CovalentBonding

Bonding models for methane, CH4. Models are NOT reality. Each has its own strengths and limitations.

Polar-Covalent bonds

Nonpolar-Covalent bonds

Covalent Bonds

Electrons are unequally shared Electronegativity difference between .3

and 1.7 Electrons are equally shared Electronegativity difference of 0 to 0.3

Covalent Bonding Forces

Electron – electron repulsive forces Proton – proton repulsive forces Electron – proton attractive forces

Bond Length and Energy

Bond Bond type

Bond length (pm)

Bond Energy(kJ/mol)

C - C Single 154 347C = C Double 134 614C C Triple 120 839C - O Single 143 358C = O Double 123 745C - N Single 143 305

C = N Double 138 615C N Triple 116 891

Bonds between elements become shorter and stronger as multiplicity increases.

Bond Energy and Enthalpy

bondsbroken bonds formedH D D

D = Bond energy per mole of bonds

Energy required Energy released

Breaking bonds always requires energy Breaking = endothermic

Forming bonds always releases energyForming = exothermic

The Octet RuleCombinations of elements tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.

Monatomic chlorine Diatomic chlorine

The Octet Rule and Covalent Compounds

Covalent compounds tend to form so that each atom, by sharing electrons, has an octet of electrons in its highest occupied energy level.

Covalent compounds involve atoms of nonmetals only.

The term “molecule” is used exclusively for covalent bonding

The Octet Rule: The Diatomic Fluorine MoleculeF

F

1s

1s 2s

2s 2p

2p

Each has seven

valence electrons

F F

The Octet Rule: The Diatomic Oxygen MoleculeO

O

1s

1s 2s

2s 2p

2p

Each has six valence

electrons

O O

The Octet Rule: The Diatomic Nitrogen MoleculeN

N

1s

1s 2s

2s 2p

2p

Each has five valence

electrons

N N

Lewis structures show how valence electrons are arranged among atoms in a molecule.

Lewis structures Reflect the central idea that stability of a compound relates to noble gas electron configuration.

Shared electrons pairs are covalent bonds and can be represented by two dots (:) or by a single line ( - )

Lewis Structures

Comments About the Octet Rule 2nd row elements C, N, O, F observe the

octet rule (HONC rule as well). 2nd row elements B and Be often have

fewer than 8 electrons around themselves - they are very reactive.

3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals.

When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.

Show how valence electrons are arranged among atoms in a molecule.Reflect the central idea that stability of a compound relates to noble gas electron configuration.

Lewis Structures

The HONC RuleHydrogen (and Halogens) form one covalent bondOxygen (and sulfur) form two covalent bonds

One double bond, or two single bondsNitrogen (and phosphorus) form three covalent bonds

One triple bond, or three single bonds, or one double bond and a single bond

Carbon (and silicon) form four covalent bonds.

Two double bonds, or four single bonds, or a triple and a single, or a double and two singles

CH

H

H

Cl

..

....

..

Completing a Lewis Structure -CH3Cl

Add up available valence electrons: C = 4, H = (3)(1), Cl = 7 Total = 14

Join peripheral atoms

to the central atom with electron pairs.

Complete octets on

atoms other than hydrogen with remaining electrons

Make the atom wanting the most bonds central

..

..

..

Multiple Covalent Bonds:Double bonds

C C

H

H

H

H

Two pairs of shared electrons

C C

H

H

H

H

Ethene

Multiple Covalent Bonds:Triple bonds

C C HH

Three pairs of shared electronsEthyne

C C HH

Acetic Acid

HO

O

HH

HC C

Two electrons (one bond) per hydrogen

Eight electrons (four bonds) per carbon

Eight electrons (two bonds, two unshared pairs) per oxygen