Chapter 9.1-9.2 Flashcards
Who developed the Valence bond theory?
Linus Pauling
Who developed the Molecular orbital theory?
Robert S. Mulliken
What is the Valence bond theory?
This theory is closely tied to Lewis’s idea that there are electron pairs between bonded atoms and lone pairs of electrons localized on a particular atom
What is the Molecular orbital theory?
Derived molecular orbitals that are “spread out”, or delocalized, over the molecule
In which situation the Valence bond theory is most useful than the Molecular orbital theory?
Is generally the method of choice to provide a qualitative, visual picture of molecular structure and bonding
This theory is particularly useful for molecules made up of many atoms
Provide a good description of bonding for molecules in their ground or lowest energy state
In which situation the Molecular orbital theory is most useful than the Valence bond theory?
Is used when a more quantitative picture of bonding is needed
Essential if we want to describe molecules in higher-energy, excited stated
What happened if two hydrogen atoms at an infinite distance apart are brought together to form a bond?
Widely separated, the two atoms do not interact.
If the atoms move closer together, however, the electron on one atom is attracted to the positive charge of the nucleus of the other atom.
A system is stable when the potential energy is lower.
What is the basis idea for Valence bond theory?
The idea that bonds are formed by the overlap of atomic orbitals is the basis for valence bond theory
What is a sigma (σ) bond?
A sigma bond is a bond in which electron density is greatest along the axis of the bond.
a sigma bond is a single bond
What is the difference of a sigma (σ) bond and a pi (π) bond?
to see the difference look a this animation
http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/bom5s2_6.swf
C2H6 : only sigma bond
C2H4: sigma bond and pi bond
C2H2: sigma bond and pi bond
Summarize the orbital overlap model of bonding in 3 points.
Orbitals overlap to form a bond between two atoms
Two electrons, of opposite spin, can be accommodated in the overlapping orbitals. Usually, one electron is supplied by each of the two bonded atoms
The bonding electrons have a higher probability of being found within a region of space influenced by both nuclei. Both electrons are thus simultaneously attracted to both nuclei
What Linus Pauling suggested in the theory of orbital hybridization?
He suggested that a new set of orbitals, called hybrid orbitals, could be created by mixing the s and p orbitals on an atom
How hybrid orbitals sets are built?
Hybrid orbital sets are always built by combining an s orbital with as many p orbitals as needed to have enough hybrid orbitals to accommodate the sigma bonding and lone electron pairs on the central atom.
How do we know if we have a sp^3 orbital? (hybrid)
By calculating with steric number.
steric number= # of sigma bonds - # of lone pairs
ex: Ch4 (methane)
steric number= 4 sigma bonds - 0 lone pairs
steric number = 4 == sp^3 bond
When the steric number =7 it means that we have a sp^3d3 bond
When the steric number =6 it means that we have a sp^3d2 bond
When the steric number =5 it means that we have a sp^3d bond
When the steric number =4 it means that we have a sp^3 bond
When the steric number =3 it means that we have a sp^2 bond
When the steric number =2 it means that we have a sp^1 bond
How do we know if we have hybrid orbitals?
For example CH4
Example CH4
Step 1: take the central atom and count the number of bounding it has
C have 2 bonds (2 bonds to H)
Step 2: Find the carbon configuration in the exited state
C: 1s2, 2s2 and 2p2
Step 3: What Linus Pauling proposed is to take one 2s orbital electron and promote it to the 2p orbital.
If we forgot the 1s orbital now we have
2s1 and 2p3
Step 4: The electrons in the 2s and 2p does not have the same energy. So we create an hybridization by promoting the 2s orbital and demoting the 2p orbital in energy.
So now we have: s,p,p,p
Together we have sp3 orbital
