(Polar molecules, Non-polar molecules, etc.)
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because we can't calculate the exact angle but we can estimate it based on it compared to something else. for example a line has a 180 degree angle and if there were to be an angle slightly smaller than a line it would just be <180
This depends on if there is greater repulsion of bonds based on polarity and lone pairs. For example, H2O has slightly less than 109.5 degrees bond angle because the lone pairs on the oxygen repel the hydrogens away from the lone pairs and closer together to each other.
If the atoms bonded to the central atom are not the same, the size and electron distribution around the atoms will differ which will affect the distribution of the electrons on the other atoms, which will affect the bond angle.
The reason why some molecules have bond angles that are "smaller" than the value of the general shape is because they have lone pairs. Molecules with lone pairs will have angles that are smaller than a given value because lone pairs have a stronger repelling effect than bonded electrons, which pushes atoms bonded to the central atom closer together. This results in smaller angles.
Bond angles can be slightly smaller than a given value when at least one region of electron density within the molecule is a lone pair of electrons. Lone pairs have a higher negative charge, so they repel the other regions more than a bonding pair would. This force of repulsion pushes the other regions to smaller angles that they normally would be observed at.
When the central atom has one or more lone pairs, the higher strength of electron pair-electron pair and electron pair-bonded pair repulsion compared to bonded pair-bonded pair repulsion results in the bonds being pushed to form smaller angles.