Mr.Luk - Review + Bonding€¦ · Model,’and’in’most’cases,’VBT’isnot’...
Transcript of Mr.Luk - Review + Bonding€¦ · Model,’and’in’most’cases,’VBT’isnot’...
AP Chemistry 12
Bonding
Valence Bond Theory
Valence Bond Theory
• Attempts to explain molecular structure based on quantum model • Results are very similar to VSEPR Model, and in most cases, VBT is not needed to explain anything extra • Having said that, the most important contribution from VBT is hybridisation
Valence Bond Theory • Recall that s-‐orbital electrons and p-‐orbital electrons have different energy levels (so do d-‐orbital electrons) v For example, when we write the electron conHiguration for C, we write 1s22s22p2
v The valence electrons in C are the 2s22p2 electrons (4 valence electrons)
Valence Bond Theory • When central atoms are bonded, however, they make bonds that are identical to nearby atoms • Problem: How can electrons of different energy levels form bonds of identical energy level? • Solution: Hybridisation
Valence Bond Theory
Valence Bond Theory • Example: CO2
v Carbon needs to make two equal bonds with oxygen
Hybridisa3on in CO2
Valence Bond Theory • Example: CH2O, BH3
v The central atom needs three equal bonds. Note that the C also needs an additional bond with oxygen
Hybridisa3on in CH2O and BH3
Valence Bond Theory
• Example: CH4, NH3 v The central atom needs four equal bonds
v Note in CH4, the four equal bonds are to H
v Note in NH3, the four equal bonds are to H and a lone pair of electron
Hybridisa3on in in CH4 and NH3
Valence Bond Theory • When central atoms are bonded to Hive or more other atoms, d-‐orbital hybridisations are theorized
• Note: hybridisation is a “convenient” theory to help explain observations. In fact, the concept of hybridization is debated and more accurate descriptions (Molecular Orbital Theory) exist. However, because it suits our purposes, we will continue using hybridization from sp to sp3.
Valence Bond Theory • To predict hybridization:
v Draw the appropriate Lewis Structure/Model v Draw the appropriate VSEPR model v Based on electron geometry (consider lone pair e-‐ as well as bonding e-‐ around the central atom), determine which orbitals need to be hybridized
• Possible hybridization schemes: v sp for two electron groups (atoms or electron pairs) v sp2 for three electron groups v sp3 for four electron groups v sp3d and sp3d2 for Hive and six electron groups. These are only possible for 3rd period elements or below (e.g. SF6, PCl5)
Valence Bond Theory • We mentioned that CH2O has a hybridization of sp2 on the central atom, what does the left over p-‐orbital do?
• Left over p orbitals from sp2 or sp are used to form double or triple bonds
• E.g. Methanal (CH2O), Ethyne (C2H2), CO2, Ethene (C2H4), NO+
Valence Bond Theory • All bonds that are formed from hybrid orbitals (or s orbitals) are called σ bonds
• Other bonds that are formed from left over p-‐orbitals are called π bonds
• Single Bond = 1 σ bond • Double Bond = 1 σ bond + 1 π bond • Triple Bond = 1 σ bond + 2 π bond
Valence Bond Theory • C2H4
Valence Bond Theory • C2H4
σ bonds π bond
Summary