Intermolecular Forces

UNDERSTANDING THE REASON BEHIND THE PHYSICAL AND CHEMICAL

BEHAVIOR OF COVALENT MOLECULES

H F

electron richregion

electron poorregion

The electronegativity of an atom can create a dipole, or polar

molecule.

Polar covalent bond or polar bond :covalent bond with greater electron density around one of the two atoms

FH

Intermolecular forces: attractive forces between molecules.

Intramolecular forces: hold atoms together, attractive forces within a molecule.

Generally, intermolecular forces are much weaker than intramolecular forces.

Intermolecular vs Intramolecular

• 41 kJ to vaporize 1 mole of water (inter)

• 930 kJ to break all O-H bonds in 1 mole of water (intra)

“Measure” of intermolecular force

boiling point

melting point

Attractive forces between polar molecules

• Partially + end of one dipole is attracted to the partially -- end of another dipole

Dipole-Dipole Forces

H FH F H F

H F H F

The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond

and an electronegative O, N, or F atom.

Hydrogen Bonding

http://www.ck12.org/user:krogers/section/Intermolecular-Forces-of-Attraction/

Why is the hydrogen bond considered a “special” dipole-dipole interaction?

Decreasing molar massDecreasing boiling point

Hydrogen Bonding in Water

Attractive forces when a polar molecule causes a nonpolar molecule to temporarily have a dipole

Induced-Dipole Forces

induced dipole interaction (by an ion)

induced dipole interaction by polar molecule

London Dispersion Forces

Attractive forces between two nonpolar molecules• In polar or nonpolar atoms, electrons are in continuous

motion.

• At any instant the electron distribution may be uneven. A momentary uneven charge can create a positive pole at one end of an atom of molecule and a negative pole at the other.

SO

O

What type(s) of intermolecular forces exist between each of the following molecules?

HBrHBr is a polar molecule: dipole-dipole forces. There are

also dispersion forces between HBr molecules.

CH4

CH4 is nonpolar: dispersion forces.

SO2

SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

Properties of Covalent Compounds

Usually soft and squishyNot soluble in water (usually nonpolar)Does not conduct electricityLow melting pointsLow boiling points

Properties of Ionic Compounds

Combination of ions (cation/anion)Hard and BrittleTightly packed solids in a crystal lattice Usually soluble in waterConducts electricity only when dissolvedHigh melting points

Metallic Bonding

Chemical bonding is different in metals than it is in ionic, molecular.

In ionic bonding, a metal and nonmetal(s) are held together by electrostatic forces

In covalent bonding, nonmetals are held together by shared pairs of electrons.

Metallic bonding, however, is the attraction between metal atoms and the surrounding sea of electrons.

Metallic Bonding

In a metal, the vacant orbitals in the atoms’ outer energy levels overlap, which allows the outer electrons of the atoms to roam freely throughout the entire metal.

These mobile electrons form a sea of electrons around the metal atoms, which are packed together in a crystal lattice.

I.E. The sea of electrons are delocalized, valence electrons that do not belong to any one atom but move freely about the metal’s network of empty atomic orbitals.

Metallic Bonding

The unique characteristics of metallic bonding gives metals their characteristic properties, listed here:

electrical conductivity

thermal conductivity

malleability

ductility

shiny appearance (luster - absorb a wide range of light frequencies )