Unit 06: Molecular Structure & Unit 06: Molecular Structure & Bonding TheoriesBonding Theories

Section B: Molecular Shapes, Section B: Molecular Shapes, Polarity, Molecular Orbital TheoryPolarity, Molecular Orbital Theory

Starring: Janice and Laura

Formal ChargeFormal Charge

The charge calculated for that atom based on the Lewis The charge calculated for that atom based on the Lewis structure of the molecule or ion using the equation...structure of the molecule or ion using the equation...

Formal Charge = [original charge of atom or ion] - Formal Charge = [original charge of atom or ion] - [number of lone pair electrons] + 1/2[number of bonding [number of lone pair electrons] + 1/2[number of bonding electrons] electrons]

Bond PolarityBond Polarity

Unless pure covalent bonding occurs, in which atoms share Unless pure covalent bonding occurs, in which atoms share an electron pair equally, the electron pair will be unequally an electron pair equally, the electron pair will be unequally shared and result in a polar covalent bond. shared and result in a polar covalent bond.

But why would the atoms not share?But why would the atoms not share?

A. Just something they didn’t learn in A. Just something they didn’t learn in Kindergarten.Kindergarten.

B. Because not all atoms hold onto their valence B. Because not all atoms hold onto their valence electrons with the same force.electrons with the same force.

C. Just keep clicking the mouseC. Just keep clicking the mouse

If you said B, you were right!If you said B, you were right!

Depending on the atom and the number of valence Depending on the atom and the number of valence electrons, the strength at which electrons can be electrons, the strength at which electrons can be kept or pulled away differs. kept or pulled away differs.

Does this sound familiar? Does this sound familiar? It should, it is called elecronegativity!It should, it is called elecronegativity! But since it isn’t in our section of chapter 6 we But since it isn’t in our section of chapter 6 we

will move on. will move on.

Molecular PolarityMolecular Polarity

In a polar molecule, electron density accumulates toward In a polar molecule, electron density accumulates toward one side of the molecule depending on the elecronegativity one side of the molecule depending on the elecronegativity of each atom. of each atom.

The result is that one part of the molecule becomes more The result is that one part of the molecule becomes more negative. negative.

If these forces cancel out, then the molecule can be neutral.If these forces cancel out, then the molecule can be neutral. Whether or not the forces cancel is usually dependant on Whether or not the forces cancel is usually dependant on

their shapes. their shapes.

Valence shell electron-pair Valence shell electron-pair repulsion (VSEPR) repulsion (VSEPR)

Is a method used to predict the shapes of covalent Is a method used to predict the shapes of covalent molecules and poly atomic ions.molecules and poly atomic ions.

It works by assuming that electrons in bond pairs and lone It works by assuming that electrons in bond pairs and lone pairs will repel each other and try to stay as far apart from pairs will repel each other and try to stay as far apart from each other as possible. each other as possible.

Generally not used for transition metals. Generally not used for transition metals.

Valence electron theoryValence electron theory

The idea that bonds are formed by the The idea that bonds are formed by the overlapping of orbitalsoverlapping of orbitals

Hybridization of OrbitalsHybridization of Orbitals

The first bond on a central atom is a s The first bond on a central atom is a s orbitalorbital

The subsequent three bonds, or electron The subsequent three bonds, or electron lone pairs are p orbitalslone pairs are p orbitals

Any other orbitals are designated as d Any other orbitals are designated as d orbitalsorbitals

Basic Molecular ShapesBasic Molecular Shapes

When there are two bond When there are two bond pairs a linear shape pairs a linear shape observed, and the orbitals observed, and the orbitals are in the same plane and are in the same plane and located 180° away from located 180° away from each other.each other.

For all basic molecular shapes, if the atoms around the For all basic molecular shapes, if the atoms around the central atom are the same, or have the same central atom are the same, or have the same electronegativity, then the molecule will be non-polar electronegativity, then the molecule will be non-polar

Orbitals Hybridized as spOrbitals Hybridized as sp

More Basic Molecular More Basic Molecular Shapes Shapes

When there are three bond When there are three bond pairs a trigonal planar or pairs a trigonal planar or triangle shape is observed. The triangle shape is observed. The orbitals are still all in the same orbitals are still all in the same plane, but are now located 120° plane, but are now located 120° away from each other.away from each other.

Orbitals Hybridized as spOrbitals Hybridized as sp22

More Basic Molecular More Basic Molecular Shapes Shapes

When there are three When there are three bond pairs a tetrahedral bond pairs a tetrahedral shape is observed. shape is observed. Because of repulsion Because of repulsion factors, the trigonal factors, the trigonal planar shape is bent away planar shape is bent away from the new orbital and from the new orbital and between each orbital between each orbital there now exits 109.5° of there now exits 109.5° of separation.separation.

Orbitals Hybridized as spOrbitals Hybridized as sp33

More Basic Molecular More Basic Molecular Shapes Shapes

When there are five bond pairs a When there are five bond pairs a Trigonal Bypyramidal shape is Trigonal Bypyramidal shape is formed with three orbitals formed with three orbitals existing on the same plane in a existing on the same plane in a triangle shape and a pair of triangle shape and a pair of orbitals in a linear formation orbitals in a linear formation perpendicular to the triangle. perpendicular to the triangle. The three orbitals forming the The three orbitals forming the base of the trigonal bypyramidal base of the trigonal bypyramidal shape are separated by 120° and shape are separated by 120° and the two orbitals existing the two orbitals existing perpendicular are 90° away from perpendicular are 90° away from the orbitals of the triangle shape.the orbitals of the triangle shape.

