Chemistry 14: Polar Molecules

1. Chemistry 14: Polar Molecules Christopher Chui Polar Molecules - C. Chui

2. Polarity • A polar covalent bond occurs when a shared pair of electrons is attracted more strongly to one of the atoms of the bond • Dipole is another name for a polar molecule • A dipole moment is a property of a polar molecule • Partial charges within a molecule are indicated by d • Dipole moment m depends upon the size of the partial charges and the distance between them, m = Qd • Melting point and boiling point are related to dipole moment Polar Molecules - C. Chui

3. Weak Forces • Weak attractive forces between the protons of one atom and the electrons of another atom are van der Waals forces • Intramolecular forces hold atoms together in a molecule; while intermolecular forces hold different molecules together • Dipole-dipole attraction exists between molecules that are permanent dipoles • A nonpolar molecule may become an induced dipole • Two nonpolar molecules may be attracted to each other • Dispersion forces (London forces) result from temporary dipoles • Van der Waals attraction can result from 1) dipole-dipole forces, 2) dipole-induced dipole forces, 3) dispersion forces Polar Molecules - C. Chui

4. Ligands • A complex ion is formed when polar molecules or negative ions cluster around a central positive ion • The polar molecules or negative ions are ligands • The number of points of attachment of the ligands around a central positive ion is called the coordination number • Water is the most common ligand • A bidentate ligand attaches at two points • Use the prefixes di-, tri-, tetra-, penta-, and hexa- to indicate the number of each kind of ligand in a name Polar Molecules - C. Chui

5. Naming Complex Ions • Neutral molecules: carbonyl, ammine, nitrosyl, aqua • Anions: bromo, cyano, chloro, fluoro, iodo, hydroxo, oxalato, oxo, thio, thiosulfato • Metal complexes: cuprate, aurate, ferrate, plumbate, argentate, stannate • Complex ions are named: 1) ligands first, in alphabetical order with prefixes; 2) central ion; 3) –ate ending, if the ion is negative; 4) Roman numeral • Ligands in the formulas of complex ions are written in a different order from ligands in the names Polar Molecules - C. Chui

6. Coordination Compounds-1 • It is possible to have a complex with a net charge of zero in a coordination compound • If a complex ion occurs in the formula of a coordination compound, place the whole complex ion in brackets • Transition metals have partially filled d orbitals that can become involved in bonding • The colors of many complex ions are the result of the splitting of the d orbitals. Energy is absorbed as electrons move from the lower to higher energy levels • The bonding structure of complex ions is similar to that of salts Polar Molecules - C. Chui

7. Coordination Compounds-2 • Gemstones have trace amounts of transition metal ions • Beryl: emerald Cr, aquamarine Fe • Corundum: ruby Cr, sapphire Fe or Ti • Quartz: amethyst Fe • The bond formed when both electrons in the shared pair come from the same atom is a coordinate covalent bond • Chromatography is a separation method that depends on the polarity of substances. The mobile phase, or mixture to be separated, passes over the stationary phase • Thin layer chromatography may be used to separate amino acids • Gas chromatography is used to analyze volatile liquids and mixtures of gases Polar Molecules - C. Chui