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Level 163

Valence Bond Theory


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Valence bond theory
A covalent bond forms when orbitals of two atoms overlap and a pair of electrons occupy the overlap region
VSEPR and VBT
The VSEPR model predicts molecular shape very well, but it has no relationship to the filling of atomic orbitals. How can we tie together the fact that covalent bonds are formed from overlap of at…
hybridization
a mathematical procedure in which the standard atomic orbitals are combined to form new atomic orbitals called hybrid orbitals that correspond more closely to the actual distribution of electrons in chemically bonded atoms
Hybrid sp orbitals
A linear arrangement of electron domains implies sp hybridization.
Hybrid sp^2 orbitals
Has 3 hybrid orbitals on the same plane, 120 degrees apart from each other leading to the trigonal-planar molecular geometry.
Hybrid sp^3 orbitals
Forms 4 sp^3 orbitals and each large lobe points toward one vertex of a tetrahedron.
sigma bonds
s bonds; arise from overlap of atomic orbitals so that the bonding electrons lie along the bond axis
pi bonds
2 sigma bonds in eliminations are converted to
Single, double and triple bonds
Almost always a single bond=sigma bond, double bond=one sigma and one pi bond, triple bond=one sigma bond and two pi bonds.
Delocalization
Molecules with 2 or more resonance structures can have pi bonds that extend over more than two bonded atoms. Electrons in pi bonds like this are said to be "delocalized". The electrons in sigma b…
# of orbitals hybridized
# of electron pair domains=
2 sp
2 electron pair domains
3 sp2
3 electron pair domains
4 sp3
4 electron pair domains
5 sp3d
5 electron pair domains
6 sp3d2
6 electron pair domains
p and p
Number of unhybridized orbitals: 2 e- domains
p
momentum
none
achiral sample rotation
90 degrees and 120 degrees
Number of unhybridized orbitals: 5 e- domains
90 degrees
Number of unhybridized orbitals: 6 e- domains
Molecular Orbit Theory (MO)
Explains the bond order observed in diatomic molecules
Can
Sigma bonds (can/cannot) rotate
Cannot
Pi bonds (can/cannot) rotate
Constructive interference
Occurs when the waves are in phase
Destructive interference
Occurs when the two waves are 180 degrees out of phase
Bond order equation
Bond Order=1/2(number of electrons in bonding molecular orbitals-# of electrons in anti bonding orbitals)
valence shell electron pair repulsion (VSEPR) theory
theory that allows prediction of the shapes of molecules based on the idea that electrons - either as lone pairs or as bonding pairs - repel one another
electron groups
general term for lone pairs, single bonds, multiple bonds, or lone electrons in a molecule
linear geometry
molecular geometry of three atoms with a 180 degree bond angle due to the repulsion of two electron groups
trigonal planar geometry
3 electron groups maximize separation by assuming 120° bond angles in a plane
tetrahedral geometry
109.5° angles between the electron groups
trigonal bipyramidal geometry
molecular geometry of six atoms with 120 degree bond angles between the three equatorial electron groups and 90 degree bond angles between the two axial electron groups and the trigonal plane
octahedral geometry
six electron groups around a central atom assume an _______, eight-sided shape given
electron geometry
geometrical arrangement of electron groups in a molecule
molecular geometry
geometrical arrangement of atoms in a molecule
trigonal pyramidal geometry
molecular geometry of a molecule with tetrahedral electron geometry and one lone pair
bent geometry
when certain atoms, such as oxygen, due to their electron configuration will almost always set their two covalent bonds in non-collinear directions
seesaw geometry
molecular geometry of a molecule with trigonal bipyramidal electron geometry and one lone pair in an axial position
T-shaped geometry
molecular geometry of a molecule with trigonal bipyramidal electron geometry and two lone pairs in axial positions
square pyramidal geometry
molecular geometry of a molecule with octahedral electron geometry and one lone pair
square planar geometry
molecular geometry of a molecule with octahedral electron geometry and two lone pairs
hybrid orbitals
_______ are eqivalent because they have the same size, shape, and energy
pi bond
A bond formed when parallel p orbitals overlap creating two regions of electron density, one above and one below the internuclear axis.
sigma bond
a bond formed when two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting the two atomic nuclei
molecular orbital (MO) theory
a more sophisticated model of bonding that can be applied equally successfully to simple and complex molecules
bonding orbital
molecular orbital that is lower in energy than any of the atomic orbitals from which it was formed
antibonding orbital
if it is occupied, the energy of the molecule is higher than for the two separated atoms
Bond Order
# of bonding electons-# of antibonding electrons/2 *More electrons in antibonding orbitals leads to a less stable molecule, which means it is weaker and is less likely to exist.