Orbitals Hybridized as spOrbitals Hybridized as sp33dd

The Last Basic Molecular The Last Basic Molecular Shape Slide Shape Slide

When there are six bond When there are six bond pairs an octahedral shape is pairs an octahedral shape is formed with four orbitals formed with four orbitals existing on the same plane, existing on the same plane, separated by 90°, and a separated by 90°, and a pair of orbitals in a linear pair of orbitals in a linear formation perpendicular to formation perpendicular to the plane. These orbitals the plane. These orbitals are located 90° away from are located 90° away from

the plane.the plane. Orbitals Hybridized as spOrbitals Hybridized as sp33dd22

Now we add electron pairs... Now we add electron pairs...

The basic shapes we just learned about, deal with only bond pairs. The basic shapes we just learned about, deal with only bond pairs. However, many molecules have free electron pairs that also effect repulsion.However, many molecules have free electron pairs that also effect repulsion. In fact electrons pairs have a greater repulsion factor than regular bond pairs do. In fact electrons pairs have a greater repulsion factor than regular bond pairs do. For Molecular shapes that have been altered by electron pairs, there is no chance for cancellation, so all of the following will be polarFor Molecular shapes that have been altered by electron pairs, there is no chance for cancellation, so all of the following will be polar

Basic Shape was Basic Shape was Trigonal Planar Trigonal Planar

Two bond pair, one electron pairTwo bond pair, one electron pair

Basic Shape: TetrahedralBasic Shape: Tetrahedral

Three bond pairs, one electron pairThree bond pairs, one electron pair Two bond pairs, two electron pairsTwo bond pairs, two electron pairs

Basic Shape was Basic Shape was Trigonal Bypyramidal Trigonal Bypyramidal Four bond Four bond

pairs, one pairs, one electron pair electron pair

Three bond Three bond pairs, two pairs, two electron pairselectron pairs

Two bond Two bond pairs, three pairs, three

electron pairselectron pairs

Basic Shape was Basic Shape was Octahedron Octahedron

Five bond pairs, one electron pairFive bond pairs, one electron pair Four bond pairs, two electron pairsFour bond pairs, two electron pairs

Molecular Orbital TheoryMolecular Orbital Theory

An alternative way to view orbitals in molecules An alternative way to view orbitals in molecules using the valence electron hybridized orbitalsusing the valence electron hybridized orbitals

Specifies that when 1s orbitals of two hydrogen Specifies that when 1s orbitals of two hydrogen atoms overlap, two molecular orbitals result from atoms overlap, two molecular orbitals result from the addition and subtraction of other overlapping the addition and subtraction of other overlapping orbitals.orbitals.

Deals with π bonds and σ bonds and their Deals with π bonds and σ bonds and their

antitheses π* bonds and σ* bonds.antitheses π* bonds and σ* bonds.

Types of BondsTypes of Bonds

σ bonds: σ bonds: – higher energy than π bondshigher energy than π bonds– exist in the x and y directionexist in the x and y direction– exist where there are single bonds in the Lewis structureexist where there are single bonds in the Lewis structure

π bonds:π bonds:– lower in energy than σ bondslower in energy than σ bonds– exist in the z directionexist in the z direction– exist where there are double bonds in the Lewis structureexist where there are double bonds in the Lewis structure

Must have both π bonds and σ Must have both π bonds and σ bonds to form a complete bonds to form a complete

moleculesmolecules

AntibondsAntibonds

Atomic orbitals form in pairs Atomic orbitals form in pairs One orbital carries the electron dense materials One orbital carries the electron dense materials

and are the π bonds and σ bondsand are the π bonds and σ bonds One orbital does not carry electron materials, and One orbital does not carry electron materials, and

these are the anti π bonds and σ bonds (use a * to these are the anti π bonds and σ bonds (use a * to denote)denote)

In order for orbitals of one atom to form bonds In order for orbitals of one atom to form bonds with orbitals of another, the number of antibonds with orbitals of another, the number of antibonds

(both π and σ) must not cancel out.(both π and σ) must not cancel out.

First Principal of Molecular First Principal of Molecular

Orbital Theory Orbital Theory Molecular orbitals are the total valence Molecular orbitals are the total valence

atomic orbitals of all atoms in the atomic orbitals of all atoms in the moleculemolecule

Second PrincipleSecond Principle

The bonding The bonding molecular orbital is molecular orbital is lower in energy than lower in energy than the parent valence the parent valence orbitalorbital

Antibonds are Antibonds are higher in energyhigher in energy

Third PrincipleThird Principle

Electrons are Electrons are assigned to assigned to orbitals of orbitals of successively successively higher energyhigher energy

Fourth PrincipleFourth Principle

Atomic orbitals Atomic orbitals combine to form combine to form molecular orbitals molecular orbitals most effectively most effectively when atomic when atomic orbitals are of orbitals are of similar energysimilar energy

Homonuclear diatomic Homonuclear diatomic molecules molecules

Diagram for Diagram for second second period period

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