Bonding Molecular orbital theory - molecular orbital theory Flashcards
constructive waves definition
waves which overlap in the same phase which produce a greater amplitude.
destructive waves definition
waves which overlap in different phases which produce no amplitude.
how are antibonding molecular orbitals formed
The electrons in atomic orbitals will act as deconstructive waves and overlap out of phase, producing a higher energy orbital where electrons will fill after the bonding orbital.
how are bonding molecular orbitals formed
The electrons in atomic orbitals will act as constructive waves and overlap in phase, producing a lower energy orbital where electrons will fill before the antibonding orbital.
wave function of bonding molecular orbital
ψ = CaXa + CbXb
wave function of molecular orbitals
ψ* = CaXa - CaXb
ψ = CaXa + CaXb
c stands for weighting co efficient.
x stands for atomic orbital wave function.
ψ stands for molecular orbital wave function.
how do you show that an orbital is an antibonding molecular orbital
you use a *
antibonding molecular orbitals features
antibonding molecular orbitals will always contain an extra nodal plane than bonding molecular orbitals due to destructive waves overlapping to produce a part of the wave function which will pass through 0.
s atomic orbitals linear combination of atomic orbitals
same sign s orbitals form σg
opposite sign s orbitals will form σu*
g meaning
g stands for even
(gerade)
when a molecular orbital remains in the same phase after being rotated 180 degrees around the bond axis.
u meaning
u stands for odd or ungerade:
this is where when you rotate the bond 180 degrees around the bond axis the phase will change.
u orbitals
σ* and π
antibonding sigma and bonding pi
g orbitals
σ and π*
bonding sigma and antibonding pi
numerical proof for u and g
if when +orbital = (x,y,z), -orbital = -(x,y,z) it is u
if when + orbital = (x,y,z), - orbital = (x,y,z) then it is g.
when would you use u and g
when you have homonuclear atoms bonding with the same type of orbital.
bond order formula
(number of electrons in bonding orbitals - number of electrons in antibonding orbitals) / 2
which molecular orbitals fill first
bonding molecular orbitals fill first since they are lower energy.
what does a bond order of 0 show
the molecular structure will not be possible.
bond order rules
as the bond order increases the bond strength increases and bond length will decrease.
oxidation of molecules effect on bond order
oxidation is a loss of electrons, which will mean that electrons will be lost from the HOMO, this will alter the bond order.
reduction of molecules effect on bond order
reduction is a gain of electrons, so electrons will fill the LUMO.
LUMO
the lowest unoccupied molecular orbital
HOMO
the Highest occupied molecular orbital.
SOMO
A singularly occupied molecular orbital
what does a SOMO mean
It means that the HOMO and the LUMO are the same orbital, meaning electrons will be lost and gained from the same orbital.
rule for orbitals considered in bond order and molecular orbital diagrams
always just use valence orbitals
eg: if we use fluorine we only consider 2s and 2p orbitals.
what does a squiggly line mean in molecular orbital diagrams
the squiggly line will represent the core orbitals which don’t matter in the final calculation.
linear combinations of atomic orbitals for p orbitals
same sign pz atomic orbitals will form σg
opposite sign pz atomic orbitals will form σu*
same sign pxy atomic orbitals will form πu
different sign pxy orbitals will form πg*
molecular orbital diagrams using p orbitals steps
- write out electronic configuration of atoms.
- identify the number of valence electrons.
- set out molecular orbital energy chart.
fill in electrons using Aufbau and hunds rule.
calculate bond order.
labelling orbitals in MO diagrams
give every molecular orbital a number starting with 1 being the lowest energy π and σ orbitals.
bond strength formula
v = 1/2π x sqrt(k/μ)
v is vibrational frequency
k is the force constant
μ is the reduced mass
linear combinations of atomic orbitals for s and p overlap
same sign pz and s will result in a σ bonding orbital.
opposite sign pz and s will result in a σ antibonding orbital.
px or py with an s orbital will result in no orbital being formed due to the difference in symmetry between the orbitals meaning that the constructive and destructive overlap of the waves will cancel each other out.
orbital energy rule affected by electronegativity
As you go along a period the positive charge of the nucleus increases, which will result in a greater difference between the σ and π orbitals
elements where σ and π orbitals will mix
boron, carbon and nitrogen due to low energy differences between 5σg and 1πu
elements where σ and π will not mix
fluorine, oxygen and neon due to higher energy gap between the 5σg and 1πu orbitals.
rule for extra contribution of bonding to molecular orbital diagrams
pz and s orbitals will contribute towards all σ orbitals.
px and py will contribute towards all π bonds.
rule for π orbitals energy
π orbitals energy wont change only the σ orbitals energy changes.
electronegativity on heteronuclear diatomic Molecular orbitals
the more electronegative atom will have lower energy orbitals and therefore contribute more towards the bonding molecular orbital.
the electropositive atom will have greater energy orbitals and contribute more towards the antibonding ,molecular orbital.
