Intermolecular Forces and Polarity

Polar Molecules and
Intermolecular Forces

Non-polar and covalent bonds

•Non polar covalent bonds

• Bonding electrons are shared equally between two atoms

•Example: Cl2

• Example: H2

Polar covalent bonds

•Bonding electrons are shared unequally.
•Electron sharing is based on electronegativity differences.

• More electronegative atom attracts the electrons more closely
• More electronegative atom acquires a slight negative charge
• Less electronegative atom acquires a slight positive charge

Polar Covalent Bonds

•Unequal sharing creates “polarized” bonds with opposite
charges.
• Example:

Two ways to show polarity in formulas

•Lower case Greek deltas:

•δ- more electronegative
•δ+ less electronegative

•Slashed arrows

•Arrow points to more
electronegative element

Selected electronegativity values

Type of bond and electronegativity

Electronegativity
difference

Guideline: Type of
Bond

Example
(electronegativity
difference)

0.0 – 0.4

Non-polar Covalent

CH4: 2.5 – 2.1 = 0.4

0.4 – 2.0

Polar covalent

HF: 4.0 – 2.1 = 1.9

> 2.0

Ionic

NaCl: 3.0 – 1.9 = 2.1

Multiple Choice

What type of bond?

C=O in CO₂?

• Polar

• Non-Polar

• Ionic

Multiple Choice

Which type of bond?

N-Cl in NCl₃?

• Polar

• Non-Polar

• Ionic

Multiple Choice

Which type of bond?

Si-H in SiH₄?

• Polar

• Non-polar

• Ionic

Multiple Choice

Which type of bond?

KCl?

• Polar

• Non-polar

• Ionic

Multiple Choice

Which type of bond?

C-F in CF₄?

• Polar

• Non-polar

• Ionic

Multiple Choice

Which bond is most polar?

• C-F

• C-Br

• C-N

• C-S

Polar Molecules

A molecule is polar if one end of the molecule is slightly negative and one end is slightly positive.

A molecule is polar if the electrons are pulled to one side of the molecule: it is asymmetric/lopsided.

​Dipoles

A dipole is a molecule with one negative end and one positive end.

Example: carbon monoxide

Example: carbon dioxide

1) Draw the Lewis structure
2) Are there any lone pairs on the central atom?
Yes? -- POLAR

​Example:
Sulfur dioxide

Draw

Draw the Lewis structure for sulfur trioxide

Multiple Choice

Is sulfur trioxide a polar molecule?

• Yes, it has a lone pair on the central atom

• Yes, the polar bonds are not symmetric about the central atom

• No, the polar bonds are symmetric about the central atom

• No, there are no polar bonds.

Multiple Choice

Is H₂O a polar molecule?

• No, the polar bonds are symmetric about the central atom

• No, there are no polar bonds

• Yes, there are lone pairs on the central atom

• Yes, the polar bonds are not symmetric about the central atom

Multiple Choice

Is CH₄ a polar molecule?

• Yes, the polar bonds are not symmetric about the central atom

• Yes, there are lone pairs on the central atom

• No, there are no polar bonds

• No, the polar bonds are symmetric about the central atom

Multiple Choice

Is NH₃ a polar molecule?

• Yes, there are lone pairs on the central atom

• Yes, the polar bonds are not symmetric about the central atom

• No, there are no polar bonds

• No, the polar bonds are symmetric about the central atom

Intermolecular Forces

Attractions between molecules

Three types:
1) London dispersion forces
2) Dipole-dipole attraction
3) Hydrogen bonding

London dispersion forces

• weakest of the three

• due to temporary dipoles caused by shifting electron clouds

• ALL molecules have London dispersion forces

• LDF is greater in larger molecules due to the larger electron clouds

Dipole-dipole bonding

• polar molecules are attracted to each other

• opposite ends attract

​Hydrogen Bonding

• only in molecules with a covalent hydrogen bond to an electronegative atom

  • H-F

  • H-O

  • H-N

• the hydrogen of one molecule is attracted to the negative end in the other molecule

• strongest of the three forces

Low melting points
Low boiling points

CH₄
Cl₂

​weak intermolecular forces

High melting points
High boiling points

H₂O
NH₃

strong intermolecular forces

IMFs and phase changes

Multiple Choice

Type of intermolecular force present in I₂, Br₂, and Cl₂.

• dipole dipole
• H-bond
• dispersion
• metallic

Multiple Select

H₂S has what kind of intermolecular force?

• dipole dipole
• dispersion
• H-bond
• ionic

Multiple Choice

Which of the following has the highest boiling point?

• H₂
• NH₃
• N₂
• O₂

Multiple Choice

Which of the following has the lowest boiling point?

• CaCl₂
• PH₃
• Cl₂
• N₂

Multiple Choice

What explains the very high melting and boiling point of water?

• Dipole-dipole forces between water molecules
• Hydrogen bonds between water molecules
• London dispersion forces between water molecules
• Molecule-ion attractions between water molecules