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ChemFinalSG - Chem 303/Organic Chemistry I Final Exam Study...

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Chem 303/Organic Chemistry I - Final Exam Study Guide 1. Atomic Structure And Theory Nucleus: Protons and Neutrons. Elements with extra neutrons are isotopes. Electron cloud: Electrons orbiting the nucleus. Areas without e- density are nodes. - First electron shell contains only an s orbital. - Second electron shell contains an s and 3 p orbitals (p orbitals are degenerate). - The outermost shell is the valence shell. - The further the electrons are held from the nucleus the higher in potential energy they are - Electron configuration obeys: Pauli’s exclusion principle, Aufbau principle and Hund’s rule. Atomic Orbital Theory - s orbital electrons behave as a fundamental vibration of an attached string. Either all upwards (+) or all downward (-). - p orbital electrons behave as first harmonic of an attached string. Electrons in each lobe are out of phase (one lobe is +, the other lobe is -). - Covalent bonds form when s and p orbitals overlap. Wave functions add and this describes new molecular orbital shape. Bond orbitals with C axis of symmetry are sigma (σ) bonds (cylindrically symmetrical). Those with C 2 symmetry are pi (π) bonds. Constructive overlap forms σ and π bonding orbitals lower in energy than the original s and p orbitals (σ being the lowest in energy). Destructive overlap forms σ * and π * anti-bonding orbitals that are higher in energy (σ * being the highest in energy). Valence Bond Theory - To form bonds, the s and p orbitals within an atom will overlap with each other to form sp 3 , sp 2 and sp hybridized orbitals (each hybridized orbital is degenerate). - The hybridized orbitals from one atom overlap with hybridized orbitals of other atoms to form bonds. When hybridized orbitals overlap, σ bonds are formed. When p orbitals (not hybridized) overlap, π bonds are formed. - sp 3 orbitals have bond angles of 109.5 o and the geometry is tetrahedral. sp 2 orbitals have bond angles of 120 o and the geometry is trigonal planar. sp orbitals have bond angles of 180 o and the geometry is linear. However, a lone pair will occupy more space in a hybridized orbital and will force the other orbitals closer together. 2. Bond Characteristics - Atoms form bonds to fill the valence shell (octet rule). - Bond length is governed by the size of the atoms and the hybridization of the atoms. - Bond strength is governed by the overlap of the orbitals and the closeness of the electrons to the nucleus. - Bond angle is governed by the size and repulsion of the electrons in orbitals. - If an atom forms more or less bonds than its valence, a formal charge (FC) will result. FC = [valence electrons] – [nonbonding electrons] – ½[shared electrons] Resonance structures describe how electrons (and thus the FC) are delocalized. The major resonance form will have the following characteristics: All octets satisfied As many bonds as possible Little charge separation Delocalize the charge Charge on most electronegative atom Each of these characteristics help stabilize the formal charge.
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