Covalent Bonding

Covalent Bonding

• Electrons are shared between two nonmetals

• Weaker attractive force than ionic bonding

Properties of Molecular Compounds

State at SATP Typically gases & liquids, but some solids

Structure Variable (waxy to brittle)

Melting & Boiling Points Low

Electrical Conductivity Nonelectrolyte

Solubility in Water Variable (low to high)

Confused?? Don’t worry…more answers to come

Covalent Bonding

• Formation of hydrogen chloride:

H + Cl

Cl

H H - Cl

Covalent bond, shared electrons

Lone pairs, valence electrons not involved in covalent bond

Structural Formula: H-Cl (lone pairs are not drawn)

Lewis Structures

H H+

ClCl

H2: or H H

Cl2: Cl

Cl

+

or Cl Cl

H H

Structural Formula: Cl-Cl

Double and Triple Bonds• Atoms can share 4 electrons to form a double bond

or 6 electrons to form a triple bond.

• The number of shared electron pairs (covalent bonds) that an atom can form is thebonding capacity.

O2:

N2:

=O O

N N

Multiple Covalent Bonds

N••

••

•N N

•

••

•

•

••

•N••

••

•

N N•

•• ••

•N N •

•

••

Multiple Covalent Bonds

C••

••

O••

• •

• •O••

• •

• •CO O

•

••

•••

••

••

••

•CO O ••

•••

••

••

••CO O

••

••

••

••

Drawing Lewis Structures1. Arrange the element symbols.

• Central atoms are generally those with the highest bonding capacity.

• Carbon atoms are always central atoms

• Hydrogen atoms are always peripheral atoms

2. Add up the number of valence electrons from all atoms.

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

4. Complete the octets of peripheral atoms.

5. Place extra electrons on the central atom.

6. If the central atom doesn’t have an octet, try forming multiple bonds by moving lone pairs.

Practice Problems

• Examples– Methanal CH2O

– Chlorate (polyatomic ion)

• p.77-81 #1-15

Strategy for Writing Lewis

Structures

Draw Lewis structures and the structural formula 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

Molecular formula

Atom placement

Sum of valence e-

Remaining valence e-

Lewis structure

For NF3

NFF

F

N = 5e-

F = 7e- X 3 = 21e-

Total 26e-

:

: :

::

: :

:: :

Zero: NF3 is uncharged

SAMPLE PROBLEM 10.1 Writing Lewis Structures for Molecules with One Central Atom

SOLUTION:

PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds responsible for the depletion of stratospheric ozone.

Step 1: Carbon has the highest bonding capacity and is the central atom. The other atoms are placed around it.

C

Steps 2-4: C has 4 valence e-, Cl and F each have 7. The

sum is 4 + 4(7) = 32 valence e-.

Cl

Cl F

F

C

Cl

Cl F

FMake bonds and fill in remaining valence electrons placing 8e- around each atom.

:

::

::

:

:

::

: ::

SAMPLE PROBLEM 10.2 Writing Lewis Structure for Molecules with More than One Central Atom

PROBLEM: Write the Lewis structure for methanol (molecular formula CH4O), an important industrial alcohol that is being used as a gasoline alternative in car engines.

SOLUTION: Hydrogen can have only one bond so C and O must be next to each other with H filling in the bonds.

There are 4(1) + 4 + 6 = 14 valence e-.

C has 4 bonds and O has 2. O has 2 pair of nonbonding e-.

C O H

H

H

H

::

SAMPLE PROBLEM 10.3 Writing Lewis Structures for Molecules with Multiple Bonds.

PLAN:

SOLUTION:

PROBLEM: Write Lewis structures for the following:

(a) Ethylene (C2H4), the most important reactant in the manufacture of polymers

(b) Nitrogen (N2), the most abundant atmospheric gas

For molecules with multiple bonds, there is a Step 5 which follows the other steps in Lewis structure construction. If a central atom does not have 8e-, an octet, then e- can be moved in to form a multiple bond.

(a) There are 2(4) + 4(1) = 12 valence e-. H can have only one bond per atom.

CCH

H H

H

:

CCH

H H

H

(b) N2 has 2(5) = 10 valence e-. Therefore a triple bond is required to make the octet around each N.

N

:

N

:

. .

..

N

:

N

:

. . N

:

N

:

Drawing Lewis Structures

COCl2 24 ve’s

HOCl 14 ve’s

CH3OH 14 ve’s

Cl C Cl

O

H O Cl

H C O H

H

H