Paramagnetic
A substance that is attracted to a magnetic field
Diamagnetic
A substance that is not attracted to a magnetic field
True.
Once you know the electron-domain geometry, you know the hybridization state of the atom. True or False?
tetrahedral
sp³ orbital geometry
sp³d geometry
trigonal bipyramidal
octahedral
Bond Angle - 90° and 180°
The atoms are in the same plane when hybridized unitl the Pz orbital is used. Explain?
There is three P orbitals. Px, Py, and Pz. Pz makes it 3D, so unless that orbital is used the molecule will be planar.
p bonds characterized by:
What are Pi bonds characterized by:
Are single bonds Pi or sigma? Why
Single bonds are always s bonds, because sigma head-on bonding results in greater overlap- therefore creating a stronger bond and with lower energy
If a molecule has 3 bonds what type would they be? and how many of that type?
In a multiple bond one of the bonds is a sigma bond and the rest are pi bonds
unhybridized
Pi bonds occur in _______ (hybridized/ unhybridized) P orbitals.
Why do we use Molecular Orbital theory rather than Valance bond theory??
Though valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals
In MO theory, we invoke the wave nature of electrons
What does it mean when they say that orbitals have wave like characteristics?
Bond Order Formula
Bond Order= (# of valence e? in free/unbonded atom) -1/2(# of bonded e?)- (# of lone pairs of e?)
Which period of elements is different from all the other ones when it comes to MO theory?
The smaller p-block elements in the second period have a sizeable interaction between the s and p orbitals
Sigma* 2P
What is the exception?
Atomic Orbital
A mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom.[1] This function can be used to calculate the probability of finding any electron o…
How are orbital in an atom is characterized
by a unique set of values of the three quantum numbers n, l, and m, which correspond to the electron's energy, angular momentum, and 3 Quantum Numbers (n represents). Any orbital can be occupie…
the electron's energy, (n)
3 Quantum Numbers (n represents)
3 Quantum Numbers (l represents)
the electron's angular momentum (l)
s-orbital
s orbital refers to an orbital with angular momentum quantum number l=0
d-orbital
d orbital refers to an orbital with angular momentum quantum number l=2
f-orbital
f orbital refers to an orbital with angular momentum quantum number l=3
molecular orbital diagram, or MO diagram
a qualitative descriptive tool explaining chemical bonding in molecules in terms of molecular orbital theory in general and the linear combination of atomic orbitals (LCAO) molecular orbital method in particular. A fundamental principle of t…
0
0
1s
1s or 2p?
Penetration
Penetration describes the proximity of electrons in an orbital to the nucleus. Electrons which experience greater penetration experience less shielding and therefore experience a larger Effective Nuclear Charge (Zeff) but shield other electrons more effectiv…
Orbital Penetration
Different orbitals have greater nuclear penetration than others. Penetration refers to how effectively electrons can get close to the nucleus. The electron probability density for s-orbitals is highest in the center of the orbital…
Shielding
Shielding describes the amount of screening from nuclear charge that one electron can do with respect to its neighboring electrons. Electrons that have greater penetration can get closer to the nucleus and effectively block…
Effective nuclear charge (Zeff)
The "positive charge" actually "felt" by an electron
Periodic Trends Due to Penetration and Shielding: ELECTRONEGATIVITY
The electronegativity of the elements is highest near flourine. In general, it increases from left to right and decreases from top to bottom.
Periodic Trends Due to Penetration and Shielding: IONIZATION ENERGIES
The ionization energies increase from left to right, and decrease from top to bottom.
Periodic Trends Due to Penetration and Shielding: ATOMIC RADIUS
The atomic radius decreases from left to right, and increases from top to bottom.
Hybrid orbitals allows us to use
valence bond theory to describe covalent bonds (sharing of electrons in overlapping orbitals of two atoms)
to describe the electronic orbitals used by the central atom in bonding
When we know the molecular geometry, we can use the concept of hybridization
NH3 VSEPR indicates
VSEPR indicates: tetrahedral geometry with one non-bonding pair of electrons (structure itself will be trigonal pyramidal)
Rule for Hybridization
rule for hybridization: