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UMass Amherst CHEM 110 - Chemsitry Chapter 9

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Chemistry Chapter 9● Valence Bond Theory: Bonds result from the pairing of unpaired electrons from valenceshell○ According to simple VBT approach, carbon should form only 2 bonds (2 unpairedelectrons). The carbon atom has 4 unpaired electrons available for bonding oncethe 2s electron is promoted to an empty 2p orbital○ Limitations of VBT■ VBT predicts O2 molecule to behave as a diamagnetic molecule● Types of Bonds:○ Sigma(σ) bonds: When drawing a covalent molecule, the first bond drawnbetween any two atoms. Cylindrically symmetric about the bond axis with nonodal plane along the bond axis.■ Bonding occurs on bonding axis.■ Sigma bond is stronger than pi bond due to better overlap of orbitals.○ Pi(π) bonds: Any multiple bonds drawn after the first bond(sigma bond).Electron density in two lobes with a single nodal plane along the bond axis.■ Bonding occurs everywhere besides the bonding axis.■ Determined by the amount of p orbitals available to fill.○ Single bond = σ-bond○ Double bond = 1 σ-bond + 1 π-bond○ Triple bond = 1 σ-bond + 2 π-bonds● Hybrid Orbitals: Orbitals that have s, p, and or d orbitals are combined together to formequal orbitals.○ Number of atomic orbitals = number of hybrid orbitals.● Determining Hybridization State:○ Step 1: How many atoms and lone pairs are attached to the atom of interest.○ Step 2: How many orbitals are needed.■ Number of orbitals = number of atoms and lone pairs attached.○ Step 3: Choose the appropriate orbitals in order of energy(lowest to highest)■ s orbital, then p orbital, then d orbital.● Molecular Orbital Theory: Overlap of atomic orbitals produces molecular orbitals.○ Uses the number of valence electrons of atoms.○ First fill up atomic orbitals using valence electrons(s, p, d, f)○ Then fill in molecular orbitals, first fill in bonding orbital, then antibonding orbitals.■ Bonding orbitals: Where the arrows go first.● Constructive manner■ Antibonding orbitals: Arrows go here after bonding orbitals are filled up,shown by a star. Has a nodal plane.● Deconstructive manner● Bond order = ½ [number of e- in bonding orbitals – number of e- in antibonding orbitals]○ If bond order = 0 then atom doesn’t exist.○ Low Bond order=Weak, Long bonds. High Bond order=Strong, Short

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