<P> VSEPR theory is used to predict the arrangement of electron pairs around non-hydrogen atoms in molecules, especially simple and symmetric molecules, where these key, central atoms participate in bonding to two or more other atoms; the geometry of these key atoms and their non-bonding electron pairs in turn determine the geometry of the larger whole . </P> <P> The number of electron pairs in the valence shell of a central atom is determined after drawing the Lewis structure of the molecule, and expanding it to show all bonding groups and lone pairs of electrons . In VSEPR theory, a double bond or triple bond are treated as a single bonding group . The sum of the number of atoms bonded to a central atom and the number of lone pairs formed by its nonbonding valence electrons is known as the central atom's steric number . </P> <P> The electron pairs (or groups if multiple bonds are present) are assumed to lie on the surface of a sphere centered on the central atom and tend to occupy positions that minimize their mutual repulsions by maximizing the distance between them . The number of electron pairs (or groups), therefore, determines the overall geometry that they will adopt . For example, when there are two electron pairs surrounding the central atom, their mutual repulsion is minimal when they lie at opposite poles of the sphere . Therefore, the central atom is predicted to adopt a linear geometry . If there are 3 electron pairs surrounding the central atom, their repulsion is minimized by placing them at the vertices of an equilateral triangle centered on the atom . Therefore, the predicted geometry is trigonal . Likewise, for 4 electron pairs, the optimal arrangement is tetrahedral . </P> <P> The overall geometry is further refined by distinguishing between bonding and nonbonding electron pairs . The bonding electron pair shared in a sigma bond with an adjacent atom lies further from the central atom than a nonbonding (lone) pair of that atom, which is held close to its positively charged nucleus . VSEPR theory therefore views repulsion by the lone pair to be greater than the repulsion by a bonding pair . As such, when a molecule has 2 interactions with different degrees of repulsion, VSEPR theory predicts the structure where lone pairs occupy positions that allow them to experience less repulsion . Lone pair--lone pair (lp--lp) repulsions are considered stronger than lone pair--bonding pair (lp--bp) repulsions, which in turn are considered stronger than bonding pair--bonding pair (bp--bp) repulsions, distinctions that then guide decisions about overall geometry when 2 or more non-equivalent positions are possible . For instance, when 5 valence electron pairs surround a central atom, they adopt a trigonal bipyramidal molecular geometry with two collinear axial positions and three equatorial positions . An electron pair in an axial position has three close equatorial neighbors only 90 ° away and a fourth much farther at 180 °, while an equatorial electron pair has only two adjacent pairs at 90 ° and two at 120 ° . The repulsion from the close neighbors at 90 ° is more important, so that the axial positions experience more repulsion than the equatorial positions; hence, when there are lone pairs, they tend to occupy equatorial positions as shown in the diagrams of the next section for steric number five . </P>

How do lewis structures help predict the shape of a molecule