liquids, solids, and intermolecular forces · intermolecular forces: the attraction forces that...

Chapter 12

Liquids, Solids, and

Intermolecular Forces

Intermolecular Forces: The Attraction Forces that Hold

Matter Together in a Condensed State (Liquids and Solids)

• The strength of the attractions between the particles of a

substance determines its state (i.e., gas, liquid, solid).

• At room temperature, moderate to strong attractive forces

result in materials being solids or liquids.

• The stronger the attractive forces are, the higher will be the

boiling point of the liquid and the melting point of

the solid.

– Other factors also influence the melting point.

Trends in the Strength of

Intermolecular Attraction

• The stronger the attractions between the atoms or molecules,

the more energy it will take to separate them.

• Boiling a liquid requires adding enough energy to overcome all

the attractions between the particles.

– However, it does not require breaking the

covalent bonds.

• The higher the normal boiling point of the liquid, the stronger

the intermolecular attractive forces.

Ionic Compounds CRYSTALS

NaCl

Ionic Bonding and the Crystal Lattice

• The extra energy that is released comes from the formation of a

structure in which every cation is surrounded by anions, and

vice versa.

• This structure is called a crystal lattice.

• The crystal lattice is held together by the electrostatic

attraction of the cations for all the surrounding anions.

• The crystal lattice maximizes the attractions between cations

and anions, leading to the most stable arrangement.

What is Polarity?

• Polarity in covalently bonded molecules refers to a

separation of charge and can describe a bond or an

entire molecule.

• Experimentally, bond polarity is measured by its dipole

moment.

• Bonds connecting atoms of different electronegativity

are polar with a higher density of bonding electrons

around the more electronegative atom giving it a partial

negative charge (designated as δ-).

• The less electronegative atom has some of its electron

density taken away giving it a partial positive charge

(δ+).

Examples

Carbon DioxideWater

µ = 1.8 D

Shapes of Molecules - Review

• The shapes of molecules are determined by

both bonds as well as the non-bonding

valence shell electrons.

• That is why Carbon Dioxide is linear and

non-polar

• Water is bent and polar due to remaining 2

sets of non-bonding lone pair electrons.

Hydrogen bonds (H-bonding)

• An especially strong dipole–dipole

attraction results when H is covalently

attached to a Oxygen, Nitrogen, or

Fluorine atom.

• These are called hydrogen bonds (H-

bonding).

• Strong intermolecular attraction. Very

Important in Biology.

DNA – The Molecule of Life