The halogens have 7 electrons in their outer shells. This is not a very stable setup, but an outer shell with 8 electrons is stable. Because of this, a halogen will tend to gain 1 extra electron to fill this space. An electron carries a 1- charge.
The location in three-dimensional space of the nucleus of each atom in a molecule defines the molecular shape or molecular geometry. Molecular shapes are important in determining macroscopic properties such as melting and boiling points, and in predicting the ways in which one molecule can react with another. A number of experimental methods are available for finding molecular geometries, but we will not describe them here. Instead we will concentrate on several rules based on Lewis diagrams which will allow you to predict molecular shapes.To provide specific cases which illustrate these rules, "ball-and stick" models for several different types of molecular geometries are shown in Table 1. The atoms (spheres) in each ball-and-stick model are held together by bonds (sticks). These electron-pair bonds determine the positions of the atoms and hence the molecular geometry.