When there are three lone pairs (m=1, n=3 or AX 3E 2), the lone pairs occupy the three equatorial positions.The molecular geometry is T-shaped with bond angles of slightly less than 120° and slightly less than 90°. When there are two lone pairs (m=3, n=2 or AX 3E 2), each lone pair occupies one of the three equatorial positions.The molecular geometry is called a see saw with bond angles of slightly less than 120° and slightly less than 90°. When there is one lone pair (m=4, n=1 or AX 4E 1), the lone pair occupies one of the equatorial positions.When all electron groups are bonds (m=5 or AX 5), the molecular geometry is a trigonal bipyramid with bond angles of 120° and 90° between adjacent ligands.There are four different molecular geometries that are possible in this category, depending upon the number of bonded groups and lone pairs of electrons: The arrangement of five groups around a central atom results in a trigonal bipyramidal electronic geometry. The justification for this preference, according to VSEPR theory, is that the lone electron pairs are more repulsive than bonding electron pairs, and thus the lone pairs prefer the less crowded equatorial positions. Lone pairs of electrons generally prefer to occupy equatorial positions rather than axial positions. The crowding of axial positions results in slight differences in bond distances crowded axial groups have longer bonds than the less crowded equatorial groups. Axial groups are thus more crowded than the equatorial positions with only two adjacent groups at 90°. The axial positions have three adjacent groups oriented 90° away in space. The three equatorial ligands are 120° from one another and are 90° from each of the two axial ligands. (Steric number = 5) In the case that there are five electron groups around a central atom, there are two different types of positions around the central atom: equatorial positions and axial positions. The bond angles in ammonia are 106.6°.įive Electron Groups (m + n = 5) Figure: Trigonal pyramidal molecules (steric number 5) possess different bond angles and lengths for axial (ax) and equatorial (eq) pendant atoms. However, the H–N–H bond angles are less than the ideal angle of 109.5° because of LP–BP repulsion. In essence, this is a tetrahedron with a vertex missing. There are three nuclei and one lone pair, so the molecular geometry is trigonal pyramidal. The Difference in the Space Occupied by a Lone Pair of Electrons and by a Bonding PairĪs with SO 2, this composite model of electron distribution and negative electrostatic potential in ammonia shows that a lone pair of electrons occupies a larger region of space around the nitrogen atom than does a bonding pair of electrons that is shared with a hydrogen atom.Ĥ. We expect the LP–BP interactions to cause the bonding pair angles to deviate significantly from the angles of a perfect tetrahedron. This designation has a total of four electron pairs, three X and one E. With three bonding pairs and one lone pair, the structure is designated as AX 3E. Repulsions are minimized by directing each hydrogen atom and the lone pair to the corners of a tetrahedron.ģ. There are four electron groups around nitrogen, three bonding pairs and one lone pair. The O-S-O bond angle is expected to be less than 120° because of the extra space taken up by the lone pair.Ģ. Thus, with two nuclei and one lone pair the shape is bent, or V shaped, which can be viewed as a trigonal planar arrangement with a missing vertex. The molecular geometry is described only by the positions of the nuclei, not by the positions of the lone pairs. In SO 2, we have one BP–BP interaction and two LP–BP interactions.Ĥ. Bonding pairs and lone pairs repel each other electrostatically in the order BP–BP < LP–BP < LP–LP. The lone pair occupies more space around the central atom than a bonding pair (even double bonds!). This designation has a total of three electron pairs, two X and one E. With two bonding pairs and one lone pair, the structure is designated as AX 2E.
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