VSEPR Theory & Molecular Geometry PDF

VSEPR Theory & Molecular Geometry Prepared by: RUFTHEL S. GUTIERREZ MT II, STEM Outline:  Lewis Structure Steps in writing Lewis Structure Resonance Structure & Formal Charge Exceptions to the Octet Rule  VSEPR Theory & Molecular Shape  Molecular Polarity & Dipole m...

VSEPR Theory & Molecular Geometry Prepared by: RUFTHEL S. GUTIERREZ MT II, STEM Outline:  Lewis Structure Steps in writing Lewis Structure Resonance Structure & Formal Charge Exceptions to the Octet Rule  VSEPR Theory & Molecular Shape  Molecular Polarity & Dipole moment  Molecular Shapes in Biological process Objectives  1. define molecular geometry and Valence Shell Electron Pair Repulsion (VSEPR) theory;  2. describe the geometry of simple compounds;  3. define polarity;  4. describe the types of molecules based on polarities;  5. determine the factors that determine the bond polarity; and  6. determine the polarity of simple compounds. Lewis structure of a Molecule or Polyatomic Ion Lewis structure of a molecule or polyatomic ion is a structural formula that shows how atoms are covalently bonded. Steps in Writing Lewis Structure of a Molecule or Polyatomic Ion Step 1.Determine the total number of valence electrons using the periodic table of elements. ( The number of valence electrons equals the A-group number.) Example nitrogen trifluoride, NF3 [1 x N(5e-)] + [3 x F(7e-)] = 5e- + 21e- = 26 valence e- Step 2.Choose the central atom and terminal atom. The least electronegative elements takes the central position. H and F do not occupy a central position. Phosphorus never occupy terminal position. Step 3. Draw a single bond from each surrounding atom to the central atom and subtract two valence electrons for each bond from the total number of valence electrons in step 1 Step 4. Distribute the remaining electrons as lone pairs (nonbonding electrons) around the terminal atoms and then around central atoms. Obey Octet rule. Step 5. If the central atom is deficient of electrons for it to comply the octet rule, convert a lone pair of terminal atom into bonding pair to produce a double or triple bond. Resonance Structure and Formal Charge Resonance structure are two or more Lewis structure of molecule or ion. Resonance structure have the same relative placement of atoms but different locations of bonding and lone electron pairs.  Formalcharge is the charge an atom have if the bonding electrons were shared equally. Rules in Determining the Plausible Lewis Structure based on FC 1. The Lewis structure having no formal charge is the most plausible. 2. The Lewis structure in which the formal charge are close to zero (+1 or -1) is more plausible than the one in which the formal charges are farther from zero (+2, -2, +3,-3 etc.). 3. If two or more Lewis structure have similar distribution of formal charges, the one in which the negative formal charges are on the more electronegative atoms is the most plausible. In the given example cyanate ion, CNO-, the resonance form III is the most plausible Lewis structure. The -1 charge is on more electronegative atom O. Exceptions to the Octet rule Octet Rule the observation that when atoms bond, they often lose, gain or share electrons to attain a filled outer shell of eight electrons. Exceptions to the Octet Rule 1. Incomplete Octet, one of the atoms involved is short of some electrons to meet the electron requirement of the octet rule. Be and B tend to form octet rule. 2. Odd – Electron Molecule, one of the atoms presenthas an odd number of electrons. Example is the nitrogen dioxide, NO2 3. Expanded Octet, an atom having more than eight electrons around. It is possible to elements belonging to period 3 through period 7. Example is sulphur hexafluoride. VSEPR Theory (Valence-shell electron-pair repulsion) VSEPR theory, state that each group of valence electrons around a central atom is located as far away as possible from the others in order to minimize repulsions. Molecular Geometry  Molecular geometry is the three-dimensional arrangement of atoms in a molecule. A molecule’s geometry affects its physical and chemical properties, such as melting point, boiling point, density, and the types of reactions it undergoes. Electron-group repulsions and The 5 Basic molecular shapes Molecular Shapes with Two Electron-Group (Linear Arrangement) Molecular Shapes with Three Electron-Group (Trigonal Planar Arrangement) Molecular Shapes with Four Electron-Group(Tetrahedral Arrangement) https://www.youtube.com/watch?v=cdo6FtSU_k8 Molecular Shapes with Five Electron-Group (Trigonal Bipyramidal Arrangement) https://www.youtube.com/watch?v=cdo6FtSU_k8 Molecular Shapes with Six Electron- Group (Octahedral Arrangement) https://www.youtube.com/watch?v=Qcy-TjJ10xk Stepwise Method for using VSEPR theory to Determine Molecular Shape Step 1. Write the Lewis structure from the molecular formula and identify the relative placement of atoms and the number of electron groups. Step 2. Assign an electron-group arrangement by counting all electron groups around the central atom, bonding plus nonbonding. Step 3. Predict the ideal bond angle from the electron-group arrangement and the direction of any deviation caused by lone pairs or double bonds. Step 4. Draw and name the molecular shape by counting bonding groups and nonbonding groups separately. Simulation of the Molecular Shapes https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule- shapes_en.html  Molecular shape and Molecular Polarity  Molecularpolarity refers to a molecules having a net imbalance of charge. The net imbalance of charge produced from a polar bonds of a molecule causes the molecules to become polar (or dipole). A nonzero (not zero)net dipole moment makes a molecule polar. Dipole moment  Dipolemoment is a measure of the strength of the polarity of a bond.  The presence of polar bonds in a molecule does not mean that the molecule is polar like in CO2 CO2 based on electronegativity difference it is a polar. CO2 is linear, zero net, nonpolar The shape of the molecule determines whether a molecules is polar or nonpolar. Ex. CO2 is linear, zero net dipole, nonpolar  Ex.2. CCl4 , tetrahedral, zero net dipole, nonpolar Ex.3. CHCl3, tetrahedral, nonzero dipole, polar Molecular Shape, Biological Receptors and the Sense of Smell  A, The olfactory area lies at the top of the nasal passage very close to the brain.  B, A blow-up of the region shows olfactory nerve cells with their hairlike endings protruding into the liquid-coated nasal passage.  C, A greater blow-up shows a receptor site on one of the endings containing an odorous molecule that matches its shape. References:  Silberberge, M.S., Chemistry 4th Edition, Mc.Graw Hill pp.365-397  Villamar Jr.,R.A. , Chemistry, st. Bernadette Publishing House Corp.,pp.95-120  Bayquen, A.V., et.al, General Chemistry, Phoenix Publishing House pp.53-55 https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule- shapes_en.html https://www.youtube.com/watch?v=nxebQZUVvTg https://www.youtube.com/watch?v=cdo6FtSU_k8 https://www.youtube.com/watch?v=Qcy-TjJ10xk