Intermolecular Forces-forces between molecules

Three (3) Types:• Instantaneous dipole-induced dipole forces (van der Waals’ forces)

• Dipole-dipole interaction• Hydrogen bonding

Structures containing covalent bonds

1.Simple molecules2.Giant Covalent Lattices (covalent networks)

3.Molecular Solids

1. Simple moleculesbonding

Atoms are joined together within the molecule by covalent bonds.

electrical

Don’t conduct electricity as they have no mobile ions or electrons.

solubility

Tend to be more soluble in organic solvents than in water; some are hydrolysed

boiling pt

Low - the forces between molecules (intermolecular forces) are weakknown as van der Waals forces - see below

boiling points• Attractions between molecules increases as the molecules get more electrons.

• as forces are weak, little energy is required to separate molecules from each other so boiling points are low

Instantaneous dipole-induced dipole forces (van der Waals’ forces)

• electrons in atoms or molecules are moving at high speeds in orbitals

• it is possible for more electrons to be on one side of an atom/molecule

• this forms a dipole where one side is slightly negative; the other slightly positive

• a dipole in one atom/molecule can then induce a dipole in a neighbouring one

Instantaneous dipole-induced dipole forces (van der Waals’ forces)

For an instant there are more electrons on the right side of the atom - a dipole is formed

The dipole on one atom induces (causes) dipoles to form on other atoms. The atoms are then attracted to each other by their oppositely charged ends

Instantaneous dipole-induced dipole forces (van der Waals’ forces)

Result • atoms/molecules become attracted to each other

• this makes them harder to separate and gives them higher boiling points

Trends • the more electrons there are in an atom/molecule the bigger the effect

Note: instantaneous dipole–induced dipole forces are also

known as London dispersion forces, dispersion forces,

London forces, or loosely van der Waals forces

Examples• layers in graphite are held together by weak van der Waals’s forces so it is soft

• the boiling point of noble gases increases down the group

HOWEVER Some molecules have boiling points much higher than one would expect!

Electronegativity ‘The ability of an atom to attract the pair of electrons in a covalent bond to itself.’

more electronegative atomless electronegative

atom a dipole(two poles)

Non-polar bond• similar atoms have the same electronegativity

• they will both pull on the electrons to the same extent

• the electrons will be equally shared

Polar bond• different atoms have different electronegativities• one will pull the electron pair closer to its end• it will be slightly more negative than average, d-• other will be slightly less negative, or more positive, d+

• a dipole is induced and the bond is said to be polar

• the greater the electronegativity difference, the greater the bond polarity.

Pauling Scale• a scale for measuring electronegativity

• values increase across periods• values decrease down groups• fluorine has the highest value

Q.6

1-15

Polar and Nonpolar Covalent BondsAlthough all covalent bonds involve sharing of electrons, they differ widely in the degree of sharing

We divide covalent bonds into:

• nonpolar covalent bonds• polar covalent bonds

D ifference in ElectronegativityBetw een Bonded Atom s Type of BondLess than 0.50.5 to 1.9G reater than 1.9

N onpolar covalentPolar covalentIons form

1-16

Polar and Nonpolar Molecules To determine if a molecule is polar, we need to determine • if the molecule has polar bonds• the arrangement of these bonds in space

Molecular dipole moment (): the vector sum of the individual bond dipole moments in a molecule• reported in debyes (D)

1-17

Polar and Nonpolar Covalent Bonds

• an example of a polar covalent bond is that of

H-Cl• the difference in electronegativity between Cl and H is 3.0 - 2.1 = 0.9

• we show polarity by using the symbols d+ and d-, or by using an arrow with the arrowhead pointing toward the negative end and a plus sign on the tail of the arrow at the positive endH Cl

d+ d-H Cl

1-18

Polar and Nonpolar Molecules these molecules have polar bonds, but each has a zero dipole moment , thus are non-polar.

O C O

Carbon dioxide = 0 D

BF

FF

Boron trifluoride = 0 D

C

Cl

ClClClCarbon tetrachloride

= 0 D

have no net dipole moment

1-19

Polar and Nonpolar Molecules these molecules have polar bonds and are polar molecules

NH

HH

OH H

W ater = 1.85D

Am m onia = 1.47D

directionof dipolem om ent

directionof dipolem om ent

Has net dipole moment

1-20

Polar and Nonpolar Molecules• formaldehyde has polar bonds and is a polar molecule

Form aldehyde = 2.33 D

directionof dipolem om ent H HC

O

Experiment• place a liquid in a burette• allow a narrow stream to run out

• place a charged rod next to the flow

• polar molecules will be attracted

Q.7