(Polar molecules, Non-polar molecules, etc.)
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Note that the parent electronic structure (considering both lone pairs and bonding pairs) of water is tetrahedral because there are 4 regions of electron density around the central oxygen - 2 lone pairs and 2 bonding pairs. For a tetrahedral geometry, each corner is equidistant from the other corners; in other words, there is not preferential location of bonding pairs or lone pairs like there is with a trigonal bipyramidal or octohedral geometry. When considering the molecular structure, we only consider the atoms, hence the shape of water is bent or angular (remove 2 corners ie. lone pairs from the tetrahedron). The lone pairs do increase the amount of electron pair repulsion however, which decreases the HOH bond angle to 104.5 degrees from the 109.5 degree angles of a normal tetrahedral geometry.
It's because oxygen has 2 lone pairs that take up space. CO2 on the other hand does not have any lone pairs that is why it is straight across. I would visualize the lone pairs as almost like another bond, so each bond needs to be placed around the oxygen molecule like a box. For example in SF4 the S is surrounded by the Fs in a box like shape. The same goes for water, but because we usually do not denote the electrons it looks like an L as opposed to a box
The H2O molecule is bent because there are 4 regions of electron density but only 2 bonding pairs. Dr. Lavelle demonstrated this in class with a foam model by starting with the tetrahedron-shaped molecule and then removing 2 of the atoms. Hope this helps!
The reason H2O is bent is due to the high amount of repulsions that is caused by the two lone pairs. As Lavelle emphasized in his lecture, the most powerful repulsion occurs between two lone pairs. Therefore, since oxygen has two lone pairs on its central atom and two bond pairs, the two bonds are repelled by the high electron density of the two lone pairs. Consequently, rather than being linear, its bond angles are positioned downwards at a 104 degree angle.
Due to the three domains of electron density, two of which being bonded pairs, and the other being a lone pair, the overall shape of H20 is bent. This is partly due to the fact that the lone pair pushes the two bonded pairs down, if that makes any sense. Hope this helped :)
H20 is bent because the two lone electron pairs that are on one side of the molecule cause a repulsion on the hydrogen atoms attached to the oxygen molecule. The repulsion is ultimately what induces the tetrahedral shape. I visualize it as each lone pair having the energy to push down (or up) on the H atoms.
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