Molecular orbital diatomics Flashcards
molecular orbitals (MOs) describe
where the electrons are found in molecule
shapes of the orbitals
relate to the electron density
if the electron density is high between two atomic nuclei, a stable bond forms –
a bonding MO
If the electron density is low between two atomic nuclei,
the positively charged nuclei repel and a bond does not form antibonding MO
As the atoms come closer together,
overlap of the AOs forms a MO
If the two AOs have the correct phases
, a bonding MO forms and the overall energy decreases
Consider the example of two s-orbitals on neighbouring atoms
- If we bring together two orbitals with the same phase, the result is a bonding sigma orbital
if the AO becomes too close
if the AO becomes too close
Electrons resides in the same space, Pauli exclusion rule.
if the AO becomes too close
Electrons resides in the same space, Pauli exclusion rule.
Bond order =
½ (number of electrons in bonding MOs – number of electrons in antibonding MOs)
This formula recognizes that electrons in antibonding MOs cancel out the bonding effect of
electrons in bonding MOs.
A single bond requires a net number of 2 bonding electrons.
A double bond requires a net number of 4 bonding electrons.
Bond order H2
For H2
the bond order is 1 and there is a single bond
The MO diagram for He2
- Each He atom contributes two 1s electrons to the occupied MO
The electronic configuration of He2
is 1s2
1s*
There is no net bonding
The electronic configurations of the Li atoms
The 1s orbitals are too close to the Li atom nuclei to
overlap with each other in Li2
* We call these core electrons and do not include them
in the MO diagram
* Hence we focus on the valence electrons (in this case
the 2s electrons)
* If we are using AOs with radial nodes such as 2s, there
is no need to include the radial nodes in the diagrams
How to tell if the molecular orbital is s or
PI - NODES
SIGMA - NO NODES
There are two pi MOs with the same energy
- One pi MO forms from overlap of px + px
(pi x
) and another from overlap of py + py
(pi y
) - The two MOs that result have the same energy (they are degenerate) but are at 90o
to each other
- Dashed lines show which AOs combine to make each MO
- The best overlap comes from
AOs of similar size
Guiding rules for drawing the MO diagrams:
- Horizontal lines represent AOs or MOs, and energy increases up the page
- We focus on the valence electrons most involved in bonding (so the “core” 1s orbitals are often not shown#
- Each pair of interacting AOs
produces both a bonding and an antibonding MO
- Hence the number of MOs formed
is the same as the starting number of AOs
- The strongest interactions are between
AOs of similar energy
For N2 and lighter homonuclear diatomic molecules of the second period elements
, a picture allowing hybridization of the 2s and 2p orbitals gives a better representation of the bonding
in B, C and N vs. O, F and Ne
the 2s and 2p orbitals are closer in energy
In homonuclear diatomic molecules (e.g. H2, N2, O2),
the MOs are also labelled with a subscript g or u
g
stands for gerade and means symmetric with respect to the centre of the molecule
u
u stands for ungerade and means antisymmetric with respect to the centre of the molecule
