Lone pairs have the same effect as bonded electron pairs. The repulsion forces of the valence electrons directly effect the size of the bond angle. The bond angle is the angle formed by two pairs of valence electrons and the central atom that connects the two. The stronger the repulsion strength, the larger the bond angle. The lone pair electrons bring a twist to the idea of molecular geometry .
Molecular geometry is the molecule's arrangement of atoms in space.
It does not describe the placement of the electrons around each atom. To help with this, we have what is called the electron-pair geometry. The electron-pair geometry accounts for the location of all bond and lone pairs of electrons. Let's look at an example. Below is a Lewis structure of NCl 3 .