Unit 9 - Molecular Geometry.pptx

Unit 9: Molecular Geometry

AP Chemistry, Kreipe

T1: use the VESPR theory and Lewis Dot Diagrams to explain molecular geometry. T2: predict the polarity of a molecule based on molecular geometry and

electronegativity.

T3: explain the concept of SP 1, 2, and 3 hybridization and its relationship to geometric structure.

T4: describe various bonds in terms of pi and sigma overlap including delocalization.

Bond

Hybridization

Overlap and Bonding

• We think of covalent bonds forming through the sharing of electrons by adjacent atoms.

• _{In a quantum mechanics approach a chemical bond is}

defined as when orbitals on the two atoms overlap.

Hybridization

• How/why is it possible for atoms to form more than 4 bonds with expanded valence shells of more than 8 electrons?

• Why does a carbon atom form a tetrahedral? If P

orbitals are on 90 degree axis, shouldn’t carbon form a square geometry?

Hybrid Orbitals

• _{Consider beryllium:} • _{In its ground}

electronic state, it

would not be able to

form bonds because it has no singly-occupied orbitals.

• _{The Lewis dot-diagram should look like this:} • _{With 0 bonding pairs and 1 non-bonding pair} • _{This means Be should NOT bond, EVER.}

• _{But what we observe in science and draw is this:} • _{With separate bonding electrons in separate}

orbitals…

Be

Hybrid Orbitals

• _{If Be absorbs the}

small amount of energy needed to promote one

electron from the 2s to the 2p orbital, it can form two bonds.

• _{It does this through}

Hybrid Orbitals

•Mixing the s and p orbitals yields two orbitals that are hybrids of the two orbitals. Called an “sp” orbital.

•These sp hybrid orbitals have two lobes like a p orbital.

Hybrid Orbitals

• _{These two “sp hybrid orbitals” would align}

themselves 180 from each other, as far apart as possible.

• _{This is consistent with the observed geometry}

Hybrid Orbitals

• _{With hybrid orbitals the orbital diagram for}

beryllium would look like this.

Hybrid Orbitals

Hybrid Orbitals

Mixing 1 s orbital and 2 p orbitals makes 3 “sp2

orbitals.”

The 3 seperate Sp2 orbitals

repel into a

Hybrid Orbitals

Hybrid Orbitals

1 s orbital and 3 p

orbitals combine

to form four “sp

”

orbitals.

The four sp

orbitals repel to

the corners of a

tetrahedral

Hybrid Orbitals

• _{For geometries involving expanded octets on the}

central atom, we must use d orbitals in our hybrids.

• This is because we have used all of the p orbitals up!

• _{Group 5 elements can make five sp}3d orbitals by

Hybrid Orbitals

• _{This leads to five sp}3d orbitals

that push apart to form a trigonal pyramidal geometry.

• …or elements in group 6 hybridize to make six sp3d2

orbitals.

VSEPR +

Geometry

VSEPR

• _{Valence-Shell Electron-Pair Repulsion Model:}

Electron bonding and lone pairs will repel each other to be as geometrically distant as possible.

• _{By assuming the electron pairs are placed as}

Electron Domains

• _{Electron Domain: An area of space occupied}

by a lone pair or a bonding pair of electrons around a central atom.

• _{Lone Pair Electrons: non-bonding (spare)}

electrons

• _{Bonding Pair Electrons: electron pair shared in}

bond

• _{Double/triple bonds count as ONE electron}

• Lets derive some geometric arrangements… together!

5 Basic Electron

Domain Geometries

• This chart summarizes the MAXIMUM

3-D geometric separation of 2, 3, 4, 5, and 6 electron domains.

• _{ALL molecules have one of these 5}

electron domain geometries.

• _{Some have different MOLECULAR}

Molecular Geometries

• _{Electron Domain Geometry: Geometry of ALL}

electron pairs (both bonding and non-bonding) around an atom.

• _{Molecular Geometries: The geometry of the}

Linear Molecular Geometries

• _{Linear: In this domain, there is only one}

molecular geometry: linear.

Trigonal Planar Molecular

Geometries

• Trigonal Planar

• _{There are two molecular geometries:} • _{Trigonal planar, if all the electron domains}

are bonding

• _{Bent, if one of the domains is a nonbonding}

Tetrahedral Electron Domain

• _Tetrahedral

• There are three molecular

geometries:

• Tetrahedral, if all are bonding pairs

• _{Trigonal pyramidal if}

one is a nonbonding pair

• _{Bent if there are two}

Trigonal Bipyramidal Electron

Domain

• Unlike other electron

geometries, for Trigonal

Bipyramidal, there are two distinct positions in this

geometry:

• _{Axial: Point straight up/down} • Equatorial: form triangle

Trigonal Bipyramidal Electron

Domain

• _Trigonal

Bipyramidal

• There are four

Octahedral Electron Domain

• _{All positions are}

equivalent in the octahedral

domain.

