repulsion

[Jina Kwon]

How does repulsion contribute to definite size and shapes?

[Maria Poblete 2C]

Let's take a molecule with tetrahedral electron geometry, for example. If 4 bonded atoms are connected to the central atom, the molecular geometry will also be tetrahedral with bond angles measuring approximately 109.5 degrees.
However, if only 3 bonded atoms and one lone pair of electrons are connected to the central atom, the molecular geometry will be trigonal planar and the bond angles will measure qualitatively less than 109.5 degrees each. This is because only three bonded atoms are connected, and the lone pair of electrons repulses the other nearby electrons on other atoms away.
So, to summarize, the presence of lone pairs around an electron will repulse other nearby electrons, which will lower the bond angle shape. Presence of lone pair electrons can also indicate how many bonded atoms there are around a central atom, which will help you determine the molecular geometry of the molecule.

[SGonzales_3L]

Lone pair-lone pair repulsion is higher than lone pair-bonded pair repulsion, which is then higher than bonded pair-bonded pair repulsion.

Thus, lone pair-lone pair repulsion around the central atom can reduce the bond angle between atoms attached to the central atom by a large amount. Lone pair-bonded pair repulsion is much less and wouldn't reduce the bond angle as much as lone pair-lone pair. Similarly bonded pair-bonded pair repulsion keeps the standard VSEPR shapes without much alteration to the bond angles.