attached to the central atomcentral atom

The actual molecule is a combination of all the The actual molecule is a combination of all the resonance forms.resonance forms.

•••• •• ••••••••

•• ••O S O O S O•••••• ••••

••••

••••

Coordinate Covalent BondCoordinate Covalent Bond A covalent bond in which one atom A covalent bond in which one atom

contributes both bonding electrons.contributes both bonding electrons.

Predicting Molecular Predicting Molecular GeometryGeometry

VSEPR TheoryVSEPR Theory– Valence Shell Electron Pair RepulsionValence Shell Electron Pair Repulsion

The shape around the central atom(s) can be predicted The shape around the central atom(s) can be predicted by assuming that the areas of electrons on the central by assuming that the areas of electrons on the central atom will repel each otheratom will repel each other

Each Bond counts as 1 area of electronsEach Bond counts as 1 area of electrons– single, double or triple all count as 1 areasingle, double or triple all count as 1 area

Each Lone Pair counts as 1 area of electronsEach Lone Pair counts as 1 area of electrons– Even though lone pairs are not attached to other atoms, they Even though lone pairs are not attached to other atoms, they

do “occupy space” around the central atomdo “occupy space” around the central atom– Lone pairs generally “push harder” than bonding electrons, Lone pairs generally “push harder” than bonding electrons,

affecting the bond angleaffecting the bond angle

ShapesShapes LinearLinear

– 2 atoms on opposite sides of central 2 atoms on opposite sides of central atom, no lone pairs around CAatom, no lone pairs around CA

– 180° bond angles180° bond angles Trigonal or Triangular PlanarTrigonal or Triangular Planar

– 3 atoms form a triangle around the 3 atoms form a triangle around the central atom, no lone pairs around CAcentral atom, no lone pairs around CA

– PlanarPlanar– 120° bond angles 120° bond angles

TetrahedralTetrahedral– 4 surrounding atoms form a 4 surrounding atoms form a

tetrahedron around the central atom, tetrahedron around the central atom, no lone pairs around the CAno lone pairs around the CA

– 109.5° bond angles109.5° bond angles

180°

120°

109.5°

ShapesShapes

Pyramidal or Trigonal PyramidalPyramidal or Trigonal Pyramidal– 3 bonding areas and 1 lone pair around 3 bonding areas and 1 lone pair around

the CAthe CA– Bond angle = 107Bond angle = 10700

V-shaped or BentV-shaped or Bent– 2 bonding areas and 2 lone pairs around 2 bonding areas and 2 lone pairs around

the CAthe CA– bond angle = 104.5bond angle = 104.500

Polarity of MoleculesPolarity of Molecules

Molecule will be NONPOLAR if:Molecule will be NONPOLAR if:– the bonds are nonpolar (Br-Br, F-F)the bonds are nonpolar (Br-Br, F-F)– there are no lone pairs around the there are no lone pairs around the

central atom and all the atoms attached central atom and all the atoms attached to the central atom are the sameto the central atom are the same

Molecule will be POLAR if:Molecule will be POLAR if:– the central atom has lone pairsthe central atom has lone pairs

Sigma and Pi BondsSigma and Pi Bonds Sigma Bond – End to end overlap along the internuclear Sigma Bond – End to end overlap along the internuclear

axisaxis Pi Bond – Parallel (side by side) overlap (can only be done Pi Bond – Parallel (side by side) overlap (can only be done

with p orbitals)with p orbitals)

2 “s” orbitals overlapping – sigma (2 “s” orbitals overlapping – sigma (σσ))

1 “s” and 1 “p” orbital overlapping- sigma1 “s” and 1 “p” orbital overlapping- sigma

2 “p” orbitals overlapping (same axis) - sigma2 “p” orbitals overlapping (same axis) - sigma

2 “p” orbitals overlapping (parallel axes) - pi2 “p” orbitals overlapping (parallel axes) - pi

HybridizationHybridization

refers to a mixture or a blendingrefers to a mixture or a blending Biology – refers to genetic materialBiology – refers to genetic material Chemistry – refers to blending of orbitalsChemistry – refers to blending of orbitals Remember, orbitals can only predict an Remember, orbitals can only predict an

area in space where an earea in space where an e-- may be located. may be located. Sometimes blending orbitals can produce a Sometimes blending orbitals can produce a

lower, more stable bonding opportunity.lower, more stable bonding opportunity. Orbital hybridization occurs through eOrbital hybridization occurs through e--

promotion in orbitals that have similar promotion in orbitals that have similar energies (i.e. same energy level).energies (i.e. same energy level).

Hybridization Cont.Hybridization Cont. Hybridization occurs WITHIN the atom Hybridization occurs WITHIN the atom

to enhance bonding possibilities.to enhance bonding possibilities. Do not confuse this concept with Do not confuse this concept with

orbital overlap (bonding).orbital overlap (bonding). Hybridization is a concept used to Hybridization is a concept used to

explain observed phenomenon about explain observed phenomenon about bonding that can’t be explained by bonding that can’t be explained by dot structures.dot structures.

EXAMPLES – draw box diagrams for EXAMPLES – draw box diagrams for Be, B, and C (use noble gas core).Be, B, and C (use noble gas core).

How do I know if my CA is How do I know if my CA is hybridized?hybridized?

If your CA is B, Be, C, Si, or Al then it If your CA is B, Be, C, Si, or Al then it is hybridized.is hybridized.

If your molecule has multiple bonds If your molecule has multiple bonds in it then it is hybridized.in it then it is hybridized.– Double bonds – spDouble bonds – sp2 2 hybridizedhybridized– Triple bonds (or 2 double bonds) – sp Triple bonds (or 2 double bonds) – sp

hybridizedhybridized