CHEM 101 THIRD WEEK NOTES
CHEM 101 THIRD WEEK NOTES Ch 101
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This 3 page Class Notes was uploaded by Lauren Faris on Friday October 14, 2016. The Class Notes belongs to Ch 101 at University of Alabama - Tuscaloosa taught by Dr. Bakker in Fall 2016. Since its upload, it has received 11 views. For similar materials see Chemistry 101 008 in Science at University of Alabama - Tuscaloosa.
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Date Created: 10/14/16
CHEM 101 THIRD WEEK NOTES By Lauren Faris Chapter 7 Cliff Notes Oxygen is paramagnetic. Hybridization is where you mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process. The valence electrons of the atoms in a molecule reside in quantum mechanical atomic orbitals. A chemical bond results when these atomic orbitals interact and there is a total of two electrons in the new molecular orbital. The electrons must be spin paired. According to valence bond theory, bonding takes place between atoms when their atomic or hybrid orbitals interact. To interact, the orbitals must do either of the following: o Be aligned along the axis between the atoms. o Be parallel to each other and perpendicular to the interatomic axis. Some atoms hybridize their orbitals to maximize bonding. More bonds = more full orbitals = more stability. Hybridizing is mixing different types of orbitals in the valence shell to make a new set of degenerate orbitals. sp, sp2 , sp3 , sp3d, sp3d2 The same type of atom can have different types of hybridization. A sigma (σ) bond results when the interacting atomic orbitals point along the axis connecting the two bonding nuclei. Either standard atomic orbitals or hybrids • s to s, p to p, hybrid to hybrid, s to hybrid, etc. • A pi (π) bond results when the bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the two bonding nuclei. Between unhybridized parallel p orbitals. The interaction between parallel orbitals is not as strong as between orbitals that point at each other; therefore, σ bonds are stronger than π bonds. Atom with two electron groups are linear shape with a 180° bond angle. Valence Bond (VB) theory predicts many properties better than Lewis theory. – Bonding schemes, bond strengths, bond lengths, bond rigidity. Resonance hybrids: VB theory presumes the electrons are localized in orbitals on the atoms in the molecule, so doesn’t really address resonance structures. In MO theory you apply Schrödinger’s wave equation to the molecule to calculate a set of molecular orbitals. The equation solution is estimated. The estimated solution is evaluated and adjusted until the energy of the orbital is minimized. The simplest guess starts with the atomic orbitals of the atoms adding together to make molecular orbitals; this is called the linear combination of atomic orbitals (LCAO) method. Because the orbitals are wave functions, the waves can combine either constructively or destructively. When the wave functions combine constructively, the resulting molecular orbital has lower energy than the original atomic orbitals. Called a bonding molecular orbital. o Designated: σ, π o Most of the electron density between the nuclei. When the wave functions combine destructively, the resulting molecular orbital has higher energy than the original atomic orbital. Called an antibonding molecular orbital. o Designated: σ*, π* o Most of the electron density outside the nuclei. o Nodes between nuclei. Use Aufbau approach for MO’s (as we did for individual atoms). Electrons in bonding MOs are stabilizing. o Lower energy than the atomic orbitals. Electrons in antibonding MOs are destabilizing. o Higher in energy than atomic orbitals. o Electron density located outside the internuclear axis. o Electrons in antibonding orbitals cancel stability gained by electrons in bonding orbitals. Bond order = ½ (# Bonding Electrons – # Antibonding Electrons). Bond order = difference between number of electrons in bonding and antibonding orbitals. Molecular orbitals (MOs) are a linear combination of atomic orbitals (AOs). MOs are named by type: σ, π, with a subscript to indicate what AOs they were formed from. When the combining atomic orbitals are identical and of equal energy, the contribution of each atomic orbital to the molecular orbital is equal. When the combining atomic orbitals are different types and energies, contributions to the MOs are different. The more electronegative an atom is, the lower in energy are its orbitals. • Lower energy atomic orbitals contribute more to the bonding MOs. Higher energy atomic orbitals contribute more to the antibonding MOs. Nonbonding MOs remain localized on the atom donating its atomic orbitals. When many atoms are combined together, the atomic orbitals of all the atoms are combined to make a set of molecular orbitals, which are delocalized over the entire molecule. This gives results that better match real molecule properties than either Lewis or valence bond theories. Band Theory: o Electrons become mobile when they make a transition from the highest occupied molecular orbital into higher energy empty molecular orbitals. o These occupied molecular orbitals are referred to as the valence band. o The unoccupied orbitals the conduction band.
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