Presentation on theme: "Lots of multisyllabic words, I know. Overlap and Bonding We think of covalent bonds forming through the sharing of electrons by adjacent atoms. In such."— Presentation transcript:
Lots of multisyllabic words, I know
Overlap and Bonding We think of covalent bonds forming through the sharing of electrons by adjacent atoms. In such an approach this can only occur when orbitals on the two atoms overlap.
Hybridization Atomic orbitals (s, p, d, f) cannot adequately explain the bonding in molecules Consider CH 4 … how do we get the tetrahedral shape out of the above orbitals
Hybridization An averaging of atomic orbitals into a new set of hybrid orbitals Consider beryllium: In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.
Hybridization Consider beryllium: if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.
Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. These sp hybrid orbitals have two lobes like a p orbital. One of the lobes is larger and more rounded as is the s orbital.
Hybrid Orbitals These two degenerate orbitals would align themselves 180 from each other. This is consistent with the observed geometry of beryllium compounds: linear.
Hybrid Orbitals With hybrid orbitals the orbital diagram for beryllium would look like this. The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.
Hybrid Orbitals Think about Boron Whats its electron configuration? How many bonds does it normally form? How?
Practice Problems Specify the electron-pair and molecular geometry for each underlined atom in the following list. Describe the hybrid orbital set used by this atom in each molecule or ion. a) BBr 3 b) CO 2 c) CH 2 Cl 2 d) CO 3 -2
Practice Problems Answers Electron-Pair MolecularHybrid GeometryGeometryOrbital Set a) Trigonal PlanarTrigonal Planarsp 2 b) LinearLinearsp c) Tetrahedraltetrahedralsp 3 d) Trigonal planartrigonal planarsp 2
Types of Covalent Bonds According to the valence bond theory, bond formation requires that two orbitals on adjacent atoms to overlap…. So what about multiple bonds… Draw C 2 H 4 Is there room in between the two atoms for two pairs of electrons to share space? NO
Types of Covalent Bonds Sigma ( σ) Formed from the overlap of hybrid orbitals. Electron density along the internuclear axis… between the centers of the atoms…
Types of Covalent Bonds Pi ( π) Formed from the overlap of unhybridized p-orbitals. Electron density is above and below the internuclear axis, between the atomic centers
Types of Covalent Bonds Draw Aceylene:C 2 H 2 In triple bonds, two sp orbitals form a bond between the carbons, and two pairs of p orbitals overlap in fashion to form the two bonds. Or there is always one sigma bond… anything more is a pi bond
Molecular Orbital Theory Molecular Orbital Theory: Atomic orbitals from all atoms combine to form new orbitals resulting in known geometries Hybridization: Atomic orbitals from single atoms combine to form new orbitals resulting in known geometries VESPER: Electrons repel; moleular shapes result. ATOMS Electrons in Orbitals s, p, d, f, etc MOLECULES Experimental Geometries Linear, trigonal planar, tetrahedral
Molecular Orbital Theory Molecular orbitals are formed by combinations of atomic orbitals from different atoms. There are bonding and antibonding orbitals Without significant electrons density between them, the nuclei will repel each other… creating antibonding orbitals
Molecular Orbital Theory First Principle of MO Theory the total number of molecular orbitals is always equal to the total number of atomic orbitals contributed by the atoms that have combined. Second Principle of MO Theory The bonding molecular orbital is lower in energy than the parent orbitals, and the antibonding orbital is higher in energy
Molecular Orbital Theory Third Principle of MO Theory Electrons of the molecule are assigned to orbitals of successively higher energy Fourth Principle of MO Theory Atomic orbitals combine to form molecular orbitals most effectively when the atomic orbitals are of similar energy
Molecular Orbital Theory # of molecular orbitals = # of atomic orbitals Equal # of bonding and antibonding orbitals Antibonding orbitals ALWAYS higher in energy than its BONDING COUNTERPART Basically, it means that these guys will fill in from the bottom up… from lower energy to higher energy…
Molecular Orbital Theory Can use this to calcualte bond order Bond Order = ½ [(# e - in bonding orbitals) – (# of e - in antibonding orbitals)] What is the bond order of Ne 2 + ? Bond order = (8-7)/2 = ½