Covalent Properties Polarity and IMF

Properties Depend on strength of forces between “particles” or separate units. In covalent substances, the units are molecules.

Properties of Covalent (Molecular) Substances Poor conductors of heat & electricity in any phase. No charged particles! Low melting & boiling points: easy to pull molecules apart from each other. Majority of solids are soft Low Hf and Hv compared to ionic & metallic substances High vapor pressure compared to ionic & metallic substances

Attractions between Molecules Intermolecular Forces Dispersion (London) or Van der Waals Occurs between NONPOLAR molecules Dipole-Dipole Forces Occurs between POLAR molecules Hydrogen Bonding Occurs between molecules with a H-F, H-O, H-N bond Memory Jogger

Attractions and Properties Some atoms are able to form multiple repeating covalent bonds with themselves. Some with other atoms. The result is a 3-D structure known as a carbon Si, O, B, N network solid

Crystal Structure of Diamond The ball-and-stick diagram shows the crystal structure of diamond, a covalent network solid. Each corner or intersection of lines represents a carbon atom. Notice the tetrahedral arrangement at each carbon atom.

Properties of Network Solids Extremely high melting points Very hard Insoluble in polar and nonpolar liquids Both solid and liquid do NOT conduct electricity….except for GRAPHITE

Graphite Structure The ball-and-stick diagram of graphite shows the that the carbon atoms form a network of hexagonal rings within each plane. The planes are stacked in layers that can slide over one another.

Bond Energy The strength of a Chemical Bond Energy required to break a chemical bond Measured in kJ/mole A large bond dissociation energy corresponds to a strong covalent bond. Covalent Bond Strength: Triple > Double > Single HBr = 367 kJ/mole

Bond Energies

Resonance Structures Sulfur Dioxide Nitric Acid

Resonance A condition that occurs when more than one valid Lewis structure can be written for a molecule or ion. Resonance structures differ only in the position of the electron pairs, never the atom positions. Examples include O3 and polyatomics: NO3-, NO2-, SO32-, and CO32-

Exceptions to the Octet Rule Odd number of valence electrons NO2, ClO2, NO Coordinate Covalent Bonds Polyatomic Ions Some compounds from with fewer than eight electrons…example is BH3 Expanded Octet Central atom contains more than eight electrons electrons…examples PCl5 , SF6

Polarity of Molecules A molecule may contain polar bonds, but not be polar! Depends on the geometry of the molecule. If molecule is symmetric, the “pull” of one polar bond is offset by the “pull” of another polar bond. It’s a tug-of-war that no one can win!

Symmetric Molecules Contain at least one mirror plane.

The green arrows do not cancel out, so water is polar! Polarity of H2O? The green arrows do not cancel out, so water is polar!

Bond vs. Molecular Polarity Water is a polar molecule because: O-H bond is polar Molecule has a bent geometric shape Polar Molecule Negative end Positive end

Polarity of Carbon Dioxide? Carbon needs double bonds between both oxygen atoms to complete its octet. Electronegativity difference for C O is 0.9 so bond is polar. Is this a polar or nonpolar MOLECULE?

Polarity of CO2? symmetry Individual C-O bond may be polar but MOLECULE is nonpolar by symmetry Draw an arrow along each bond pointing to the more electronegative atom. If the arrows cancel out, the molecule is NONPOLAR.

Symmetric vs. Asymmetric H2O is bent. Electron cloud is lopsided. H2O is polar. CO2 is nonpolar. Can’t tell the ends apart.

Symmetry of Larger Molecules CF4 is fairly symmetric. Overall, it is nonpolar. The arrows cancel out.

Use the structural formula to predict Molecular Polarity! H  C  H  

Ethane = C2H6 Ethene = C2H4 Ethyne = C2H2 These molecules are symmetric and the electron cloud is the same on both ends. Overall, they are nonpolar.

Summary Molecules containing only nonpolar bonds are always nonpolar. H2 , O2 , CH4 Molecules containing polar bonds with symmetrical charge distributions are nonpolar. CO2, CCl4 Molecules containing polar bonds with asymmetrical charge distributions are polar. HF , H2O , NH3

Perspective Drawings Front of plane Back of plane Partial bond

Tetrahedron CH4 H C METHANE H H Carbon’s s and p orbitals overlap with hydrogen’s s orbital to form a hybrid orbital known as a sigma bond. Tetrahedron CH4 H C H H METHANE

Lewis Dot Structures Unshared electron pairs Shared electron pairs