Valence Shell Electron Pair Repulsions (VSEPR) is a theory that is used to estimate shapes. It is based on the premise that the valence electrons of each adjacent atom will repel each other (electrostatic repulsion) strongly and therefore cause the peripheral atoms to move as far from each other as possible. This short video gives you an introduction to the VSEPR theory.

For more about molecular structure, follow the link below and review the Lewis Dot Diagrams of BF₃ and O₃ along with CH₄, NH₃ and H₂O.

VSEPR