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Keys to the Chapter 3 Looking closely at electron shifts that convert one resonance form into another, you will notice several patterns that repeat themselves over and over. The two most common types of electron movement are shown in the resonance forms of carbonate on page 18 of your text. "Bond to atom": A pair of electrons moves from being a bond to becoming a lone pair on one of the atoms involved in the original bond. The bond is thus broken. "Atom to bond": A lone pair of electrons on an atom moves to become a bond with a neighboring atom. A TT bond is thus formed. Again referring to carbonate on page 18, notice that violation of the octet rule must be avoided at all costs (not the least of which will be to your grade on an exam): If a pair of electrons moves toward an atom al- ready containing an octet, another pair of electrons must at the same time move away from that atom. The guidelines in the textbook for determining the relative degree to which each resonance form contributes to the actual structure are the ones you will use the most when dealing with the most common atoms in organic chemistry. It is somewhat abbreviated. Some additional considerations regarding resonance forms (and some reminders of the basic rules) follow. ALWAYS TRUE (these are your reminders): 1. Individual contributing resonance forms do not exist. Only the resonance hybrid, which is a "weighted average" or blend of the contributing forms, is real. 2. All resonance forms of a single chemical species must have the same total number of valence electrons and the same total charge. 3. Second-row atoms (i.e., up through neon) can never exceed an octet in their valence shell in any resonance form. In other words, the rules for drawing Lewis structures apply to drawing resonance forms. 4. Atom positions and geometries do not change from one resonance form to another-there is only one geometry and set of atomic positions for the actual chemical substance being represented. USUALLY TRUE (these are mostly implied by the guidelines in the textbook-we're just spelling them out): 1. Resonance forms differ only in the positions of and/or nonbonding electrons. The σ bond electrons normally stay put. 2. We convert one resonance form into another by moving electron pairs from places where there is an excess of electron density to places where there is an electron deficiency. 3. Resonance forms with the most covalent bonds are usually more important contributors than resonance forms with fewer covalent bonds (but don't forget that octets take priority). 4. Resonance forms with fewer charged atoms are usually more important contributors. 5. Atoms in the third row (P, S, Cl) and below (Br, I) are not limited to octets of electrons in their outer shells. In fact, Lewis structures with 10 or 12 valence electrons are frequently written for these elements. 1-6, 1-7, and 1-8. Orbitals Atomic orbitals are a convenient way to represent the distribution of electrons in atoms. Note that the + and signs associated with parts of these orbitals do not refer to electrical charges. They refer to mathematical signs of functions (wave functions) associated with the distribution of the electrons. Molecular orbitals are similar but are spread out over more than one atom. They provide an alternative to the Lewis electron-dot method for picturing bonds. The number of molecular orbitals involved in describing a bond is always exactly equal to the number of atomic orbitals contributed by the individual atoms. Overlap of atomic orbitals results in bonding. and sometimes also nonbonding molecular orbitals. Bonding orbitals are always lower in energy (more stable) than the original constituent atomic levels, and antibonding orbitals are always higher in energy. Thus bonding electrons will be more stable than electrons in nonbonding atomic orbitals and will give rise to strong bonds. Electrons in antibonding orbitals will reduce bonding. Hybrid orbitals are derived by mixing atomic wave functions. They are used to explain the geometrical shapes of molecules. Hybridization provides several advantages for bonding. With the larger lobe of the hybrid