Polarity

Polar Covalent

• Remember ionic bonds transfer electrons, while covalent bonds share electrons.

• All atoms do not attract their electrons in a bond to the same degree.

• One can determine the degree to which electrons are transferred or shared using the electronegativity difference

Polar Covalent• Remember…electronegativity is a measure

of an atom’s ability to attract the pair of electrons it shares with another atom.

• In general, metals have lower electronegativities than non metals

• Each atom’s electronegativities has been determined by experiences and can be found in your textbook on pg 71.

EN Differences

• To find ΔEN, subtract the EN of 2 atoms. EN must be a real number. (ie. No negatives!)

• If the ΔEN is:• 3.3-1.7 = an ionic bond• 1.6-0.5 = polar covalent• 0.5-0.0 = non polar covalent

• Determine the classification of these bonds:A) B & P B) S & O C) Li & O D) Mg & N

=0.15 non polar =0.86 polar =2.46 ionic =1.73 ionic

Polar vs Nonpolar CovalentNonpolar Covalent Bonds

• Between two atoms with similar EN

• Electrons are shared equally between the two

• Nuclei from each atom are attracting the electrons with equal force

• There is little or no positive & negative charges present

Polar Covalent Bonds• Between two atoms with

moderate difference in EN• Electrons are not shared

equally• Results in a localized

negative and positive charge• The atom with higher EN

will be slightly negative and other will be slightly positive

• Bigger the difference, the more polar the bond

Dipole Moments

• In polar molecules, we indicate their positive or negative partial charges using δ- and δ+ and their polarity or “dipole moment”

Hδ+ Fδ-

Hδ+ Hδ+

Oδ-

• Dipole Dipole moments occur when there is an instant in time when the electrons are distinctly unequally shared.

Polar Molecules

• In addition to bonds being polar or non polar, so can molecules based on their shape.

• If polar bonds are present, to determine if a molecule is polar, we look at its shape and whether or not it is symmetrical.

Polar Molecules

Polar Molecule

Water is polar because the lone pair pushes the bonded pair together and so the molecule is not symmetrical (Asymmetrical)

Non Polar Molecule

All the hydrogens in methane are pulling at the same strength (same EN) so the molecule is symmetrical

Which molecules are polar? (Indicate partial charges for polar molecules)

a) NI3 b) SiCl4 c) CHCl3

ΔEN = 0.38= non-polar Bonds

δ+δ-

Asymetrical, but NO POLAR BONDS! Non-polar Molecule

ΔEN = 1.26 = Polar Bonds

δ-

δ-

δ-

Symmetrical, non-polar molecule

ΔENC-Cl = 0.61 = Polar BondsΔENH-C = 0.35 = non-polar Bonds

δ+

δ-

δ-

δ-

Asymetrical, with polar bonds, polar molecule

Polar vs Nonpolar CharacteristicsMolecule

TypeMolecule

ShapeForces of Attraction

Relative Melting/

Boiling Point

Soluble in H2O

Polar Less symmetry

Relatively Strong

Relatively High

Yes

Non Polar High Symmetry

Weak Very low No