• _{There are three}

molecular geometries:

• _Octahedral

• Square pyramidal

Now you try…



• Sketch the Lewis dot diagram, and determine the

electron geometry as well as the molecular geometry of:

MgF₂ H₂O NH₃ SCl₄ BCl₃

EDG:Linear Tetrahedral Tetrahedral Octahedral Trig planar

A few tweaks to bond angles…

• _{Nonbonding pairs are}

physically larger than bonding pairs.

• _{Therefore, their repulsions}

Multiple Bonds and Bond

Angles

• _{Double and triple bonds}

place greater electron density on one side of

the central atom than do single bonds.

• _{Therefore, they also}

affect bond angles slightly by repelling adjacent electron

Multiple Geometries, One

Molecule

In larger molecules, it makes more sense to

talk about the geometry about each particular

atom rather than the geometry of the

Now you try…



• Determine the molecular geometry around all of the central atoms in the amino acid (protein building

block) Glycine. H₂N-CH₂-COOH

Trig Pyramidal Trig planar Tetrahedral

Molecular

Polarity

Polarity

• _{In Unit 8 we discussed bond}

dipoles.

• _{Calculated by the difference}

in electronegativity of the two bonding elements

• _{Represent with an arrow}

pointing toward the higher EN atom

Polarity

dipoles, in a vector-like fashion, one can determine the overall dipole moment for the molecule.

Most of you should understand vectors from Physics last

year!

Molecule Dipole: If the sum of all bond

Polarity Examples

• _{So basically, any molecule that has}

asymmetrical atoms of different

electronegativity or has asymmetrical lone pairs of e- will likely be “polar.”

• _{Molecules that are very symmetrical tend to be}

non-polar.

Now you try…



• _{Determine whether the following molecules are polar or}

non-polar. If polar than draw the net polarity directional arrow.

• _CCl4

• 4 Bonding Pairs and 0 non-bonding pairs = Tetrahedral, all Cl dipoles cancel

• _Non-Polar • _PCl3

• _{3 BP and 1 NBP = trigonal pyramidal, non-bonding pair is more}

(-), polar

• _H2O

• 2 BP and 2 NBP = Bent, 2 NBP are more (-) region, polar

• _CO2

E- Cloud Bond

Overlaps

Overlap and Bonding

• _{Increased overlap}

brings the electrons and nuclei closer

together while simultaneously

decreasing electron-electron repulsion.

• _{However, if atoms get} too close, the

Sigma (



) Bonds

• _{Sigma bonds are characterized by}

• _{Head-to-head overlap.}

Pi (



) Bonds

•

Pi bonds are

characterized by

• _{Side-to-side overlap.} • _{Electron density}

Single Bonds

Single bonds are always  bonds, because  overlap is greater, resulting in a stronger

Multiple Bonds

Multiple Bonds

• _{In a double bond}

containing molecule

like formaldehyde

(shown at left) an sp2

orbital on carbon

overlaps in  fashion

with the

corresponding orbital on the oxygen.

• _{The unhybridized p}

Multiple Bonds

In triple bonds, as in acetylene, two sp

orbitals form a 

bond between the carbons, and two pairs of p orbitals

overlap in  fashion to form the two 

Delocalized Electrons: Resonance

Delocalized Electrons: Resonance

• _{In reality, each of the four} atoms in the nitrate ion has a p orbital.

• _{The p orbitals on all three}

oxygens overlap with the p

orbital on the central nitrogen.

• _{The pi bonding electrons in}

the p orbitals are

Molecular

Orbitals

Molecular Orbital (MO)

Theory

Though valence bond theory effectively

conveys most

observed properties of ions and molecules,

there are some concepts better represented by

Molecular Orbital (MO)

Theory

• _{In MO theory, we invoke the wave nature}

of electrons.

• _{If waves interact constructively, the}

Molecular Orbital (MO)

Theory

MO Theory

2 the two electrons go into the bonding

molecular orbital.

• _{The bond order is one}

half the difference

between the number of bonding and anti-bonding electrons.

• _{Bond order =}

MO Theory

For hydrogen, with

two electrons in

the bonding MO

and none in the

antibonding MO,

the bond order is

MO Theory

• _{In the case of He2, the}

bond order would be

1

2 (2 - 2) = 0

• _{Therefore, the bonding and}

MO Theory

• _{For atoms with both s}

and p orbitals, there are two types of

interactions:

• _{The s and the p}

orbitals that face

each other overlap in

 fashion.

MO Theory

• _{The resulting MO}

diagram looks like this.

• _{There are both}  _and

 bonding molecular orbitals and * and

* anti-bonding

MO Theory

• _{The smaller p-block}

elements in the second period have a sizeable

interaction between the s

and p orbitals.

• _{This flips the order of the s}