Molecular Geometries and Bonding Theories

Molecular Shapes Lewis structure – does not indicate 3D shape The shape of a molecule plays an important role in its reactivity and properties By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule. Indicate 3D shape

What Determines the Shape of a Molecule? Bond angles! (the angles between atoms) Electron pairs repel each other bonding OR nonbonding pairs So electron pairs are placed as far as possible from each other This lets us we can predict the shape of the molecule (ie, the angles between the atoms)

Electron Domains We can refer to the electron pairs as electron domains. Each of the following counts as ONE electron domain: A single bond A nonbonding pair (or lone pair) A multiple bond (double or triple bond) The central atom in this molecule, A, has four electron domains.

Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.” In other words, the best structure is the one where electron domains are most spread out.

Electron-Domain Geometries These are the electron-domain geometries for two through six electron domains around a central atom.

Electron-Domain Geometries Just count the number of electron domains in the Lewis structure The electron-domain geometry will be that which corresponds to the number of electron domains

Molecular Geometries The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs. After finding electron-domain geometry, count the bonding electron domains to determine molecular geometry

Linear Electron Domain In the linear domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

Trigonal Planar Electron Domain There are two molecular geometries: Trigonal planar - if all the electron domains are bonding Bent - if one of the domains is a nonbonding pair

Tetrahedral Electron Domain For tetrahedral electron domain, there are three molecular geometries: Tetrahedral - if all are bonding pairs Trigonal pyramidal - if one is a nonbonding pair Bent - if there are two nonbonding pairs.

Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: Axial Equatorial

Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. In other words, nonbonding electrons will always occupy equatorial positions

Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: Trigonal bipyramidal Seesaw T-shaped Linear

Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: Octahedral Square pyramidal Square planar

Nonbonding Pairs and Bond Angles Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater This tends to decrease bond angles in a molecule.

Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds. Therefore, they also affect bond angles.

Larger Molecules In larger molecules, We can assign a geometry for each “central” atom

Polarity Nonpolar molecule A bond dipole is due to unequal sharing between 2 atoms in a covalent bond (a way to quantify bond polarity) shows the direction the electrons are being pulled The overall dipole moment of a molecule is a sum of its bond dipoles But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. If the bond dipoles are equal in magnitude but opposite in direction, they cancel each other  nonpolar molecule If the overall dipole moment is not zero  polar molecule In other words, if the central atom of the molecule is symmetrically surrounded by identical atoms, it will be nonpolar. Otherwise it is polar. Polar molecule

Polarity 4. 1. 3. 5. 2.

Examples Predict both the electron-domain geometry and the molecular geometry for the following molecules. Finally, indicate whether each is polar or nonpolar. Electron-domain geometry Molecular geometry Polar/Nonpolar FCl2+ AsF5 AsF3 ICl2- TeF6

Examples Predict both the electron-domain geometry and the molecular geometry for the following molecules. Finally, indicate whether each is polar or nonpolar. Electron-domain geometry Molecular geometry Polar/Nonpolar FCl2+ AsF5 AsF3 ICl2- TeF6 Trigonal planar Bent Polar Trigonal bipyramidal Trigonal bipyramidal Nonpolar Tetrahedral Trigonal pyramidal Polar Trigonal bipyramidal Linear Nonpolar Octahedral Octahedral Nonpolar

Overlap and Bonding We think of covalent bonds forming through the sharing of electrons by adjacent atoms. This can only occur when orbitals on the two atoms (called atomic orbitals) overlap (or share space). H: 1s1 Cl: [Ne]3s23p5

Overlap and Bonding But simply overlapping atomic orbitals doesn’t explain the angles in tetrahedral, trigonal bipyramidal, and other geometries. The 109.5° angles can’t be explained this way. The 3 p orbitals from the C atom H ≠ C H H H s orbital from each of 4 H atoms The overlapped atomic orbitals are at 90° angles to each other here.

Hybrid Orbitals So instead of simply overlapping, such atomic orbitals hybridize (mix) to form new orbitals called hydrid orbitals. Note: The total number of orbitals remains constant.

Hybrid Orbitals Consider beryllium: In its ground electronic state, it would not be able to form bonds because it has no unpaired electrons. But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds because now there are two unpaired electrons.

Hybrid Orbitals Hybridize Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. These sp hybrid orbitals have two lobes like a p orbital. One of the lobes is larger and more rounded (like an s orbital).

Hybrid Orbitals These two degenerate orbitals align themselves 180 from each other. This is consistent with the observed geometry of beryllium compounds: linear. So a linear arrangement of electron domains implies sp hybridization, and vice versa.

Hybrid Orbitals Using a similar model for boron leads to… …three degenerate sp2 orbitals.

Hybrid Orbitals With carbon we get… …four degenerate sp3 orbitals.

Hybrid Orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.

Hybrid Orbitals This leads to five degenerate sp3d orbitals… …or six degenerate sp3d2 orbitals.

Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom (and vice versa). Steps to determining hybridization of an atom: Draw Lewis structure Determine electron-domain geometry Use this chart to determine the corresponding hybridization of the central atom.

Examples Complete the following chart: Molecule or ion Electron-domain geometry Hybridization of central atom Dipole moment? Yes or no. SiCl4 BrF4- Left off

Examples Complete the following chart: Molecule or ion Electron-domain geometry Hybridization of central atom Dipole moment? Yes or no. SiCl4 Tetrahedral sp3 No BrF4- Octahedral sp3d2