Section 6.5 – Molecular Geometry The properties of molecules depend on the bonding and the molecular geometry, the 3- dimensional arrangement of the atoms in space.

Molecular Polarity This is the uneven distribution of molecular charge, and it is determined by the polarity of each bond, along with the geometry of the molecule.

Molecular Polarity and Symmetry 2 or more lines of symmetry  Nonpolar molecule; electrons distributed symmetrically Only 1 line of symmetry  polar molecule; asymmetrical electron distribution creates a dipole H H H Cl Cl Cl Non-polar Polar Non-polar

Dipole A dipole is created by equal but opposite charges that are separated by a short distance We show a dipole using an arrow pointing towards the more electronegative atom δ+ indicates the partial positive charge, and δ- indicates the partial negative charge H H δ+ H Cl δ- Cl Cl Non-polar Polar Non-polar

Identify the polar molecules, and indicate the dipole… δ+ H H Cl H C H Cl C Cl δ- Polar Non-polar

Identify the polar molecules, and indicate the dipole… .. δ- .. H O H δ- H N H .. H δ+ Polar δ+ Polar

Dipoles Give each molecule oppositely charged ends that can be attracted to each other. As the electronegativity difference increases, attractive force strength increases = stronger dipoles. H Cl Polar δ+ δ- 2.1 3.0 δ+ δ- H F 2.1 4.0 polar

Intermolecular Forces These are the forces of attraction that occur between molecules. They vary in strength but are generally weaker than regular bonding (ionic, covalent, or metallic).

Intermolecular Forces Are responsible for holding molecules together in the solid and liquid phase Caused by the attraction of the + end of one molecule to the – end of another molecule

What do you notice? (p.190) Nonpolar covalent H2 -253 O2 -183 Cl2 -34 Bonding Type Substance BP (Celcius) Nonpolar covalent H2 -253 O2 -183 Cl2 -34 C6H6 80 Polar Covalent PH3 -88 NH3 -33 H2O 100 HF 20 Ionic NaCl 1413 MgF2 2239 Metallic Cu 2567 Fe 2750 W 5600

Melting and Boiling Points a measure of the force of attraction between molecules in covalent compounds between ions in ionic compounds and between atoms in metals

Melting and Boiling Points As a substance is heated, the kinetic energy of its particles increases. At the MP, the energy is enough to overcome the attractive forces holding the particles in fixed positions (in a solid). The particles are able to move/flow past each other, but the particles are still held close together.

Melting and Boiling Points As a substance is heated, the kinetic energy of its particles increases. At the BP, the energy is enough to overcome the force of attraction holding the particles close together in the liquid. The particles are able to pull away from each other and enter the gas phase.

Melting and Boiling Points The higher the boiling point, the stronger the forces between particles. Remember particles means molecules for covalent substances, ions for ionic substances, atoms for metals

3 Types of Intermolecular Forces Dipole – Dipole Forces Hydrogen bonding (special case of D-D) London Dispersion Forces

Dipole Force The strongest intermolecular forces exist between polar molecules. The δ+ end of one molecule is attracted to the δ- end of another molecule. The direction of the arrow is pointed to the negative pole, the crossed tail indicates the + side

Dipole-dipole forces These are the forces of attraction between polar molecules. Example: bp for F2 is -188°C bp for HF is 20°C bp for HCl is -85°C Which is the stronger dipole-dipole force?

Ion-dipole forces These are the forces of attraction between an ion and the partial charge on a polar molecule. This explains why ionic compounds dissolve in polar liquids. Generally stronger than dipole-dipole forces

Induced Dipoles The electrons of a nonpolar molecule can be temporarily attracted by a polar molecule. This is weaker than a regular dipole-dipole force. This can be very important in the solubility of gases in water.

Usually Represented By Hydrogen Bonding A very special type of dipole-dipole force in which a hydrogen atom that is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of another molecule. Usually Represented By Dotted Lines

Usually Represented By Hydrogen Bonding Only happens in molecules where H is bonded to O, F, or N, and sometimes Cl Usually Represented By Dotted Lines

Explains High bp’s of Some Compounds Gives the H atom a large positive charge, and it’s small size allows it to come very close to the unshared pair of electrons on an adjacent molecule.

Explains properties of water High surface tension Why ice floats on water

Extremely Important in Biochemistry

London Dispersion Forces This is a weak attractive force resulting from electron motion and the creation of an instantaneous dipole. Important for noble gases and nonpolar molecules. Explains why nonpolar gases can be liquefied.

London Dispersion forces Act between all atoms and molecules However, dispersion forces are much weaker than dipole-dipole forces Dispersion forces are the only forces acting among noble-gas atoms, and non-polar molecules.

increase with increasing atomic or molar mass. Dispersion forces Substance Bp (1atm, °C) H2 -253 O2 -183 Cl2 -34 He -268 Ar -185 Increase with the increased number of electrons in the interacting atoms or molecules increase with increasing atomic or molar mass.

Recall the bonding lab and evaporation rates… Explain why pentane and hexane evaporated quickly, but the other substances did not. Explain why pentane evaporated more quickly than hexane. Hexane, C6H14 (covalent) ~ 40 sec Water, H2O (covalent) > 5 min. Acetic Acid, (C2H4O2) (covalent) > 5 min. Pentane, C5H12 (covalent) ~ 25 sec Isopropyl Alcohol, C3H8O (covalent) > 5 min.