The shape of a molecule is determined by electron-pair repulsions in the valence shell. A lone pair occupies larger space than a bond pair because it is not shared by two nuclei. Thus the lone pair-lone pair repulsion is greater than the lone pair-bond pair repulsion, which in turn is greater than the bond pair-bond pair repulsion. The presence of lone pairs causes distortion of bond angles and hence a deviation from an ideal shape. The extent of distortion depends upon the orientation of the lone pairs around the central atom. In a trigonal bipyramid, the lone pairs occupy equatorial positions than the apical ones. In AB_n type molecules, as the electronegativity of A increases, the bond pairs come closer and the repulsion between them increases. On the other hand, as electronegativity of B increases, the bond pairs get farther and repulsion decreases.