Electonegativity, Polar

Bonds, and Polar Molecules

Some Definitions

• Electronegativity: the ability of an

atom to attract bonding electrons to

itself.

• Intramolecular forces: the

attractive force between atoms and

ions within a compound.

• Intermolecular forces: the

attractive force between molecules.

Polar Covalent Bonds

• When a chemical bond is formed, it

is not always exclusively ionic or

covalent.

• In the case of shared electrons

between two identical atoms, the

electrons are shared equally.

• However, this is not the case for a

compound like hydrogen chloride,

where electrons are shared between

two different elements.

• In this situation, the sharing is

unequal, as the bonding electrons

spend more time near one atom

than near the other.

• Due to this unequal sharing of

electrons, one atom will have a

slightly positive charge while the

other will have a slightly negative

charge.

• We indicate these slight charges by

+ and -. (The Greek letter delta, ,

indicates “a small difference.”)

• The bond is somewhere between an

ionic bond and a covalent bond and

is called a polar covalent bond.

• Does HCl have a polar covalent

bond? NH3?

• To determine if the bond is ionic,

covalent or polar covalent, measure

the difference in electronegativity of

the atoms.

• EN > 1.7 - ionic bond

• EN < 1.7 - covalent bond

• What type of bond do the following

compounds have:

– CsF, HF, and F2

Polar Molecules

• A molecule that is slightly positively

charged at one end and slightly

negatively charged at the other

because of electronegativity

differences is classified as a polar

molecule.

• Not all molecules containing polar

covalent bonds are polar molecules.

• Take a look at CH4 and HCl.

• polar molecule: a molecule in

which the uneven distribution of

electrons results in a positive charge

at one end and a negative charge at

the other end

• non-polar molecule: a molecule in

which the electrons are equally

distributed among the atoms,

resulting in no localized charges

Intermolecular Forces

• There are three kinds of

intermolecular forces.

• Two of these are classified as van

der Waals forces.

Dipole-Dipole Forces

• Dipole-dipole forces: an attractive

force acting between polar

molecules

• As a polar

molecule, hydrogen

fluoride has a

negatively charged

fluoride end and

a positively charged

hydrogen end.

• When two hydrogen fluoride

molecules are next to one another,

the positive end of one molecule is

attracted to the negative end of the

next molecule.

dipole-dipole force

London Dispersion Forces

• London dispersion forces: an

attractive force acting between all

molecules and unbonded atoms,

including nonpolar molecules.

Hydrogen Bonds

• Water, a polar molecule, consists of one

atom of oxygen bound by single covalent

bonds to two hydrogen atoms.

• Its structure is simple, but water exhibits

some rather unusual properties:

– higher than expected melting and boiling

points,

– high vapour pressure,

– high surface tension, and

– the ability to dissolve a large number of

substances.

• To explain these properties, we must

consider the intermolecular forces that

exist between water molecules.

• As a result of the large difference in

electronegativity between the hydrogen

and oxygen, the O-H bonds in a

molecule of water are highly polar

covalent.

• As a result, the hydrogen atoms of one

water molecule exert a strong force of

attraction on the oxygen atom of

neighbouring water molecules.

• This is known as a hydrogen bond.

• Hydrogen bonds occur among highly

polar molecules containing F-H, O-H and

N-H bonds.

• Although a hydrogen bond is similar to a

dipole-dipole force, it is stronger than any

of the van der Waals forces.

Special Properties of Water

• Unusually high Melting and Boiling

Points

• Low Density of Ice

• High Surface Tension (a phenomenon

that leads to the formation of a skin-like film on

the surface of a liquid)

• High Specific Heat Capacity (the

quantity of the energy that a certain mass of a

substance can absorb, and warm up by 1ºC)

Hydrogen Bonds in

Biochemistry

• Hydrogen bonds play a significant

role in determining the shape and

function of large, biologically

important molecules.

• Proteins consist of hundreds or even

thousands of atoms. The chain of atoms

folds into very specific three-

dimensional structures because of

attractions between different parts of the

chain.

• A DNA molecule is

made up of two long

chains of structures

called nucleotides.

• Hydrogen bonds

readily form

between the two

helixes, holding

them together like

the rungs of the

ladder