Explanations:- The shape of the molecule depends on bonding groups and lone pair of electrons present in the central atom. We do the sum of these bonding groups and lone pair of electrons for the shape of the molecule. The presence of lone pair of electrons affects the shape (molecular geometry) of the molecule.
For example, methane is tetrahedral as four hydrogen atoms are bonded to carbon and there is no lone pair of electrons present on C. Here, the sum of bonding groups and lone pairs is 4.
On the other hands, in water, two H atoms are bonded to the central oxygen atom and also there are two lone pair of electrons present on oxygen atom. The sum is also 4 here but the molecular geometry is bent. The difference in geometries of these two molecules is because of the repulsion between the two lone pairs present on oxygen atom.
If we take an example of boron hydride then three H atoms are bonded to B and there is no lone pair of electrons present on B. The sum is 3 and so it's shape is trigonal planar.
So, the number of bonded atoms as well as the number of lone pairs present on the central atom decides the shape of the molecule.
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