Electronegativity and Polarity Chemistry

A Strange Observation… AlCl 3 – 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

Electronegativity Pauling’s electronegativity scale F

Electronegativity F 4.0

Electronegativity Pauling’s electronegativity scale Li 1.0 F 4.0

Electronegativity Pauling’s electronegativity scale Li 1.0 Be 1.5 F 4.0

Electronegativity Pauling’s electronegativity scale Li 1.0 Be 1.5 B 2.0 F 4.0

Electronegativity Pauling’s electronegativity scale Li 1.0 Be 1.5 B 2.0 C 2.5 F 4.0

Electronegativity Pauling’s electronegativity scale Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 F 4.0

Electronegativity Pauling’s electronegativity scale Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 O 3.5 F 4.0

Electronegativity Pauling’s electronegativity scale Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 O 3.5 F 4.0 Na 0.9 Mg 1.2 Al 1.5 Si 1.8 P 2.1 S 2.5 Cl 3.0

Electronegativity Pauling’s electronegativity scale HHe Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 O 3.5 F 4.0 Ne Na 0.9 Mg 1.2 Al 1.5 Si 1.8 P 2.1 S 2.5 Cl 3.0 Ar

Electronegativity Pauling’s electronegativity scale H 2.1 He Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 O 3.5 F 4.0 Ne Na 0.9 Mg 1.2 Al 1.5 Si 1.8 P 2.1 S 2.5 Cl 3.0 Ar

Electronegativity Pauling’s electronegativity scale H 2.1 He - Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 O 3.5 F 4.0 Ne - Na 0.9 Mg 1.2 Al 1.5 Si 1.8 P 2.1 S 2.5 Cl 3.0 Ar -

The following chart may be used to determine the type of bond in a compound. ElectronegativityType ofType of DifferenceBond Sharing 0.0 – 0.4NonpolarEqual PolarUnequal >1.9IonicNone, transfer

H F If two different atoms share a bond, one will pull more strongly on the bonding electrons.

H F

H F

H F

H F

H F

H F

H F

H F

H F The bonding electrons carry negative charge.

H F The 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 electrons as in the formation of an ion.

H F There is an unequal sharing of electrons.

H F The partial charge is denoted by a + or – and the Greek letter delta,   –  +

A polar bond is a bond in which the bonding electron pair is shared unequally. A polar molecule is a molecule with regions of partial negative (  –) and partial positive (  +) charge.

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

FF Shared pair (bonding pair)

FF What is the electronegativity of each atom?

FF Both atoms have equal pull, so the bonding pair is shared equally. Non-polar covalent bond

Example: Determine the type of bond in hydrogen chloride.  EN=|EN H – EN Cl | =| |  EN =0.9 Polar covalent bond ++ -- Rule: Lower EN  + Higher EN  - H Cl ++ --

Example: Determine the type of bond in water.  EN=|EN H – EN O | =| |  EN =1.4 Polar covalent bond ++ -- Rule: Lower EN  + Higher EN  - ++ -- H O H

Example: Determine the type of bond in ammonia.  EN=|EN N – EN H | =| |  EN =0.9 Polar covalent bond ++ -- Rule: Lower EN  + Higher EN  - ++ -- N H H H

Example: Determine the type of bond in methane.  EN=|EN C – EN H | =| |  EN =0.4 Nonpolar covalent bond C H H H H No shift of e- No poles No charges