In our last post we discussed the geometries of molecules without lone pair of electrons. We know that, in this model, the valence shell , is considered as a sphere with electron pairs on the spherical surface, at maximum distance from one another. The maximum distance is necessary to make sure that there is minimum repulsion between the two electron pairs.
However, due to the presence of non-bonding / lone pair of electrons ,molecules depart from ideal electronic geometry/bond angles (as predicted by VSEPR model).
Let us discuss how these non- bonding pairs affect molecular geometries.
The effect of Non-bonding/lone pair of electrons.
The presence of lone pair of electrons in a molecule results in deviation from ideal geometries/ bond angles, as predicted by VSEPR theory.Such deviations can be explained on the basis of electron domains(the region, which an electron pair occupies in space.)
The reasons for deviation from ideal geometries are –
1.Non-bonding/lone pairs occupy more space than bonding pairs –
The bonding pair of electrons are a part of a bond between two atoms, they are diffused through orbitals of A-B s. Thus, they are away from the nucleus of one single atom (as they are shared, they lie between two nuclei) and are attracted by both nuclei.Thus, the electron domain reduces in size, as it has positive nuclei at both the ends pulling it. Whereas a lone pair is on a single atom and is only attracted by a single nucleus.So having a positive nucleus only at one end keeps the electron domain “fatter” as compared to that of a bonding pair domain.
2. The intensity of repulsion between different electron pairs is as follows –
Lone pair – Lone pair > Lone pair – Bonding pair > Bonding pair – Bonding pair electrons, i.e
LP – LP > LP- BP > BP – BP.
As the lone pairs occupy more space, the electron domains (region in which the electrons lie) of two lone pairs on a single atom come close to each other.Thus, the repulsion between them is more. This is true for LP-BP interactions too.The BP electron domains are the farthest apart from each other and thus there is minimum repulsion between them.
3. Repulsions occurring at angles more than 90 °are not significant.
4. Bonding pairs of electronegative substituents occupy less space than the electropositive ones as these electrons are strongly attracted by the electronegative atom.Thus, the repulsion between two BP’s becomes less and so the bond angle becomes less.
In the above example, fluorine is more electronegative than hydrogen.
So, BP of electrons between N – F are attracted more towards F atom → repulsion between N-F bonding electron pairs decreases → the N-F bonds come closer to each other as a result of decrease in repulsion between them → Bond angle decreases.
5. Electron pairs in filled shell repel stronger than electron pairs in incomplete shell.
When there is an incomplete shell (of similar energy) in an atom the electron pair from the filled shell can undergo diffusion to the vacant orbitals. Hence, the LP – BP repulsion diminishes dramatically as the electron pairs in a vacant shell repel less. Thus, the bond angles around the central metal decrease.
Less repulsion between LP -BP → the bonds come closer → decrease in bond angle.
6.Triple bonds repel other bonding-electrons more strongly than double bonds and double bonds repel other bonding-electrons more strongly than single bonds.
Based on the above facts let us now study some examples –
Deviation from tetrahedral geometry /109.5 °bond angle – We studied in the earlier post, that four electron domains exhibit a tetrahedral geometry with bond angle 109.5°. So, methane has a bond angle of 109.5°. However when lone pairs are present in a molecule, the measure of this ideal bond angle (109.5 °) changes and so does the geometry of the molecules.
e.g.– Ammonia has a pyramidal shape(bond angle = 107.3 °) with one lone pair of electron domain and water molecule has a bent shape( bond angle = 104.5 ° ) as it has 2 lone pair of electron domains.This change in geometry is due to the presence of one and two lone pair of electrons on ammonia and water respectively. The LP occupies vertices of the tetrahedron .
As the lone pair is ‘fatter’ , it occupies more space and thus it pushes the bonding pair domains closer to one another.Thus, the bond angle and geometry changes .
(Imagine three slim people comfortably sitting in a car’s rear seat. What happens when one fat person replaces a slim one? The two slim people are pushed closer to make room for this fatter person. Similar thing happens with the electron domains too).
The following table shows us deviations from the ideal geometries –
The above chart shows how the geometry changes with the introduction of one and two lone pairs of electrons on the central atom.
Steric numbers of 7 or greater are less common.
In our next post we continue discussion on VSEPR and study some examples.Till then,
Be a perpetual student of life and keep learning…
Good day !
References and Further reading –
Image source –
4.By Benjah-bmm27 – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2035653