Structure And Bond Theory Flashcards
What’s the importance of resonance?
A single Lewis structure is inadequate
The actual structure can be considered as the average of its resonance structures - where charge and bond length is equally distributed accordingly
What are hypervalent molecules?
Atoms that seemingly exceed the octet
Often include larger p-block elements from the 3rd period and below which are surrounded by electronegative atoms
What does VSEPR stand for?
Valence Shell Electron Pair Repulsion
What shape does 5 regions of electron density result in?
Trigonal bipyrmidal
Why does water have a smaller bond angle than predicted?
Water has a smaller bond angle due to the lone pairs which repel more strongly than bond pairs
They require more space and so push bond pairs closer together
105.5˚ < 109.5˚ predicted
Why is the trigonal bipyramidal geometry special?
Because the 3 equatorial regions are more spacious than the 2 axial regions
Thus equatorial regions will be preferred by e- regions demanding more space - lone pairs
What’s the special case where a lone pair occupies a equatorial region in the trigonal bipyramidal geometry called?
Disphenoidal
SF4
What’s the bond angle of the tetrahedral geometry? Give an example
109.5˚ - CH4
What’s the bond angle of the trigonal pyramidal geometry? Give an example
107˚ - NH3
What does Valence bond theory state?
Atomic orbitals overlap to form individual bonds
Electrons are treated as being confined to individual bonds
What does Molecular bond theory state?
Atomic orbitals of the whole molecule with matching symmetry are combined to form molecular orbitals.
MO theory gives high accuracy of results for the structures and energies of molecules and solids
Why does bond formation lower the energy in VB theory?
Because of the added electrostatic interaction between the nuclei and the shared electron
What happens for bonding in VB theory?
For a covalent bond, each atom will contribute one or more valence orbitals that will overlap with the other atom.
Overlapping orbitals interfere constructively, increasing amplitude and hence e- density in region between nuclei.
When can constructive orbital overlap happen?
Only when orbitals are ‘in phase’
What are the differences between ∂ bonds and pi bonds?
In ∂ bonds there is one region of overlap
In pi bonds there are two regions of overlap, there is a nodal plane that passes through both nuclei
