Chapter 14 - Covalent Bonding: Orbitals Flashcards
What are the two widely used bonding models?
Localized electron model and Molecular orbital model
Explain the localized electron model
Molecule is pictured as a group of atoms sharing electron pairs between
atomic orbitals
Hybrid orbitals, which are combinations of the “native” atomic orbitals,
are often required to account for the molecular structure
Explain the hybrid orbitals in the localized electron model.
Four electron pairs (tetrahedral arrangement) require sp3 orbitals - A set of four sp3 hybrid orbitals can be created through the linear combination of one s and three p atomic orbitals
Three electron pairs (trigonal planar arrangement) require sp2 orbitals – A set of three sp2 hybrid orbitals can be created through the linear combination of one s and two p orbitals
Two electron pairs (linear arrangement) requires sp orbitals - - A set of two sp hybrid orbitals can be created through the linear combination of one s and one p atomic orbital
Name and explain the two types of bonds
Sigma: electrons are shared in the area centered on a line joining the
atoms
Pi: a shared electron pair occupies the space above and below the line
joining the atoms
Explain the MO theory, its benefits and limitations
A molecule is assumed to be a new entity consisting of positively charged
nuclei and electrons
The electrons in the molecule are contained in molecular orbitals, which
in the simplest form of the model are constructed from the atomic orbitals
of the constituent atoms
The model correctly predicts relative bond strength, magnetism, and
bond polarity
It correctly portrays electrons as being delocalized in polyatomic molecules
The main disadvantage of the model is that it is difficult to apply qualitatively
to polyatomic molecules
Explain how MOs are defined in terms of energy
A bonding MO is lower in energy than the atomic orbitals from which
it is constructed. Electrons in this type of MO are lower in energy in
the molecule than in the separated atoms and thus favor molecule
formation.
An antibonding MO is higher in energy than the atomic orbitals from
which it is constructed. Electrons in this type of MO are higher in energy
in the molecule than in the separated atoms and thus do not favor
molecule formation.
Explain how MOs are classified based on shape/symmetry
Sigma (s) MOs have their electron probability centered on a line passing
through the nuclei
Pi (p) MOs have their electron probability concentrated above and
below the line connecting the nuclei
How do you find BO in MO theory?
Bond order = (number of bonding electrons - number of antibonding electrons) / 2
Explain spectroscopy and the two kinds discussed in class
Provides a method for obtaining information about the identity, structure,
and properties of substances.
Electronic spectroscopy: provides information about the spacings of the
electronic energy levels.
Vibrational spectroscopy: typically in the infrared region (IR spectroscopy);
gives evidence for a particular atom pair in a molecule.
Explain bond rotation in VB theory
- In single bonds, atoms can freely rotate around the sigma bond because it is cylindrically symmetrical; different structures called conformers
- In double bonds, atoms cannot freely rotate around the pi bonds as they are not cylindrically symmetrical; different structures called isomers
- Rotation around a double bond would require breaking and then re-forming the pi bond
What are the limitations of VB theory?
• VB theory and hybridization are not bad at predicting the geometries of p-block compounds
• Often incorrectly predicts electron configurations
o For example, while predicted to have 4 unpaired electrons, O2 has only 2
• Cannot predict the energies of orbitals in molecules
• Also gets structure of water wrong
Explain HOMO
- Highest occupied molecular orbital (HOMO) – the highest energy MO that is occupied by electrons in the ground state
Explain LUMO
- Lowest unoccupied molecular orbit (LUMO) – the lowest energy MO that is not occupied by electrons in the ground state
Explain HOMO-LUMO gap
- HUMO-LUMO gap – the difference in energy between the HOMO and LUMO, which also represents the lowest energy electronic transition available for that molecule. These transitions are what UV-Vis spectrophotometry makes use of.
