What is the main idea behind VSEPR?

The simple idea behind VSEPR theory is that valence electrons around a central atom (either as lone pairs or involved in bonding pairs) will repel one another. When they do this, they will take on specific geometries that try to maximize the distance between these electron groups.

This is the basis for the geometries of chemical covalent compounds.

3 Electron Groups: If your atom has 3 electron groups around it, these electron groups will take on a geometry that looks like a flat triangle....trigonal planar.

4 Electron Groups: 4 electron groups around a central atom will cause the geomtry of these groups around the central atom to be that of a tetrahedron, with ~109.5 degrees between groups.

5 Electron Groups: The geometry of the groups around the central atom in this case will be two-fold......3 groups will be in a flat triangle, and two other groups will be orthoganol to this plane. Trigonal Bipyramidal.

6 Electron Groups: Octahedral, with electron groups in 6 directions, all 90 degrees apart.

Now of course there are differences in name and bond angle between things that have only bonds vs. lone pairs, etc....but be it bonds or lone pairs, the geometries around the central atoms of these electron groups will be the above. And that is the basic idea behind VSEPR theory.

The main idea behind VSEPR (Valence Shell Electron Pair Repulsion) theory is that the shape of a molecule is determined by the arrangement of electron pairs around the central atom, which repel each other to be as far apart as possible, resulting in a geometry that minimizes electron pair repulsions.