Intermolecular Forces (IMF)

Section 6.5 p. 20311/20/14

IMF General Info

• The impact that intermolecular forces have on molecules can be seen on how boiling points are impacted.– Reference Table 7 p. 204

• Generally, these forces are weaker than actual chemical bonds, but are still important in examining the properties of molecules.

Dipoles and Polarity

• The strongest IMF exists between polar molecules.

• Dipole = equal but opposite charges separated by a short distance.– Is represented by an arrow with its head pointing

to the negative pole.– Ex: H - Cl

Dipole and Polarity

• Polarity of diatomic molecules is determined by 1 Bond.

• Polarity for molecules with more than 1 Bond is determined by orientation of each bond (aka molecular geometry / shape).– Figure 26 p. 205

• Ex: CO2 -> bond polarities between each C = O cancel each other out bc the extend equally and symmetrically in opposite directions.

London Dispersion Forces

• Even Noble gas molecules that are nonpolar experience weak IMF attraction.

• This is because electrons are in constant motion and may cause the electron distribution to be uneven within an atom / molecule.

• When this happens instantaneous dipoles are formed = “London dispersion forces.”

London Dispersion Forces (LDFs)

• These forces increase with the number of electrons present.

• So there is a trend: – LDFs increase with increasing atomic #– Ex: Low boiling point of Noble Gases.

Practice Problems IMF

• Complete p. 210-211 #33-42, 43, 45, 46, 50

Resonance Structures

• Some molecules cannot be represented adequately by a single Lewis Structure.

• Ex: Ozone – O3

• In this example the two structure of ozone are constantly alternating and therefore is no average structure but 2 plausible version.

• We use a double headed arrow in between the Lewis Structures to represent this.