• VSEPR Model: Valence shell electron-pair repulsion → used to predict the geometries of covalent


         ○ States that structure of a molecule is principally determined by minimizing electron-pair repulsions between atoms

               ■ The lone and bonded pairs around an atom will be positioned as far apart as possible to minimize repulsion

  • Things to know

         ○ Lone pairs take up more space than bonded (shared pairs) .

               ■ So replacing bonded pairs with squeezes bonding pairs together and causes bond angles to become slightly less

         ○ Multiple bonds count as one shared pair (electron domain/steric number)

  • Note: sp hybridized & sp2 hybridized = flat→ atoms bonded are in the same plane; sp³+ = not flat → atoms bonded must exist in a diff plane
  • Electron Geometry: determines shape; count bonds (shared pairs) and lone pairs `
  • Molecular Geometry: what you see; count only bonds
  • The number of pairs determines:

           ○ Bond angles; Underlying structure; Polarity

  • The number of atoms determines: actual shape


Hybridization and the Localized Electron Model

  • Hybridization: mixing of the atomic orbitals to form special orbitals for bonding

          ○ Reason: orbitals blend bcuz leads to minimal energy for the molecule and suitable geometry

  • Atomic orbitals (e.g s, p, d, f) are only present in single, unbonded atoms
  • When atoms form bonds, their atomic orbitals form hybrid orbitals
  • Model summarized: an atom in a molecule might adopt a different set of atomic orbitals (called hybrid orbitals) from those it has in the free state

          ○ Makes sense: bcuz assumes that the individual atoms respond as needed to achieve the minimum energy for the molecule

                 ■ The electrons will be arranged to give each atom a noble gas configuration, where possible, and to minimize electron-pair repulsions.

Sp3 Hybridization

  • Can say that the central atom undergoes sp3 hybridization or is sp3 hybridized                             
  • Tetrahedral electron geometry = Sp3 hybridization ○ Formed from 1 s and 3 p orbitals:

Sp2 Hybridization

  • Trigonal planar arrangement (three effective pairs) of atomic orbitals → sp2

          ○ The plane of the sp2 hybridized orbitals is determined by which p orbitals are used 

            ● The three sp2 orbitals on each carbon can be used to share electrons

Sp Hybridization

  • Linear geometry (2 effective pairs) = sp hybridization

Dsp3 Hybridization

  • Trigonal bipyramidal arrangement (five effective pairs) around atom imply dsp3 hybridization
  • Note: AP exam won’t ask about hybridization for domains 5 and 6

D2sp3 Hybridization

  • Octahedral arrangement (six electron pairs) around an atom imply d2sp3 hybridization of the atom.

Sigma vs Pi Bonds

  • Sigma bond: bond formed from overlapping linear hybridized orbitals

          ○ Bonds are formed from electron pairs shared in an area centered on the line running between the atoms (internuclear axis)

          ○ Can be described as being localized (doesn’t move around) → electrons stay put between the 2 atoms

  • Pi bond: formed from overlapping unhybridized and parallel p orbitals

          ○ Only appear in a multiple bond

              ■ Double bond = one sigma + one pi

              ■ Triple bond = one sigma + 2 pi

          ○ Pi bonds cause atoms to be in the same plane → atom with double or triple bond will be in the same plane b/c they can’t rotate

          ○ In situation where resonance exists: there is a pair of [delocalized] electrons (pi bond) resonating between two locations

Hybridization Trick

  • H = ½ (V + M – C + A)

         ○ V = # of valence electrons

         ○ M = # of monovalent atoms bonded

               ■ Monovalent = valency of one

         ○ C = cationic charge

         ○ A = Anionic charge

  • With compounds with more than one central atom, to find H of single atom, divide value by number of atoms