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Chem 40, Spring 2014 Section 1 Handout 1.) Lewis Dot Structures Though we will be discussing the shortcomings of Lewis dot structures and VSEPR theory in detail, they are nevertheless very useful ideas; with practice, drawing molecules will become second nature. Drawing a Lewis structure: 1. Count the total number of valence electrons (don’t forget charges!) 2. Place the largest and/or most electropositive element in the center 3. Surround the central atom with the more electronegative atoms 4. Draw single bonds between the central atom and each peripheral atom (2 e- each) 5. Place electrons (lone pairs) around the outside atoms to fill their octets 6. Any remaining electrons are placed on the central atom 7. Use the lone pairs on the peripheral atoms to make double bonds if the central atom has an incomplete octet or a formal positive charge 8. Draw resonance structures Things to keep in mind when drawing Lewis structures: First-row elements (Li-Ne) CANNOT have expanded octets. (Why?) Atoms from the left half of the periodic table often have incomplete octets, and that’s OK! For the purposes of this class, you ONLY need to draw no-bond resonance structures for fluorides that have central atoms with exceeded octets. A complete Lewis dot structure includes ALL contributing resonance structures. Formal charge = (# valence e- in a free atom) – (bonds) – (non-bonding e-) If you need additional help with drawing Lewis dot structures, consult Appendix D of your textbook. 2.) VSEPR: Predicting molecular geometry VSEPR gives us a good “first guess” of molecular geometry. The “VSEPR geometries” depend only on the number of atoms (X) and lone pairs (or radicals) (E) surrounding an atom, and are summarized in the table below. “Steric number” refers to the sum of the number of atoms and the lone pairs (or radicals). Steric number 2 0 lone pairs 1 lone pair 2 lone pairs 3 lone pairs linear 3 trigonal planar bent tetrahedral trigonal pyramidal trigonal bipyramidal see-saw octahedral square pyramidal pentagonal bipyramidal pentagonal pyramidal 4 bent 5 T-shaped 6 7 8 square antiprismatic Lone pairs always occupy the “largest” site. square planar linear 3.) Symmetry and Point Groups A symmetry element (E, Cn, , i, Sn) is different from a symmetry operation (E, Cnm, , i, Snm). There are several conventions that we will use when naming symmetry elements: 1.) The principal axis of rotation (Cn) is always taken as the Cartesian z-axis. The xz plane contains the most atoms possible. 2.) When performing rotation operations, the directionality of the rotation does matter for a few applications (such as rotational spectroscopy). We will use the counterclockwise rotation convention used by the textbook for problem sets and exams. 3.) Indistinguishable symmetry elements (for example, the C2 axes of borane or the indistinguishable v planes of PtCl42-) share the exact same label and are represented by a coefficient in front of the symmetry element (3C2 or 2v). If there are different, distinguishable symmetry elements of the same type (for example, there are three types of C2 axes in PtCl42- -one is the z-axis, one set goes through the chlorines, one set bisects the chlorines), we differentiate them with primes (C2, C2’, C2’’). The single-prime symmetry elements contain more atoms than the double-prime elements. 4.) The superscripts h, v, and d differentiate -planes. Horizontal planes (h) are by definition the xy-plane; there cannot be multiple h-planes. Vertical planes (v) contain the z-axis. Dihedral planes (d) bisect C2 axes, and often (but not necessarily) contain the z-axis. Flow Chart for assigning point groups 4.) Practice Problems Draw complete Lewis dot structures for the following molecules and polyatomic ions, including all resonance contributors, lone pairs, and formal charges. Indicate the expected geometry of the molecule or ion. NO3- RnCl22- SbCl5 NCO- GeF2 PCl3 ICl3 SbF52- Identify all the symmetry elements present in the following molecules, and assign the point group of each. Cl 3+ OC NH2 H2N H2 H2N Co N N NH2 H2 CO Fe OC CO Cl F F Ph Me P Me F Sb P Ni Me Me Cl Fe Cl F Ph F