Electronegativity and Polarity Chemistry. A Strange Observation… AlCl 3 – Aluminum Chloride Is this an ionic or a covalent compound? How can we tell?

Electronegativity and Polarity

Chemistry

A Strange Observation…

• AlCl3 – Aluminum Chloride

• Is this an ionic or a covalent compound?• How can we tell?

• Despite consisting of a metal and a non-metal, aluminum chloride is actually a covalent compound…

• WHAT GIVES?

Don’t Believe Everything You Hear…

• Remember when we thought covalent bonds were so nice…

• Turns out, some of them aren’t so nice…

Reality Sets In…

• Covalent bonds fall into two categories depending on how the electrons are shared between the atoms.

• Polar covalent – unequal sharing of e-• Nonpolar covalent – equal sharing of

e-

So How Do You Tell the Difference?

I can tell you!!!

Really – just because I look crazy doesn’t mean I don’t know

my crap – use ELECTRONEGATIVITY

Your new boy – Linus Pauling

Electronegativity

What is it?

Electronegativity is the measure of an atom’s ability to attract electrons unto itself in a covalent bond

The higher the value, the more electronegative the element

ElectronegativityPauling’s electronegativity scale

F


F4.0


Li1.0

F4.0


Li1.0

Be1.5

F4.0


Li1.0

Be1.5

B2.0

F4.0


Li1.0

Be1.5

B2.0

C2.5

F4.0


Li1.0

Be1.5

B2.0

C2.5

N3.0

F4.0


Li1.0

Be1.5

B2.0

C2.5

N3.0

O3.5

F4.0


Li1.0

Be1.5

B2.0

C2.5

N3.0

O3.5

F4.0

Na0.9

Mg1.2

Al1.5

Si1.8

P2.1

S2.5

Cl3.0


H He

Li1.0

Be1.5

B2.0

C2.5

N3.0

O3.5

F4.0

Ne

Na0.9

Mg1.2

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ar


H2.1

He

Li1.0

Be1.5

B2.0

C2.5

N3.0

O3.5

F4.0

Ne

Na0.9

Mg1.2

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ar


H2.1

He-

Li1.0

Be1.5

B2.0

C2.5

N3.0

O3.5

F4.0

Ne-

Na0.9

Mg1.2

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ar-

The following chart may be used to determine the type of bond in a compound.

Electronegativity Type of Type ofDifference Bond Sharing

0.0 – 0.4 Nonpolar Equal0.5 - 1.9 Polar Unequal>1.9 Ionic None, transfer

H FIf two different atoms share a bond,

one will pull more stronglyon the bonding electrons.

H F

H F

H F

H F

H F

H F

H F

H F

H FThe bonding electrons carry negative

charge.

H FThe closer they get to the fluorine atom,

the more negative it gets.The farther they get from the hydrogen,

the more positive it gets.

H F+

_

But the charge is only partial.Hydrogen has not lost the electronsas in the formation of an ion.

H FThere is an unequal sharing of electrons.

H FThe partial charge is denoted by a + or – and the Greek letter delta, d

d – d +

A polar bond is a bond in which thebonding electron pair is shared unequally.

A polar molecule is a moleculewith regions of partial negative ( d –)

and partial positive ( d +) charge.

The degree of sharing (equal to unequal)is determined by the electronegativitydifference between the two atoms.

F FShared pair

(bonding pair)

F FWhat is the electronegativity of each atom?

F FBoth atoms have equal pull,

so the bonding pair is shared equally.

Non-polar covalent bond

Example:Determine the type of bond in hydrogen chloride.DEN = |ENH – ENCl|

= |2.1 - 3.0|

DEN = 0.9

Polar covalent bond

d+ d-

Rule:Lower EN d+Higher EN d-

H Cld+d-

Example:Determine the type of bond in water.

DEN = |ENH – ENO|

= |2.1 - 3.5|

DEN = 1.4

Polar covalent bond

d+ d-


d+d-H O

H

Example:Determine the type of bond in ammonia.

DEN = |ENN – ENH|

= |3.0 - 2.1|

DEN = 0.9

Polar covalent bond

d+d-


d+d-N

HH

H

Example:Determine the type of bond in methane.

DEN = |ENC – ENH|

= |2.5 - 2.1|

DEN = 0.4

Nonpolar covalent bond

C

HH

H

H

No shift of e-No polesNo charges

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Electronegativity and Polarity Chemistry. A Strange Observation… AlCl 3 – Aluminum Chloride Is this an ionic or a covalent compound? How can we tell?