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How come for trigonal planar geometry and for tetrahedral geometry if the number of bonded electrons (coordination number) is 2, the molecular geometry is bent but for trigonal bipyramidal when the number of bonded electrons is 2, the molecular geometry is linear - not bent? Thank you in advance!
Hi! Whether a molecule is bent or linear depends on if it has a lone pair. Lone pairs push the other bonds away, creating a bent or angular shape. For example, CO2 is a linear molecule because it does not have a lone pair, but H2O is a bent molecule because of the lone pairs on the oxygen atom.
The bent shape is a result of the lone pair. In a trigonal planar shape where there is one lone pair present, the molecular shape is bent because the lone pair repels or pushes away the two surrounding atoms.
In addition to the great explanations above, the lone pairs become very important in differentiating certain compound models due to their repulsive forces, which also directly changes the bond angles. They are tricky but important for understanding the compounds better.
When there are five areas of electron density and only two bondings pairs, the molecular shape is linear since the unbonded electrons are in the equatorial plane and they cancel each other's repulsion, leaving the bond angle to be 180. For the tetrahedral and trigonal planar geometry, when there are two bonds, the shapes are bent because the unpaired electrons repel.
The bent shape comes from the lone electron pair because that region counts as an area of electron density. That lone pair causes a repulsion for the other electrons which push the other bond angles closer together.