Chem II Chapter 9:

Why is this important? the size and shape of a molecule of a particular substance, together with the strength and polarity of its bonds, largely determine the physical and chemical properties of that substance.

The geometry of a molecule includes a description of the arrangements of the atoms in the molecule. Remember: we are dealing with covalent bonded (polar and nonpolar) compounds that consist of molecules…not ionic crystals

Since atoms are always in motion, when we talk about shapes of molecules or polyatomic ions, we mean the shape based on the average location of the atoms

Molecules have shapes and sizes that are defined by the angles and distances between the nuclei of their component atoms The overall shape of a molecule is determined by its bond angles BF3

Molecular shape is only meaningful when there are at least three atoms If there are only two atoms, the molecule must always be linear in shape as the centers of the 2 atoms will always be in a straight line. H Cl

Molecules with a central atom (“A”) surrounded by two or more atoms of the same type (“X”) are denoted AXn AX2 molecules AX3 molecules CO2 H2O NH3 SnCl3-

5 fundamental shapes for Axnmolecules

The possible shape of the AXn molecule is determined by the value of n AX2 molecules can be either linear or bent Three possible shapes for AX3 molecules

Can we predict the exact shape? To determine the shape, we use a simple theory called VSEPR (often read as "vesper"), which stands for Valence Shell Electron Pair Repulsion.(≈90% accuracy) VSEPR theory states the shapes of different AXn molecules or ions depends on the number of “electron domains” surrounding the central “A” atom

Electron domains are the regions of electron density where electron pairs are most likely to be found Each nonbonding pair (referred to as a lone pair), single bond, and multiple bond produces an electron domain around the central atom The Lewis structure of NH3 has four electron domains around the central nitrogen atom. (3 bonding pairs & one lone pair)

The number of electron pairs surrounding an atom, both bonding and nonbonding, is called its steric number. Steric number = (number of lone pairs on central atom) + (number of atoms bonded to central atom)

Electron domains are negatively charged and repel each other In other words, electron domains try to stay out of each other’s way VSEPR theory is based upon the idea that the best arrangement of a given number of electron domains is the one that minimizes the repulsions among them

The "AXE method" of electron counting is commonly used when applying the VSEPR theory A represents the central atom and always has an implied subscript one The X represents the number of bonds between the central atoms and outside atoms Note:Multiple covalent bonds (double,triple, etc) count as one X. E represents the number of lone electron pairs surrounding the central atom

We will use electron domain geometry To determine molecular geometry The molecular geometry describes the arrangement of the atoms only and not the lone pairs of electrons

Molecules with 2 electron domains on central atom Electron domain geometry is linear Bonding domains = 2 Non-bonding domains = 0 Molecular geometry is linear

Linear Molecular geometry

Molecules with 3 electron domains on central atom Electron domain geometry is trigonal planar Bonding domains = 3 Non-bonding domains = 0 Molecular geometry is trigonal planar

Trigonal planar geometry

Molecules with 3 electron domains on central atom Electron domain geometry is Trigonal planar Bonding domains = 2 Non-bonding domains = 1 Molecular geometry is Bent

Molecules with 4 electron domains on central atom Electron domain geometry is Tetrahedral Bonding domains = 4 Non-bonding domains = 0 Molecular geometry is Tetrahedral

In geometry, a tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral",

Molecules with 4 electron domains on central atom Electron domain geometry is Tetrahedral Bonding domains = 3 Non-bonding domains = 1 Molecular geometry is Trigonal pyramidal

Trigonal pyramid geometry

Molecules with 4 electron domains on central atom Electron domain geometry is Tetrahedral Bonding domains = 2 Non-bonding domains = 2 Molecular geometry is Bent

Expanded valence shells Third row elements (“Period 3” e.g., Al, Si, P, S, Cl) and higher often have more than four valence shell orbitals filled with Lone Pairs and/or Bond Pairs; this is called"expanded valence". This means more than 8 electrons can surround a third-row element. P Although the octect rule is useful for understanding bonding in molecules, certain atoms can expand their valence shells to accommodate extra electrons

