week 3 Flashcards
what are molecular orbitals made of
atomic orbitals
adding wavefunctions of AOs across the molecule
number of AOs = …
number of MOs
what 2 things must AOs fulfil to form MOs
AOs must match interns of symmetry
AOs must be close enough in energy to overlap
- the closer in energy, the larger the interaction
AOs must be of similar energies
in which two ways can AOs overlap
constructively/in-phase
- peaks and troughs of wave line up perfectly
- leads to a wave that is the sum of both waves
destructively/out-of-phase
- waves are oppositely aligned
- positive peaks of one coincide with negative peaks of the other
- waves cancel out, leads to a node
see w3 vid13 for examples
what does a sigma orbital suggest about an orbital
sigma means the orbital is symmetric about the internuclear axis
what is used to denote an anti-bonding orbital
star - *
describe the orbital formed wen two in-phase 1s AOs overlap
a sigma gerade orbital
- sigma bc symmetric about internuclear axis
- gerade because symmetric wrt inversion centre
what is a nodal plane in an orbital
region where there is 0 probability of finding electron density
what is more energetically stable, bonding or anti-bonding MOs
bonding MOs
what does bond order indicate
how is it calculated
overall strength of bonding interaction
= 1/2(no. of bonding e⁻- no. of anti-bonding e⁻)
describe the in phase and out of phase overlap of the 2pz orbital
2pz orbitals will overlap in a horizontal manner with the heads of the lobes overlapping into each other ∞↔︎∞like that
in phase
- sigma gerade orbital σg
out of phase
- sigma ungerade σu*
describe the in-phase and out of phase overlap of the 2px/y orbitals
why are they π orbitals
px and py form π orbitals because
- the resulting MOs change sign when rotated 180° across internuclear axis
- pi-orbitals are anti-symmetric wrt rotation around internuclear axis
they overlap like this = 8↔︎8 with the lobes vertical
in phase
- lobes at the top and at the bottom are the same sign
- forms πu orbital
out of phase
- pi gerade anti bonding orbital πg*
what does increased overlap between AOs mean for the energy of MOs
more overlap between AOs = bigger energy separation between bonding MO and anti-bonding MO
which have more overlap, pz or px/py
pz have greater overlap
so larger energy separation between bonding and anti-bonding MOs
see w3 vid14 for examples fo MO diagrams
what is sp mixing
when MOs of the same symmetry label interact with each other
describe the sp mixing in the 2nd energy level
what effect does this have on orbital energy?
σg MO from 2s orbitals and σg MO from 2pz orbitals will interact
just like two AOs of similar symmetry can combine, MOs of similar symmetry can combines
results in one MO of higher energy and one of lower energy
- σg (2pz) will become destabilised and increase in energy, past the πu MOs of px and py
- σg (2s) becomes more stable and moves down in energy
the same is observed for the two σu* orbitals from 2s and 2pz
what is needed for sp mixing to happen
in which period 2 elements is this observed and in which ones is it not
2s and 2p energy gap must be small enough
in B, N and C, it is observed, energy gap is small enough
not seen in O and F because energy gap is too big
what is the relationship between bond order and:
- bond distance
- bond dissociation enthalpy
- higher bond order = greater bond distance
- higher bond order = higher bond dissociation
how do the following vary as you go down a group:
- bond order
- bond distance
- bond dissociation
- bond order is the same
- bond distance increases
- bond dissociation generally decreases
when constructing MO diagrams for heteronuclear diatomic, what is not necessary
gerade and ungerade because there is no centre of inversion in heteronuclear molecules
MO diagrams for heteronuclear diatomics:
which element will the bonding MO have more character of
will have more character of the more electronegative element
see w3 vid16 fo examples of heteronuclear diatomic MO diagram
MO diagrams for heteronuclear diatomics:
which element will the anti-bonding MO have more character of
will have more character of the less electronegative element
define electronegativity
give the trends of it
the ability of an atom in a molecule to attract electrons to itself = greek latter kai ϗ
decreases down groups
increases left to right
how does electronegativity affect stability of orbitals
more electronegative = more stable orbital
electrons are held more ‘tightly’ towards highly attractive atom and so more stable
