Electron Configurations Ionic & Covalent Bonds Review VSEPR Bonding & Molecular Shapes When a nonmetal and a metal form a compound: Valence electrons of the metal are lost and the nonmetal gains these electrons forming an Ionic Bond. Bond. When two nonmetals form a compound: They share electrons to achieve a Noble gas electron configuration forming a Covalent Bond. Bond. Ionic Compounds Ionic compounds are formed when electron(s) are transferred. Electrons go from less electronegative element to the more electronegative forming ionic bonds. Covalent Compounds Share electrons. pair = bond. Octet rule ( duet for hydrogen) Lewis structures: Notice the charges: In one case they balance, can you name the compound? In the other they do not, can you name the polyatomic ion? It has a Formal charge. Important Bond Numbers (Neutral Atoms!) one bond two bonds G.N. Lewis Photo Bancroft Library, University of California/LBNL Image Library Footnote: G.N. Lewis, despite his insight and contributions to chemistry, was never awarded the Nobel prize. three bonds four bonds H F Cl Br I O N C Notes from Lewis Lewis s notebook and his Lewis Lewis structure.
Formal Charge VSEPR Model Valence Shell Electron Pair Repulsion Equals the number of valence electrons of the free atom minus [the number of unshared valence electrons in the molecule + / the number of shared valence electrons in the molecule]. Adding/subtracting atoms and electrons. See examples on the board. Formal charge = number of valence electrons (number of lone pair electrons +/ number of bonding electrons) VSEPR Model The molecular structure which surrounds a given atom is determined principally by minimizing electron pair repulsions through maximizing separations. VSEPR Model The molecular structure of a given atom is determined principally by minimizing electron pair (bonded &free) repulsions through maximizing separations. Some examples of minimizing interactions. Some examples of minimizing interactions. Predicting a VSEPR Structure. Draw Lewis structure.. Put pairs as far apart as possible. 3. Determine positions of atoms from the way electron pairs are shared. 4. Determine the name of molecular structure from positions of the atoms.
Orbital Geometry Important Linear in Trigonal Planar Organic Compounds Trigonal Planar Molecular Geometry Bond Angle # of lone pairs Linear Trigonal Planar Bent Trigonal Pyramidal Bent Seesaw T-shape Linear 3 Square Pyramidal Square Planar 3
Summary: Representing Molecules Formulas, Structures & Models Lewis Structures / VSEPR Molecular Models Using all of the tools. Computer Generated Models Covalent Compounds Equal sharing of electrons: nonpolar covalent bond, same electronegativity (e.g., H) Unequal sharing of electrons between atoms of different electronegativities: polar covalent bond (e.g., HF) http://chemconnections.llnl.gov/organic/chem6/labs/vsepr/j-index.html 4
Bond Dipole & Dipole Moment Dipole moments are experimentally measured. Polar bonds have dipole moments. dipole moment (D) = µ = e x d (e) : magnitude of the charge on the atom (d) : distance between the two charges Molecular Polarity & Dipole Moment Bond Polarity A molecule, such as HF, that has a center of positive charge and a center of negative charge is polar, polar, and has a dipole moment. moment. The partial charge is represented by δ and the polarity with a vector arrow. H F δ+ When identical polar bonds point in opposite directions, the effects of their polarities cancel, giving no net dipole moment. When they do not point in opposite directions, there is a net effect and a net molecular dipole moment, designated δ. δ Resultant Molecular Dipole = Resultant Molecular Dipole > 5
An electrically charged rod attracts a stream of chloroform but has no effect on a stream of carbon tetrachloride. Acetic acid Proteins & Small Molecules The interaction of a large protein bio-polymer, acetylcholinesterase, acetylcholinesterase, with a relatively small molecule of acetylcholine. A general process similar to the way that scientists think we smell. Is acetic acid polar or non-polar? Will acetic acid be miscible in water? Historical view of a few smell receptors. Molecular Shape & Smell Theory 4 October 4 The Nobel Assembly at Karolinska Institutet has today decided to award The Nobel Prize in Physiology or Medicine for 4 jointly to Richard Axel and Linda B. Buck for their discoveries of "odorant receptors and the organization of the olfactory system" 6
S-(+)- caraway R-(-)- spearmint They discovered a large gene family, comprised of some, different genes (three per cent of our genes) that give rise to an equivalent number of olfactory receptor types. These receptors are located on the olfactory receptor cells, which occupy a small area in the upper part of the nasal epithelium and detect the inhaled odorant molecules. 7