chemical bonding compounds are formed from chemically bound atoms or ions. bonding involves only the...

Chemical Bonding

• Compounds are formed from chemically bound atoms or ions.

• Bonding involves only the valence electrons.

Chemical Bonding

• Ionic Compounds

– Ionic Radii

– Lattice Energy

• Molecular Compounds

– Covalent bonds

– Bond Order

– Bond Strength

– Lewis Structures

Lewis Symbols

Lewis symbols show the valence electrons as dots arranged around the atomic symbol.

hydrogen:

sodium:

chlorine:

Na

H

Cl

The Octet Rule

Atoms tend to gain, lose, or share electrons until they have eight valence electrons.

Ionic and Molecular Compounds• Formation of sodium chloride:

• Formation of hydrogen chloride:

A metal and a nonmetal transfer electrons to form an ionic compound. Two nonmetals share electrons to form a molecular compound.

Na + Na+ [ ]Cl

Cl

H + Cl

Cl

H

Ionic CompoundsIonic compounds consist of a lattice of positive and negative ions.

NaCl:

Ionic Bonds

An ionic bond is simply the electrostatic attraction between opposite charges.

Ions with charges Q1 and Q2:

The potential energy is given by:

d

Q1

Q2

dQQ

E 21

Ionic Sizes

The Lattice Energy

• The lattice energy is the enthalpy change required to separate one mole of an ionic compound into its ions:

NaCl(s) Na+(g) + Cl(g) H = 788 kJ

• Lattice energy increases with increasing ionic charges and decreasing ionic sizes.

KCl(s) K+(g) + Cl(g) H = 701 kJ

MgCl2(s) Mg+2(g) + 2 Cl(g) H = 3795 kJ

The Born-Haber CycleThe Lattice energy can be calculated with Hess’s law and the following steps:

Hf(NaCl,s)

Hf(Na,g)

Hf(Cl,g)

I1(Na)E(Cl)

Latticeenergy

H

NaCl(s)

Na(s) + ½Cl2(g)

Na+(g) + Cl-(g)

Na(g) + Cl(g)

Na(g) + ½Cl2(g)

Na+(g) + e- + Cl(g)

Estimating Lattice EnergyArrange with increasing lattice energy:

KCl

NaF

MgO

KBr

NaCl 788 kJ

671 kJ

3795 kJ

910 kJ

701 kJ

d

K+ Cl

K+ Br

d

dQQ

E 21

Molecular CompoundsThe simplest molecule is H2:

The pair of shared electrons constitutes a covalent bond.

Increased electron density draws nuclei together

Lewis Structures• Covalent bonding in a molecule is repre-

sented by a Lewis structure.

• A valid Lewis structure should have an octet for each atom except hydrogen.

H H+

ClCl

H2: or H H

Cl2: Cl

Cl

+

or Cl Cl

H H

Bondingelectrons

Nonbonding electrons

Lewis StructuresDraw Lewis structures for:

HF:

H2O:

NH3:

CH4:

H F

or H F

H O H

or H O H

H N HH

or H N H

H

H C HH

H

or H C H

H

H

Double and Triple Bonds• Atoms can share four electrons to form a

double bond or six electrons to form a triple bond.

• The number of electron pairs is thebond order.

O2:

N2:

=O O

N N

Electronegativity• Polarity refers to a separation of positive and

negative charge. In a nonpolar bond, the bonding electrons are shared equally:

HCl:

• In a polar bond, electrons are shared unequally because of the difference in Zeff.

H2,Cl2:

Electronegativity• Electronegativity refers to the ability of an

atom in a molecule to attract shared electrons.

• The Pauling scale of electro-

negativity:

QuickTime Movie

Bond PolarityA polar bond can be pictured using partial charges:

= 0.9

ElectronegativityDifference Bond Type

0 - 0.5 Nonpolar

0.5 - 2.0 Polar

2.0 Ionic

2.1 3.0

+

H Cl

Drawing Lewis Structures• Sum the valence electrons from all atoms.

Add one for each negative charge and subtract one for each positive charge.

• Draw a skeleton structure with atoms attached by single bonds.

• Complete the octets of atoms bound to the central atom.

• Place extra electrons on the central atom.

• If the central atom doesn’t have an octet, try forming multiple bonds.

Drawing Lewis Structures

COCl2 24 ve’s

HOCl 14 ve’s

ClO3 26 ve’s

CH3OH 14 ve’s

Cl C Cl

O

H O Cl

O Cl O

O

H C O H

H

H