Topic 2 Valence Bond & Topic 2: Molecular Orbital Theory: Homonuclear Diatomics
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How to tell the electronegativity of an atomic pair. This shows how much … and … character is has. The closer to … the more … character.
Δχ = χx – χY covalent ionic 0 covalent
Lewis Theory
Method of describing the arrangement of valence electrons in molecules. Uses dots and crosses to represent the number of valence electrons associated with the nuclei. If possible, all electrons in a molecule should appear in pairs. Single electrons signify a radical species.
Why can it be energetically favourable to promote electrons to a higher energy orbitals
Bond formation gains energy for the system (releases energy)
Hybrids formed for: s + p s + 2p s+ 3p
2 sp 3 sp2 4 sp3
Molecular orbital theory
Current approach to describe bonding in molecules and predict trends such as bond length and dissociation energy differences, magnetism and chemical reactivity (not geometry) Atoms have AO’s, some with electrons and some without, so molecules must have MO’s Electrons go into Molecular Orbitals and these are filled from the lowest energy state progressively upwards
MO theory has a … approach, as an electron in an Atomic Orbital A can be described by a … (the mathematical function that replaces the classical concept of trajectory andwhose square defines probability of position), A, so too can an electron in a Molecular Orbital of amolecule AB by a wavefunction AB
quantum mechanical wavefunction
How to Determine Molecular Wavefunctions?
Solve Schrödinger’s equation for exactly 1 electron system like H2+ More complex, use the Linear Combination of Atomic Orbitals (LCAO) approach
The LCAO Approach Interaction criteria:
• the energies of the atomic orbitals must be similar • the symmetries of the atomic orbitals must be compatible • the overlap between the atomic orbitals must be significant
Place the atomic nuclei of the molecule in their … Consider the Atomic Orbitals and determine ways that they can interact to form Molecular Orbitals through … The total number of MO’s to be created is equal to the number of AO’s in the basis set If 2 AO’s combine to form a new …, they will also form a new … Antibonding MO’s have a … between the nuclei. When all MO’s have been constructed they can be ranked in energy to create the … which is then filled with …
equilibrium (lowest energy) positions simple linear combinations via addition and subtraction bonding MO antibonding MO nodal plane (change in phase) MO Energy Level Diagram electrons
Aufbau Principle
Orbitals of lowest energy are filled first
Pauli Exclusion Principle
no two electrons in the same atom can have identical values for all four of their quantum numbers
Hund’s Rule
if there are several orbitals of the same energy (degenerate) then the electrons will arrange themselves so that there is the maximum number of unpaired electrons
The number of AO’s and MO’s is …
the same
Bonding MO’s leads to … electron density between nuclei while antibonding MO’s leads to …
more less
The MO probability function for (H2+) = is wave function
2(H2+)
1 = 1* = is wave function
N(c1A + c2B) N(c1A - c2B) N and N are normalisation constants to ensure the probability of finding an electron in the orbital over all space = 1 c1 and c2 are overlap coefficients that reflect the “parentage” of the MO from the AO’s
Orbitals that are symmetric about the internuclear axis are labelled as … Orbitals that are asymmetric about the internuclear axis are labelled as … Antibonding orbitals are labelled with a …, have a … between the nuclei and the plane is orthogonal to the internuclear axis
sigma pi * nodal plane (plane of zero probability)
bond order = Stable molecular have a bond order … Bond order is proportional to … and …
(number of electron in bonding MO - anti bonding MO) / 2 >0 bond dissociation energy 1/bond length
s-s overlap forms
bonding orbital sigma anti bonding orbital sigma*
s-p overlap forms
s + head on p (e.g. pz) : bonding orbital sigma anti bonding orbital sigma* when s + side on p (e.g. px) : non-bonding
p-p overlap forms the orbitals …(3)
When head on (e.g. pz + pz) : bonding orbital sigma anti bonding orbital sigma* When side on (e.g px + px/py + py) : bonding orbital pi anti bonding orbital pi* When no overlap (e.g. px + pz) : non-bonding
Both sigma/pi and sigma */pi * orbitals have … while only … has …
internuclear axis sigma/pi nodal plane perpendicular to internuclear axis
MO energy ranking
sigma 1s sigma* 1s sigma 2s sigma* 2s sigma 2pz pi 2px and pi 2py pi* 2px and pi* 2py sigma* 2pz
Sketch and name the 3 bonding and 3 antibonding molecular orbitals from p-orbital combinations
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