COVALENT BONDING

When atoms of nonmetal bond with each other, the atoms share the electrons to lower the energy.

A chemical bonding that result when atoms share electrons is

called covalent bonding or covalent bond.

Lewis symbols are often used to keep track of electrons in a covalent bond. Two electrons shared between two atoms are shown as a pair of dots (:) or dashes (—) placed between the symbols of the two bonded atoms. The formation of H2 from hydrogen atoms, for example, can be depicted as follows.

H. + H. H H:H. + H. H H:

H. + H. H H

H H:H H: atau H H

H. + H. H H:H. + H. H H:

H. + H. H H

or

H H:H H: atau H H

is the Lewis structure of H2O

Since electrons are shared, in the H2 molecule, each H atom is considered to have two electrons.

H H:

Circle indicates that there aretwo electrons in the valenceshell of each hydrogen atom

Shared electron pair

Hydrogen, with just one electron in its K shell, can complete

valence shell by obtaining a share of just one electron from another

atom such as given in the following example.

H F.

Circle indicates that there aretwo electrons in the valenceshell of hydrogen atom and eightelectrons in the valence shell offluorine atom

:

::

Shared electron pair

.

Sharing electrons causes in the HF molecule, hydrogen atom is

considered to have two electrons, while fluorine atoms is

considered to have eight electrons.

Many nonhydrogen atoms share one or more electrons with the

same or different atom to attain eight electrons in its valence

shell. These atoms obey the octet rule such as given in the

following examples.

+ F . :+ . :

or

..

..: F.... :F ..

..

..: F....:

+ F .+....: F

..

..:F.....: F

..

..:

+ F.+ .

or

..

..: F....:Cl.

..

..: Cl....: .

+ F.+....: F

..

..:Cl.....: Cl

..

..:

or

:

C HH

H C H

H

H

or

:

HH N

:

H

H

H

HH

orO::

HH O

:

H

H..

.. ....

..

or

:

C HH C H

H

H

orN

:

HH N

:

H

H

H

HH

orO::

HH O

:

H

H..

.. ....

..

H

... .........

O:

:: C: O:

::: or O

:: C O

::

C O.: or C O: :: : :.

The Lewis structures such as given above are

representation of a molecule that shows valence electrons as

dots.

The covalent bond can also be formed between atoms in polyatomic ions such as hydronium ion, H3O+,

ammonium ion, NH4+, and hydoxide ion, OHˉ. Their Lewis

structures are give in the following pictures.

orN HH

H N H

H

H

or

.

O

:

HH O

:

H

H

H + H +

H+

H+

:O

:: H

-or O:

:: H

-.

.. ... .

. .

. ... ..

There are two kinds of pair of electron in the molecule or polyatomic ion, namely bonding pair (BP) and nonbonding pair or lone pair (LP) as shown in the following figure.

:orO::.

HH O

:

H

H

LP

BP

BP

. . .

Some simple rule about covalent bond and the octet rule

(1) Elements in the period 2 having fewer than 4 electrons (lithium, beryllium, and boron) are not necessary to obey the octet rule such as shown in the following examples.

Be HH

C H

H

H

Li

F....: F

..

..:B

F..: :

(2) The maximum number of electrons in the valence shell of elements in the period 2 is eight. Some examples are given in the following figures.

F....: F

..

..:B

F..: :

F..

: :

F....: F

..

..:Be

F..: :

F..

: : 2-

-

(3) The elements in the period 3 or more are not necessary to obey the octet rule. In a molecule and polyatomic ion, the number of electrons in the valence shell of these atoms may be more than eight electrons such as

shown in the following examples.

F....:

F..: :

F..

: :

PF

FS

F

F

F

F

I

F

FF

PF5 SF6 IF7

F..: : F..: :

F..

: : F..

: :

F....:

..

.. ....

..

..

....

....

....

....

.... ....

..

....

....

..

..

.... F..

....

In simple molecules or simple polyatomic ions there is an

atom acting as a central atom, whereas all atoms bond to the

central atom are called substituents.

The central atom is bonded to two or more other atoms.

CCl

ClCl

Cl

BF

F

F F

SubstituentsCentralAtom

CentralAtom

CCl4 BF4

Atoms bond only to one atom are also called terminal atom.

CCl

ClCl

Cl

BF

F

F F

Terminal atomCentralAtom

CentralAtom

The central atom is usually written first in the formula of

molecule or polyatomic ion such as in BeCl2, CH4, NH3,

NO3ˉ, ClO4ˉ, and NH4+.

H atom is always terminal atom.

Single and multiple covalent bonds

O:

:: C: O:

::: or O

:: C O

::

C O.: or C O: :: : :.

Double covalent bond

Triple covalent bond

C C

H

H

H

H

H

H

C C

H

H

H

H C CH H

ethane ethene or ethylene ethyne or acetylene

Single covalent

bond

Triple covalent

bond

Double covalent

bond

C C

H

H

H

H

H

H

C C

H

H

H

H C CH H

ethane ethene or ethylene ethyne or acetylene

bond

1 bondand

1 bond

1 bondand

2 bond

C C

H

H

H

H

H

H

ethane

Covalentbondlength

Covalentbond

length

The distance between the two nuclei that are held by a chemical bond is

called covalent bond length.

The net number of pairs of bonding electrons between two covalently bonded atoms

is called bond order.

C C

H

H

H

H

H

H

C C

H

H

H

H C CH H

ethane ethene or ethylene ethyne or acetylene

Bond order = 1 Bond order = 2 Bond order = 3

The covalent bond length between two atoms decreases with increasing the bond order.

C C

H

H

H

H

H

H

C C

H

H

H

H C CH H

Bond order = 1 Bond order = 2 Bond order = 3

The bond length of C C > C C > C C

Coordinate covalent bond The electrons used in the formation of a covalent bond may come from one atom such as shown in

the following example

N

H

H

H

N

H

H

H

H+ H or

+

.. .. N

H

H

H

H

+

This kind of bond is called a coordinate covalent bond or dative covalent bond.

N

H

H

H

+ B

F

F

F

N

H

H

H

..

..

..

..

.. ..

..

.. ..

B

F

F

F

..

..

..

..

.. ..

..

.. ..

..

The coordinate covalent bond is also formed when

ammonia reacts with boron trifluoride

Nonpolar and polar covalent bond

A covalent bond involves the sharing of at least one pair of electrons between two

atoms. The covalent bonds in H2, F2, O2, and N2

O O N NH H F F

are called nonpolar covalent bonds since the bonded atoms have

the same electronegativity.

The covalent bonds in HF, H2O, NH3, and CCl4

H F

HO

H

C

Cl

Cl

Cl

ClH N H

H

are called polar covalent bonds since the bonded atoms have different electronegativities.

In polar covalent bond the electron pair is pulled toward the more electronegative atom. In H–F bond, since the F atom is

more electronegative than the H atom, the bonding electron pair is pulled toward F. The bonding electron pair spends most of

their time around the F atom. This induces partial positive charge (+) on H

atom and partial negative (-) charge on F atom.

H F + _

Ionic or covalent bond?

“When is a bond considered to be ionic or covalent?”

Bond between two atoms whose electronegativities differ by at least 1.7

unit (in Pauling scale) may be considered to be ionic bond, and bond between two atoms whose electronegativities differ by less than 1.7 unit may be considered to be

covalent bond.