Intermolecular Forces Understanding the Reason behind the physical and Chemical Behavior of Covalent Molecules

The electronegativity of an atom can create a dipole, or polar molecule. electron rich region electron poor region d+ d- Polar covalent bond or polar bond : covalent bond with greater electron density around one of the two atoms

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

Dipole-Dipole Forces Attractive forces between polar molecules Partially + end of one dipole is attracted to the partially -- end of another dipole H F H F H F H F H F

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

Decreasing boiling point Why is the hydrogen bond considered a “special” dipole-dipole interaction? Decreasing molar mass Decreasing boiling point

Hydrogen Bonding in Water

Induced-Dipole Forces Attractive forces when a polar molecule causes a nonpolar molecule to temporarily have a dipole induced dipole interaction by polar molecule induced dipole interaction (by an ion)

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.

CH4 is nonpolar: dispersion forces. What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: dispersion forces. S O SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

Properties of Covalent Compounds Usually soft and squishy Not soluble in water (usually nonpolar) Does not conduct electricity Low melting points Low boiling points

Properties of Ionic Compounds Combination of ions (cation/anion) Hard and Brittle Tightly packed solids in a crystal lattice Usually soluble in water Conducts electricity only when dissolved High 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 )