![]() ![]() Overlap is above and below the bond axis, not directly between the bonded atoms. energy of isolated p orbitals bond order of a pi bond = (2)-(0) 2 = 1 bond 2p b π * = 2p a-2p b = antibonding MO = LCAO = linear combination of atomic orbitals ∆E = bond energy There is a big energy advantage for a pi bond over two isolated p orbitals. polar covalent bonding, metallic bonding, delocalization, sigma & pi bonds, bond length & energy. Problem 1 – What would the MO pictures of He 2, H 2 +, H 2-and He 2 + look like? Would you expect that these species could exist? What would be their bond orders? node = zero electron density because of opposite phases 2p a π bond LUMO HOMO π = 2p a + 2p b = bonding MO = potential energy higher, less stable lower, more stable LUMO = lowest unoccupied molecular orbital HOMO = highest occupied molecular orbital Similar phase of electron density (no node) adds together constructively. (See two examples of student responses in the picture. For example, the ethylene molecule has 5 sigma bonds and 1 pi bond. There is a node between the bonding atoms (zero electron density). The pi bond is a covalent bond created by the lateral overlap of atomic orbitals. Sigma star (σ*) antibonding molecular orbital – Normally this orbital is empty, but if it should be occupied, the wave nature of electron density (when present) is out of phase (destructive interference) and canceling in nature. A sigma bonds is always the first bond formed between two atoms. Sigma (σ) bonding molecular orbital-Shared electron density is directly between the bonding atoms, along the bonding axis. Let me just use the Greek symbols services. And so, uh, this is often considered a double bond or a triple bond, where atrial wand is simply to pie plus one sigma. This type of covalent bond is formed by the overlap of bonding orbitals along the internuclear. ![]() ![]() energy of isolated atoms bond order (H 2 molecule) = (2)-(0) 2 = 1 bond 1s b H H H H H H σ * = 1s a-1s b = antibonding MO = LCAO = linear combination of atomic orbitals node = zero electron density because of opposite phases ∆E = bond energy There is a big energy advantage for a hydrogen molecule over two hydrogen atoms. But essentially for a single bond, you combine two atomic orbitals to make a new orbital, and pie bonding is essentially a Sigma bond, plus a pie bond. For example, the methane molecule contains 4 C-H sigma bonds. The atomic orbitals overlap along the inter-nuclear axis and involve end-to-end or head-on overlap. Bond order = (number of bonding electrons)-(number of antibonding electrons) 2 = amount of bonding 1s a hydrogen molecule = H 2 LUMO HOMO σ = 1s a + 1s b = bonding MO = potential energy higher, less stable lower, more stable LUMO = lowest unoccupied molecular orbital HOMO = highest occupied molecular orbital Similar phase of electron density (no node) adds together constructively. Sigma Bond: This type of covalent bond is formed by the axial overlapping of half-filled atomic orbitals. ![]()
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