An example of bent molecular geometry that results from tetrahedral electron pair geometry is H2O. The water molecule is so common that it is wise to just memorize that water is a BENT molecule.
The oxygen has 6 valence electrons and thus needs 2 more electrons from 2 hydrogen atoms to complete its octet. This then leaves two lone electron pairs that are not bonded to any other atoms. The two hydrogen atoms and the two lone electron pairs are as far apart as possible at nearly 109o bond angle. This is tetrahedral electron pair geometry.
The 2 lone electron pairs exerts a little extra repulsion on the two bonding hydrogen atoms to create a slight compression to a 104obond angle.The water molecule is bent molecular geometry because the lone electron pairs, although still exerting influence on the shape, are invisible when looking at molecular geometry. The molecule is two dimensional and bent as opposed to the beryllium hydride case which was a linear or straight line molecular geometry because it did not have a lone electron pair.
In this example, O3, the Lewis diagram shows O at the center with one lone electron pair and two other oxygen atoms attached. This shows trigonal planar for the electron pair geometry and and bent the molecular geometry. The one lone electron pair exerts a little extra repulsion on the two bonding oxygen atoms to create a slight compression to a 116obond angle from the ideal of 120o. Compare it to the beryllium hydride (BeH2) which has 2 hydrogen atoms and no lone electron pairs. Ozone is a protective molecule by absorbing UV radiation when high in the stratosphere. However when close to the surface, ozone is produced as part of photochemical smog air pollution and is a powerful lung irritant.
In this example, NO2, the Lewis diagram shows nitrogen at the center with one lone electron. The nitrogen and and one oxygen are bonded through a double bond which counts as "one electron pair". Hence the molecule has three electron pairs and is trigonal planar for electron pair geometry. The one lone electron exerts a less repulsion than normal on the two bonding oxygen atoms so they are able to spread out more to a 134o bond angle from the ideal of 120o.
The Lewis diagram is as follows:
N = 5 e-
O = 6e- x 2 = 12e-
Total electrons = 17
All oxygen atoms have an octet of electrons. Nitrogen atom does not have an octet because the whole molecule is short an electron. This electron deficient feature of the molecule make is very reactive because it will try to react with some other molecule to complete the octet. Nitrogen dioxide is a main component of photochemical smog air pollution. It is coproduced from the high temperature combustion of gasoline in the presence of air which also contains nitrogen.
- Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook3