Unit 6: Theories of Covalent Bonding and Intro. To Organic Chemistry

Lewis Structures VSEPR Theories of Covalent Bonding

Valence Bond Theory Molecular Orbital Theory

Organic Chemistry Functional Groups Nomenclature Simple Reactions

Chemical Bonds

Octet Rule: Atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons.

When ionic compounds are formed, electrons are gained or lost.

When molecular compounds are formed, electrons are shared.

Chemical Bonds

Chemical bond: strong attractive force that exists between atoms (or ions) in a compound ionic bonds covalent bonds metallic bonds

Chemical Bonds

Ionic Bond: the electrostatic force of attraction between oppositely charged ions in an ionic compound metal cation (+) non-metal anion (-)

The Na+ and Cl- ions in a salt (NaCl) crystal are held together by electrostatic attraction.

Chemical Bonds

Covalent Bonds: the attractive force between atoms in a molecule that results from sharing of one or more pairs of electrons non-metals

H2O :

Cl2 :

H-O and Cl-Cl bonds result

from sharing of electrons

OH H

Cl Cl

Lewis Symbols

Valence electrons are involved in chemical bonding: electrons residing in the incomplete

outer shell of an atom

For main group elements, the number of valence electrons for an element = group number of the element N (group 5A) has 5 valence electrons Br (group 7A) has 7 valence electrons

Lewis Symbols

Lewis symbols (electron-dot symbols) are used to depict valence electrons in an atom or ion chemical symbol for the element dot for each valence electron

dots are placed on all 4 sides of the chemical symbol

all four sides of the symbol are equivalent

up to 2 dots (electrons) per side

Lewis Symbols

Example: Draw the Lewis symbol for oxygen.

Example: Draw the Lewis symbol for carbon.

Covalent Bonding

Lewis structures (also called electron-dot structures) can be used to represent the covalent bonds that are present in a molecule.

The formation of H2:

H + H H H or H H

Covalent Bonding

Components of Lewis (electron-dot) structures:

Elemental symbol for each atom Bond between atoms depicted using a

solid line Unshared electron pairs are shown

around the appropriate atom

Covalent Bonding

Single bond: one pair of shared electrons

Double bond Two pairs of shared

electrons

Triple bond Three pairs of shared

electrons

Cl Cl

O C OO C O

N N

Drawing Lewis Structures

To draw a Lewis structure:

Add up the valence electrons from all atoms For a cation (+), subtract 1 electron for

each positive chargeNH4

+ : 5 + 4 (1) -1 = 8 e-

For an anion (-), add 1 electron for each negative chargeCN- : 4 + 5 + 1 = 10 e-

Drawing Lewis Structures

Write the chemical symbols for each atom showing which is attached to which using a single bond (-). Sometimes (but not always) the order in

which the formula is writtenHCN: H-CN

Central atom (often written first) surrounded by other atomsCommonly the least electronegative element (except H) will be the central atom

CCl4 : C with 4 Cl attached to it

Drawing Lewis Structures

Add electron pairs to the atoms bonded to the central atom first until each has an octet of electrons.

Remember, H only gets 2 electrons

Drawing Lewis Structures

Place any leftover electrons on the central atom. Sometimes results in more than an

octet on the central atom

Drawing Lewis Structures

If there are not enough electrons to give the central atom an octet, try multiple bonds. Use one (or more) unshared pairs of

electrons from an outer atom to form double (or triple) bonds

H C N H C N

Drawing Lewis Structures

Example: Draw the Lewis structure for COCl2.

Drawing Lewis Structures

Example: Draw the Lewis structure for the carbonate ion.

Drawing Lewis Structures

Example: Draw all possible resonance structure for CO2.

Drawing Lewis Structures

Example: Draw all possible resonance structures for SCN-. Which resonance structure is the major contributor to the resonance hybrid?

VSEPR

Lewis structures show the number and type of bonds between atoms in a molecule. All atoms are drawn in the same plane

(the paper). Do not show the shape of the molecule

VSEPR

The valence-shell electron-pair repulsion model (VSEPR) can be used to predict the shape of an ABn molecule when A is a main group element.

ABn

where A = central atom, main group elementB = outer atomsn = # of “B” atoms

Examples: CO2, H2O, BF3, NH3, CCl4

VSEPR

VSEPR counts the number of electron domains around the central atom where electrons are likely to be found and uses this number to predict the shape.

Electron domains: regions around the central atom where electrons are likely to be found.

Two types of electron domains are considered: bonding pairs of electrons nonbonding (lone) pairs of electrons

VSEPR

Bonding pairs of electrons: electrons that are shared between two atoms

Cl

Cl C Cl

Cl

Bonding pairs

Bonding pairs

VSEPR

Nonbonding (lone) pairs of electrons: electrons that are found principally

on one atom unshared electrons

H N H

H

Nonbonding pair

Ammonia (NH3) has 4 electron domains:

H N H

H

VSEPR

3 bonding pairs

1 nonbonding pair

VSEPR

Electron domains tend to repel each other regions of high electron density like charges repel each other

According to VSEPR, the best arrangement of a given number of electron domains is the one that minimizes repulsions between them.

