Experiment Seven - Molecular Geometry

Transcription

1 Experiment Seven - Geometry Introduction Although it has recently become possible to image molecules and even atoms using a highresolution microscope, our understanding of the molecular world allows us to determine what these molecules will look like, long before they re seen. To this point all of our compounds have been represented as two dimensional Lewis Dot Structures, a simple drawing of electron behavior that provides insight into molecular bonding. In addition to atom connectivity, Lewis structures can also help determine the three-dimensional shape, polarity, and ultimately behavior of a molecular species. These drawings are a simple, powerful tool in our pursuit to make sense of the atomic world. This experiment will require understanding of Lewis structures, electronegativity, molecular geometry, polarity, and ultimately intermolecular forces. Lewis Structures These two-dimensional drawings are the basis of understanding for molecular substances, and are incredibly important for most fields of chemistry. The Lewis molecules have been developed from the dot structures, which represent valence electrons for each atom. It s important to recognize the two different types of electrons: 1. pairs (also called lone pairs) these electron are not involved in forming a bond between two atoms, so they are nonbonding electrons 2. Unpaired electrons these electrons can be shared with another unpaired electron on a neighboring atom to create a covalent bond, so they are bonding electrons. The Lewis structure puts these electrons together forming a dash between unpaired electrons forming a covalent bond. It is possible for two atoms to share more than two electrons, when this occurs a multiple bond is formed. Unpaired electrons are represented by two dots attached to an atom and are incapable of bonding, but still provide structural rigidity. Below is an example of a Lewis structure and its features are highlighted: 69 P a g e

2 Rules for Creating Lewis Structures: (know how to make these structures) 1. Count total electrons by adding up valence electrons from all atoms present. If the molecule has a negative charge, add the charge to the total electrons. If the electron has a positive charge, subtract the charge from the electron total. Total electrons used after each subsequent step. Structure MUST only contain number of electrons found in this step!!! Never add or subtract electrons, use what you have. 2. Draw a skeleton structure. The less electronegative atom is the central atom, (if carbon is present create a chain of all the carbons in the molecule). Draw a single bond between the central atom/s and the remaining atoms. 3. Satisfy octet of peripheral atoms. Add electrons to the peripheral atoms until they have 8 electrons. Hydrogen only ever has 2 electrons. 4. Satisfy central atom/s octet. Add remaining electrons to central atom/s. If you run out of electrons before central atom is satisfied, use multiple bonding. Halogens and Hydrogen will never have multiple bonds. Element # of bonds # of lone pairs C 4 0 N 3 1 O 2 2 X (halogens F, Cl, Br, I) 1 3 Expanded and contracted octet. In a few cases it is possible for atoms to have an expanded or contracted octet. This is just the nature of the central atom and it s important to be aware of them if they present themselves. Expanded octet: Compounds that contain an element larger than silicon (typically seen in sulfur, phosphorus, arsenic, etc.) can expand their octets and actually support more than eight electrons. If you see one of these as the central atom, it s okay to add additional bonds or electron pairs to complete the Lewis structure. Contracted octet: Compounds that contain smaller elements than carbon (typically Boron and Beryllium) have a limited octet; boron supports six electrons while beryllium supports four. In this case simply ignore the need to create double bonds or add any additional electrons to the central atom when drawing the Lewis structures. 70 P a g e

3 Geometry Determining molecular geometry is based on the Valence Shell Pair Repulsion (VSPER) theory, which states valence shell electron pairs are arranged as far from one another as possible. That allows a limited number of possible structures, based on the number of electron pairs/bonds surrounding the central atom. This electron geometry also provides bond angles to the molecule. Using the electron geometry as the scaffold for the final structure the number of bonds attached to the central atom will provide the molecular geometry. geometry is what we would see if we used a high powered microscope or other spectroscopic techniques to observe the molecule. The molecular shape takes into account the bonds (regardless of magnitude: single, double, triple) and electron pairs to provide a final three-dimensional geometry. Predicting molecular geometry comes in a few steps: 1. Draw Lewis structure 2. Determine electron domains around the central atom (multiple bonds count as one domain) 3. domains, lone pairs and bonds, determine the electron geometry. 4. The molecule will keep this shape, based on the number of bonds provides the molecular geometry. It s what you can see for a shape. 71 P a g e

4 Polarity and egativity polarity is going to be broken down into two concepts: 1. egativity Each electronegative bond must have a Dipole Arrow added to it to show the flow of electrons. You MUST have polar bonds in order for a molecule to be polar. 2. Geometry using the polar bond vectors, look at the geometry and decide if there will be a location of and + area around the atom. If the vectors cancel one another (all pull out, or pull in opposite directions) then the bonds are polar but the molecule is not! Dipole moment = polar molecule because of uneven distribution of electrons 72 P a g e

5 Compound = H2O # Valence electrons = Compound = HCl # Valence electrons = Lewis Structure: 73 P a g e

6 Compound = NH3 # Valence electrons = Compound = NH4 + # valence electrons = 74 P a g e

7 Compound = CCl4 # Valence electrons = Compound = SO2 # Valence electrons = 75 P a g e

8 Compound = CO2 # Valence electrons = Compound = CH2O # Valence electrons = 76 P a g e

9 Compound = CH3OH # Valence electrons = EXPANDED OCTET Compound = SO4 2- # Valence electrons = 77 P a g e

10 78 P a g e