Elements have a greater tendency to expand their valence: ClF3 PCl5

Sulfuric acid FC of sulfur = 6 − 0 −½(12) FC of sulfur = 0

Draw the Lewis dot structure for the molecule and count the total number of electron densities or steric number (single bonds + multiple bonds + lone pair electrons + unpaired electrons) For molecules or ions that have resonance structures, you may use any one of the resonance structures. Using VESPR to determine molecular geometry #1 #2 Determine electron-density geometry by arranging electron densities to minimize repulsions #3 Using only bonding pairs from the electron-density geometry, determine molecular geometry

The lone pair electron domain is spread out more Lone pairs repel more than bond pairs

Triple bonds repel other bonding-electrons more strongly than double bonds. Double bonds repel other bonding-electrons more strongly than single bonds.

VESPR model can be extended to predict the shapes of complex molecules

Bond polarity is a measure of how equally the electrons are shared in a bond As the difference in electronegativity between the two atoms increases, so does the polarity

The overall dipole moment of a molecule is the sum of its bond dipoles. The quantitative measure of the amount of charge separation is called the dipole moment

In CO2 the bond dipoles are equal in magnitude but exactly oppose each other. The overall dipole moment is zero.

Why are dipoles and bond polarity important? Molecular polarity affects physical properties such as boiling and melting points

The VESPR model is a simple tool for predicting the shapes of molecules but does not explain why bonds exist between atoms Valence-bond theory pictures individual atoms, each with its own orbitals and electrons coming together and forming covalent bonds of the molecule. Bonding involves the overlap of valence orbitals on the central atom with those of the surrounding atoms.

Review Electron orbital notation identifies the valence orbitals

sigma (σ ) bonds are single covalent bonds. They result from the overlap of two s orbitals, an s and a p orbital, or two head-to-head p orbitals.

pi (π) bonds result from the sideways overlap of p orbitals, and pi orbitals are defined by the region above and below an imaginary line connecting the nuclei of the two atoms pi bonds never occur unless a sigma bond has formed first, and are always part of a double or triple bond.

Whenever there is a double bond it is made up of one sigma (direct orbital overlap) bond and one pi (lateral orbital overlap) bond.

Triple bonds have two pi bonds arranged at 90º to one another brought about by the lateral overlap of one pair of py orbitals and one pair of pz orbitals.

Problem #1 The experimental shapes and bond angles do not correspond to the theoretical expectation The 'p' orbitals are oriented at 90º to one another and yet there are few molecules that show a bond angle of 90º Problem #2 The valence electrons in the s-orbital cannot bond without interfering with the p orbitals.

Hybridization is a model that allows us to combine the atomic orbitals and then produce four degenerate orbitals to be used for bonding.

Hybridization is the ‘mixing’ or ‘blending’ of standard atomic orbitals to form new bonding orbitals that will accommodate the spatial requirements in a molecule. The hybrid orbitals are formed by combining s, p and d orbitals

How does hybridization occur? Example: methane (CH4), 1 Carbon binds with 4 Hydrogens. The carbon atom itself has only 2 electrons available for bonding in the 2p subshell. In order for 4 hydrogens to bind there need to be 4 electrons available for bonding, which cannot be achieved with the current orbital arrangement.

This leads to the creation of a new ‘hybridized orbital’, called sp3. The pull of a hydrogen nucleus results in an electron being excited from the 2s subshell into the 2p subshell, where it is available for bonding.

Sigma bonds are generally formed through hybridization: Pi bonds are formed by unhybridized p-orbitals

sp2 refers to a hybrid orbital being constructed from one s orbital and two p orbitals. hybridized orbitals are notated as sp, sp2, sp3, spx, etc.

Valence-bond theory will correspond with the VESPR geometry

The End