Electron domain geometry: The arrangement of electron domains

around the central atom

Electron Domain Geometries

You must be able to draw these!

You must know these!

Electron Doman Geometries

AB

B

B

A

B

BB

B

Trigonal planar

Tetrahedral

A

B

BBB

B

A

B

BB

B

B

B

Trigonal bipyramidal octahedra

l

VSEPR

In order to determine the electron domain geometry: draw the Lewis structure count the total # of electron domains

multiple bonds = 1 electron domain determine the electron-domain

geometry

VSEPR

Example: Predict the electron domain geometry of IF5.

VSEPR

The electron domain geometry does NOT tell you the actual shape of the molecule.

Molecular geometry: the arrangement of the atoms in space

Molecular geometry is a consequence of electron-domain geometry.

VSEPR

Example: Identify and draw the molecular geometry of IF5.

VSEPR

Example: Identify and draw the electron domain and molecular geometries for I3

-.

Valence Bond Theory

Covalent bonds form when atoms share electrons Electron density is concentrated

between the nuclei

Two common theories are used to explain the properties of molecules in terms of the bonding that exists between atoms. Valence bond theory Molecular orbital theory

Valence Bond Theory

According to valence bond theory, an electron pair bond is formed between two atoms when a valence atomic orbital on one atom overlaps with a valence atomic orbital on another atom.

Overlap: share a region of space 2 electrons of opposite spin share

common space between the nuclei

Valence Bond Theory

Overlap can occur between two s orbitals, two p orbitals, or one s and one p orbital:

Overlap between two s orbitals

overlap

Valence Bond Theory

Overlap between one s and one p orbital

overlap

Valence Bond Theory

Overlap between two p orbitals

overlap

Valence Bond Theory

The previous bonds are called bonds. electron density is concentrated

symmetrically along an imaginary line connecting the two nuclei (internuclear axis)

Internuclear axis

Valence Bond Theory

bonds are formed when sideways overlap occurs between two p orbitals that are oriented perpendicular to the internuclear axis. Overlap regions lie both above and

below the internuclear axis

Valence Bond Theory

bonds are involved in the formation of double and triple bonds

Single bond:one bond

Double bond:one bond and one bond

Triple bond:one bond and two bonds

Valence Bond Theory

Sometimes, simple overlap between s and/or p orbitals can’t explain the actual shape or properties of compounds.

Consider a BeF2 molecule:

F Be F

Valence Bond Theory

F

1s22s22p5

F

1s22s22p5

Be

1s22s2

One unpaired e- One unpaired e-

No unpaired electrons

How can Be form covalent bonds with F if it doesn’t have unpaired electrons???

Valence Bond Theory

Be

1s22s12p1

2 unpaired electrons

Promote one of the 2s e- to a 2p orbital

Be

1s22s2

no unpaired electrons

Valence Bond Theory

F

1s22s22p5

F

1s22s22p5

Be

1s22s12p1

One unpaired e- One unpaired e-

2 unpaired electrons

Predicted overlaps:2s (Be) – 2p (F)2p (Be) – 2 p (F)

Implies two different kinds of Be – F bonds!

Valence Bond Theory

Both of the Be – F bonds in BeF2 are identical!

The solution: Mix the Be 2s orbital with one of the Be

2p orbitals to form two hybrid orbitalsatomic orbitals formed by mixing 2 or more atomic orbitals on an atom

Valence Bond Theory

Two sp hybrid orbitals are formed when one s and one p orbital are hybridized.

Valence Bond Theory

When Be forms covalent bonds with two F, each sp hybrid orbital on the Be atom overlaps with a p orbital located on a F atom.

Be

1s sp 2p

F

1s22s22p5

F

1s22s22p5

Valence Bond Theory

The BeF2 molecule is linear.

sp hybridization implies that the electron domain geometry around the central atom is linear.

Valence Bond Theory

Other hybrid orbitals involving s, p, and d orbitals are possible.

s, p two sp hybrid orbitals s, p, p three sp2 hybrid

orbitals s, p, p, p four sp3 hybrid orbitals s, p, p, p, d five sp3d hybrid

orbitals s, p, p, p, d, d six sp3d2 hybrid

orbitals Know these!!!

Valence Bond Theory

Each type of hybrid orbital is associated with a particular type of electron domain geometry. the same geometry that would be

predicted by VSEPR

You must know the electron domain geometry associated with each type of hybrid orbital.

Valence Bond Theory

Hybrid Electron DomainOrbital Set Geometry

sp linearsp2 trigonal planarsp3 tetrahedral

sp3d trigonal bipyramidalsp3d2 octahedral

Valence Bond Theory

Given the formula for an ABn compound, you must be able to: Draw the Lewis structure Determine the electron domain

geometry Sketch the electron domain geometry Identify the types of hybrid orbitals

present on the central atom

Valence Bond Theory

Example: What is the electron domain geometry of AlH4

-? What type of hybrid orbitals are present?

Valence Bond Theory

Example: What is the hybridization of the I atom in ICl2

-?