12:23 < Back Models Lab for Chem 182 CHEM182 24826 - S24 - Gen Inorganic Chem II-L I. Background and Theory The polarity of a molecule is an important factor in determining its molecular properties. Molecular polarity is determined by the shape of the molecule, or how the atoms are arranged in space within the molecule. By looking at the arrangement of atoms in space within the molecule, whether the individual polar bonds within a molecule either cancel each other or reinforce one another can be determined. If the polar bonds within a molecule cancel each other, the molecule is classified as a nonpolar molecule. If the polar bonds within a molecule reinforce one another, then the molecule is classified as a polar molecule. The polarity of the molecule will affect how the molecule interacts with itself as well as other substances, leading to differences in physical and chemical properties, such as boiling point, surface tension, and solubility. The polarity can also be used as a predictive tool for these different properties, thus it is important to be able to determine the polarity for various compounds. To determine the polarity of the molecule, the correct 3D structure of the molecule must be known. Prior to the development of x-ray diffraction, the determination of the correct 3D structure for a molecule was done by assigning each central atom ◄ Previous Next 2 DOO Dashboard Calendar To Do Notifications Inbox 12:23 Back Models Lab for Chem 182 CHEM182 24826 - S24 - Gen Inorganic Chem II-L actermine the polarity various compounds. To determine the polarity of the molecule, the correct 3D structure of the molecule must be known. Prior to the development of x-ray diffraction, the determination of the correct 3D structure for a molecule was done by assigning each central atom within a molecule a geometry based on a model known as VESPR, or the Valence Shell Electron Pair Repulsion model. Recall that a central atom is any atom that is bound to two or more other atoms. Only central atoms have a geometry. In VSEPR, the geometry around a central atom is arrived at by placing the bonds (the electron pairs) as far apart as possible, since electrons have like charges that repel each other. When considering the bonding electron pairs for the geometry of an atom, the double bonds (two electron pairs) or triple bonds (three electron pairs) are treated the same as single bonds (one electron pair). For example, consider carbon dioxide, where the central atom carbon has two double bonded oxygen atoms attached to it. The electrons in one double bond repel the electrons in the other double bond, causing them to spread out away from one another as much as possible. When this spreading occurs, the double bonds are at completely opposite sides of the carbon atom, creating 180° bond angles, and a geometry called linear. By knowing the number ◄ Previous Next ▸ 2 DOO Dashboard Calendar To Do Notifications Inbox 12:23 Back Models Lab for Chem 182 CHEM182 24826 - S24 - Gen Inorganic Chem II-L one another as much as possible. When this spreading occurs, the double bonds are at completely opposite sides of the carbon atom, creating 180° bond angles, and a geometry called linear. By knowing the number of atoms bound to the central atom and the number of lone pairs on the central atom (as determined from a proper Lewis structure based on the octet rule), the geometry of that central atom can be predicted. Using these known geometries and the relationship of the atoms in space within each of these geometries, the correct 3D structure of a molecule can be predicted and used to determine the polarity of the molecule as a whole. For a VSPER review, here are some Tyler Dewitt videos: • https://www.youtube.com/watch? v=nxebQZUVvTg&list=PL3hPmOZdYhyw5GnOn UgH7809B6W7sLP3s https://www.youtube.com/watch? v=8TI bDWCAmo II. Procedure: Construct a data table in your notahaal (like the on ◄ Previous Next 2 DOO Dashboard Calendar To Do Notifications Inbox 12:23 < Back Models Lab for Chem 182 CHEM182 24826 - S24 - Gen Inorganic Chem II-L II. Procedure: Construct a data table in your notebook (like the one below) and record the following for each of the listed compounds: 1. Determine total number of valence electrons and state. 2. Draw a Lewis dot structure and check that you have used the required number of dots (valence electrons). 3. All molecules and Best Resonance = All Formal Charges are Zero (2nd Best Resonance = smallest non-zero F.C. and negative F.C. on most electronegative atom) Resonance structures are attempts to show the structure of a molecule that does not have bonds that fit within the rules of Lewis structures. The atoms are in the same position with respect to each other, but electrons are shifted around in an attempt to display the actual bonding present. Resonance structures are often found when an oxygen atom is present in a molecule. Electrons in oxygen can easily shift from single to double, and sometimes to triple, bonds as shown in the carbon dioxide structures below. Remember that we deal in electron probability, not absolutes. To accurately illustrate the structure of such a molecule, all its resonance structures should be ◄ Previous Next ▸ 2 DOO Dashboard Calendar To Do Notifications Inbox 12:23 Back Models Lab for Chem 182 CHEM182 24826 - S24 - Gen Inorganic Chem II-L electrons are shifted around in an attempt to display the actual bonding present. Resonance structures are often found when an oxygen atom is present in a molecule. Electrons in oxygen can easily shift from single to double, and sometimes to triple, bonds as shown in the carbon dioxide structures below. Remember that we deal in electron probability, not absolutes. To accurately illustrate the structure of such a molecule, all its resonance structures should be drawn. Formal charge can be used to distinguish the most stable (probable) resonance structure when multiple resonance structures are possible. The one(s) with zero or the lowest formal charge is the most probable structure and the form in which the molecule exists most of the time. Electrons occasionally shift into one of the other, less probable, resonance structures. Formal charges are calculated for each atom in a molecule or ion, according to the following formula: Formal Charge = # of valence electrons - # of unbonded electrons - # of bonded electrons Example: CO2 Sometimes a model failure happens. Resonance structures are an attempt t show bonds somewhere -between single and double for single and triple) ◄ Previous Next 2 DOO Dashboard Calendar To Do Notifications Inbox