CHM151LL VSEPR and Molecular Geometry Tables

CHM151LL: VSEPR and Molecular Geometry Tables Valence-Shell Electron-Pair Repulsion (VSEPR) model Lewis structures show the two-dimensional distribution of atoms and electrons. The molecular geometry, or three-dimensional shape of a molecule or polyatomic ion, can be determined using valence-shell electron-pair repulsion (abbreviated VSEPR and pronounced “VES-per”) theory, in which the basic principle is valence electrons around a central atom stay as far apart as possible to minimize the repulsions. For diatomic molecules (i.e., those made up of two atoms), the shape has to be linear. For molecules with three of more atoms, the shape depends on the number and type of electrons (bonding versus nonbonding) around the central atom. Each electron domain attached to a central atom will repel the other electron domains in its environment. There are two types of electron domains: 1) A bonded atom and 2) a nonponding pair of electrons. It does not matter whether a bonded atom is atached with a single, double or triple bond, each noncentral atom counts as one electron domain on the central atom. Electron Domain Geometries There are five basic electron domain geometries. Linear The arrangement of 2 electron domains the two outer atoms are 180° from each other – Trigonal – – planar The arrangement of 3 electron domains three outer atoms at the corners of an equilateral triangle each outer atom is 120° from the other two outer atoms Tetrahedral The arrangement of 4 electron domains – (tetra = four) since four-sided, or four faces – maximum distance between electrons requires 3D structure with 109.5° between each outer atom – each outer atom is 109.5° from the other outer atoms Trigonal bipyramidal The arrangement of 5 electron domains – trigonal = three outer atoms form planar triangle around central atom – bipyramidal = two outer atom directly above and below central atom, connecting outer atom forms two 3-sided pyramids – equatorial positions: corners of planar triangle – 3 of outer atoms are at equatorial positions, 120° from each other – axial positions: above and below central atom – 2 atoms are at axial positions, 90° from equatorial atoms Octahedral – – – The arrangement of 6 electron domains (octa=eight) connecting the B atoms eight faces all outer atoms are 90° away from each other the terms "axial" and "equatorial" do not apply because all six positions are identical since the molecule is completely symmetrical page 1 of 8 The Electron Domain Geometries Name Number of Electron Domains Molecular Geometry and Bond Angles Hybridization of the Central atom 180 linear 2 sp trigonal planar 3 sp2 tetrahedral 4 120 109.5 sp3 90 trigonal bipyramidal 5 120 sp3d 90 octahedral 6 sp3d2 page 2 of 8 Molecular Geometries from each Electron Domain Geometry Since electron pairs cannot be seen, the electron domain geometries are theoretical. The molecular geometries, also called shapes of molecules, are determined experimentally by X-ray diffraction. Even though the lone pairs cannot be seen, they still repel the bonding pairs of electrons. In fact, they are actually more repulsive than bonding pairs, so they can compress the bond angles in the molecules where they are present. For molecules where the central atom has lone pairs, we can write a general formula that also includes the lone pairs represented by the letter E, as shown below. A=central atom B=outer atoms E=lone pairs on the central atom The various molecular geometries for these types of molecules are shown in tables and described on the following pages: Molecular Geometries from each Electron Domain Geometry Electron Domain Geometry # of Outer Atoms # of Lone Pairs General Formula 3 0 AB3 2 1 AB2E Molecular Geometry Name trigonal planar 120 120 Trigonal planar <120 bent or angular trigonal planar 120 page 3 of 8 Tetrahedral 109.5 109.5 4 0 AB4 tetrahedral 3 1 AB3E trigonal pyramidal Tetrahedral 109.5 <109.5 Tetrahedral 109.5 2 2 bent or angular AB2E2 <109.5 page 4 of 8 Molecular Geometry from Trigonal Planar Electron Domain Geometry AB2E: bent – start with AB3 molecule (trigonal planar) and replace a B atom w/ lone pair – lone pair electrons push bonding electrons away bond angles are now less than 120° Molecular Geometries from Tetrahedral Electron Domain Geometry AB3E: trigonal pyramidal (central atom + 3 outer atoms make a pyramid) – start with AB4 molecule (tetrahedral) and replace a B atom w/ lone pair – lone pair electrons push bonding electrons away bond angles are now less than 109.5° AB2E2: bent – start with AB4 molecule (tetrahedral) and replace 2 B atoms with 2 lone pairs – lone pair electrons repel each other and the bonding electrons bond angles are now less than 109.5° Molecular Geometries from Trigonal Bipyramidal Electron Domain Geometry AB4E: seesaw – start with AB5 molecule and replace one B atom with one lone pair – a B atom can be taken from an axial or an equatorial position – from axial: lone pair is 90° from three equatorial B atoms and – from equatorial: lone pair is 90° from two Axial and 120° from two other equatorial B atoms – taking a B atom from equatorial position maximizes space between lone pair and B atoms – lone pair electrons push bonding electrons away bond angles are now less than 90° and less than 120° AB3E2: T-shaped – start with AB5 molecule and replace two B atoms with two lone pairs – both B atoms taken from equatorial positions to maximize distance between the lone pairs – lone pair electrons repel each other and the bonding electrons bond angles for remaining atoms are now less than 90° AB2E3: linear – start with AB5 molecule and replace three B atoms with three lone pairs – taking three B atoms from equatorial positions maximizes space between the \ lone pairs, keeping them 120 from one another – lone pair electrons repel each other and the bonding electrons, but since the bonding electrons are balanced on either side of the 3 lone pairs bond angle is exactly 180° page 5 of 8 Molecular Geometries from each Electron Domain Geometry Electron Domain Geometry # of Outer Atoms # of General Lone Formula Pairs Molecular Geometry Name Trigonal bipyramidal 90 90 120 5 0 AB5 4 1 AB4E Trigonal 120 Bipyramidal Trigonal bipyramidal 90 120 See-saw <90 Trigonal bipyramidal <90 90 120 3 2 AB3E2 T-shaped Trigonal bipyramidal 90 180 120 2 3 AB2E3 linear page 6 of 8 Molecular Geometries from each Electron Domain Geometry Electron Domain Geometry # of # of Outer Lone Atoms Pairs General Formula Molecular Geometry Name Octahedral 90 90 6 0 AB6 octahedral Octahedral 90 90 5 1 AB5E 4 2 AB4E2 square pyramidal Octahedral 90 90 square planar page 7 of 8 Molecular Geometries from Octahedral Electron Domain Geometry AB5E: square pyramidal (central atom + 5 B atoms make 4-faced pyramid) – start with AB6 (octahedral) and replace one B atom with one lone pair – all six outer positions are identical, so any outer atom can be removed square pyramidal shape – lone pair electrons push bonding electrons away bond angles are now less than 90° AB4E2: square planar (central atom + 4 B atoms form square all in 1 plane) – start with AB6 (octahedral) and replace 2 B atoms with 2 lone pairs – remove first B atom then remove the second B atom 180° away from the first to maximize the distance between the lone pairs square planar shape – bond angles are now exactly 90° since lone pairs balance each other page 8 of 8