| water molecule, H2O |
| nitrogen dioxide molecule, NO2 |
| nitrite ion, NO2- |
| sulfur dioxide molecule, SO2 |
The illustrations and discussion below show how the presence of non-bonding electron pair(s) on a molecule's central atom affects the geometry of the molecule and the bond angle of its atoms.
Geometry of Water, H2O
As shown in its Lewis structure below, the central atom of water has four pairs of electrons: two bonding pairs and two non-bonding or lone pairs.
These pairs of electrons are presumed to take a tetrahedral arrangement in space.
Here, the water molecule is depicted as being superimposed in a tetrahedral outline.
The lone pairs of electrons are relatively closer to the nucleus of the central atom and they tend to crowd the two bonding pairs together so that the H-O-H bond angle is less than the ideal tetrahedral angle of 109.5°.
- shape of water: bent or angular
- H-O-H bond angle: 104.5°
Geometry of Nitrogen Dioxide, NO2
The central atom of nitrogen dioxide, nitrogen, has two sigma bond electron pairs and an unpaired non-bonding electron as shown in its Lewis electron dot structure below.
In order for this group of electrons to be farthest from each other, it is predicted that they take a trigonal planar arrangement.
Shown in the next illustration is the nitrogen dioxide molecule superimposed on a trigonal planar outline.
The observed O-N-O bond angle of 134.1° in nitrogen dioxide molecule, being closer to trigonal angle of 120°, confirms this prediction.
The opening out of the O-N-O bond angle is due to the less crowding affected by the half filled non-bonding orbital of the central atom.
This half-filled orbital accounts for the nitrogen dioxide (NO2) being paramagnetic.
- shape of nitrogen dioxide: bent or angular
- O-N-O bond angle: 134.1°
Geometry of Nitrite Ion, NO2-
An addition of an electron to the central atom of nitrogen dioxide (NO2) creates the nitrite anion, NO2-.
In the above diagram, the resonance structures of nitrite ion is shown. The oxygen atoms in red partial shades are atoms with negative formal charge.
As is the case with the nitrogen dioxide, the arrangement of the two sigma bond electron pairs and the lone electron pair of nitrite ion's central atom lies on a trigonal plane.
The only difference is the observed O-N-O bond angle of 115° for the nitrite ion. This bond angle now being much closer but less than the trigonal angle of 120° is attributed to the filling up of the non-bonding orbital of the nitrite ion's central atom.
The lone electron pair is relatively closer to the central atom and occupies more space than the adjacent bonding electron pairs do.
- shape of nitrite ion: bent or angular
- O-N-O bond angle: 115°
Geometry of Sulfur Dioxide, SO2
Referring to the contributing Lewis structures of sulfur dioxide below, there is a -1 formal charge on one of the oxygen atoms and +1 formal charge on the sulfur atom which cancel each other out, leaving the molecule with zero net charge.
Given the two sigma bond electron pairs and one lone electron pair on the central atom of sulfur dioxide, as shown in the figure above, SO2 is predicted to have a trigonal planar geometry.
A trigonal planar outline is shown below superimposed on the sulfur dioxide molecule.
Given this geometry, it is expected that SO2 molecule has an angular shape. This is supported by the fact that sulfur dioxide has a dipole moment.
The O-S-O bond angle is expected to be somewhat less than the trigonal angle of 120° due to the presence of a lone electron pair on the central atom.
- shape of sulfur dioxide: bent or angular
- O-S-O bond angle: less than 120°
