Chapters 8 and 9 Octet Rule Breakers Shapes
Bond Energies Bond Energy (review): The energy needed to break one mole of covalent bonds in the gas phase Breaking bonds consumes energy; forming bonds releases energy. Using bond energies to estimate ΔH rxn : 2
Exceptions to the Octet Rule Molecules with atoms having fewer than eight electrons = electron deficient Example: BF 3 Molecules having an odd number of electrons = free radicals Example: NO (next few slides) Molecules with atoms having more than an octet = expanded valence shell. Example: SF 6 (next few slides) 3
Odd-Electron Molecules Nitric oxide (NO): Odd number of valence electrons (11) Formal charges: N = [5 (3 + 2)] = 0 O = [6 (4 + 2)] = 0 Structure is reasonable, except for the lack of octet on N atom. Free radical: Odd-electron molecule with an unpaired e in its Lewis structure Very reactive! 4
Expanded Valence Shell Expanded shells observed for elements with principle quantum number n greater than or equal to 3. The extra electrons are put into d orbitals. Example: SF 6 5
Expanded Valence Shell (cont. 1) Elements in 3rd row or below Expand valence shell by using empty d orbitals Expanded valence shells occur: In molecules having strongly electronegative elements (e.g., O, S, Cl) When an expanded shell decreases formal charge on central atom 6
Practice: Expanded Valence Shells What is the Lewis dot structure of SO 4 2 ion? Does this ion have resonance structures? Total valence electrons: S = 6; O = 4 6 = 24 Ionic charge = 2 Total valence electrons = 32 Minimize formal charge, expand the octet on S 7
Shapes of Molecules Shape depends on VSEPR, Valence Shell Electron Pair Repulsion Lone pairs of electrons repel lone pairs the most Lone pairs repel bond pairs Bond pairs repel bond pairs Give all electron pairs as much space as possible!
Hybridization is the mixing of atomic orbitals to generate a new set of orbitals that form covalent bonds with other atoms. For every bonded atom and/or lone pair, a hybridized orbital is required. Mix s, p, d orbitals to get the number of bonded atoms and lone pairs. Follow VSEPR and spread out the hybridized orbital as much as possible.
Carbon Dioxide O=C=O Look at the central atom. Count up how many other atoms are bonded to the central atom Count up the lone pairs on the central atom. Bonded atoms + lone pairs = SN, steric number. What is the SN for carbon dioxide? A. 2 B. 3 C. 4 D. 8 E. None of these
Carbon Dioxide O=C=O Look at the central atom. Bonded atoms + lone pairs = SN, steric number. What is the SN for carbon dioxide? 2 For SN = 2, we need two hybridized orbitals s + p à 2 sp Start 2 orbitals (s + p) and make 2 orbitals (sp + sp) Use VSPER to spread them out. What is the shape for carbon dioxide? A.linear B.trigonal planar C. tetrahedral D. octahedral
Carbon Dioxide O=C=O Look at the central atom. SN = 2, hybridization = sp Use VSPER to spread out the orbitals. What is the shape for carbon dioxide? Linear The bond between hybridized orbitals is a sigma (σ) bond. The second bond is a pi (π) bond between p orbitals. After supper (σ), you get pi (π)
Linear: sp Hybridization Hydrogen cyanide: HCN, H-CΞN 13
sp 2 Hybridization formaldehyde, SN = 3, s + p + p = 3sp 2 H H C=O What is the geometry of an sp 2 hybridized molecule? A. Linear B. Tetrahedral C. Trigonal planar D. Trigonal pyramidal 14
sp 2 Hybridization formaldehyde, SN = 3, s + p + p = 3sp 2 H H C=O What is the geometry of an sp 2 hybridized molecule? A. Linear B. Tetrahedral 15 C. Trigonal planar D. Trigonal pyramidal
Hybridization: sp 3 Orbitals Methane, CH 4, SN = 4, s + p + p + p = 4 sp 3 orbitals Shape: tetrahedral 16
Other sp 3 Hybrid Examples Note: Lone pairs (non-bonding) Shape: trigonal pyramidal Shape: bent 17
Trigonal Bipyramidal: sp 3 d Hybridization Formed by mixing one s, one d, and three p orbitals. Example: PF 5 five sigma bonds Shape: Trigonal bipyramidal 18
Octahedral: sp 3 d 2 Hybridization Formed by mixing one s, two d, and three p orbitals. Example: SF 6 six sigma bonds Shape: octahedral 19