Week 4 chemistry notes
Week 4 chemistry notes Chem 107
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This 3 page Class Notes was uploaded by Kelly Johnson on Friday February 12, 2016. The Class Notes belongs to Chem 107 at West Chester University of Pennsylvania taught by Jacqueline Butler in Winter 2016. Since its upload, it has received 65 views. For similar materials see General Chemistry for Health Science in Chemistry at West Chester University of Pennsylvania.
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Date Created: 02/12/16
Unit 2- Chapter 3 continued 1. Drawing Lewis Structure and VESPR Theory a. Octet Rule i. All atoms must have 8 electrons around them to be stable 1. Hydrogen is an exception, as it only requires 2 b. Lewis structure i. The Lewis Structure of an atom is a way to represent its electron content and ability to form bonds with other atoms 1. Remember that each atom wants to look like a noble gas and have 8 electrons surrounding it ii. How it is made? 1. Determine the number of electrons that atom has a. This is only the number of valence electrons in that atom 2. Arrange the atoms into a molecule a. Normally the first atom listed is the center atom, but this is not always true b. Carbon will always be in the middle 3. Attach each atom with bonds a. One bond is equal to two electrons 4. Distribute the remaining electrons as non-bonding pairs around the rest of the molecule (each atom should have its 8 electrons when including the bonding pair) 5. Use multiple bonds if this is necessary iii. Writing Lewis Structure 1. Steps a. Find the sum of the valence electrons of all atoms in the polyatomic ion or molecule b. The central atom is the least electronegative element that isn’t hydrogen. Connect all other atoms by single bonds. Subtract the number of electrons you use to form these bonds. c. Fill the octets of the outer atoms, still keeping track of how many electrons you have left to use. d. Fill the octet of the central atom. Still subtracting this from your original sum. e. If you run out of electrons before you fill all of these octets, there must be a multiple bond somewhere in the molecule. 2. Examples are online and in the class PowerPoints c. Updates on the Octet Rule i. Beryllium, Boron and Hydrogen do not follow the Octet Rule 1. Be wants 4 electrons 2. B wants 6 electrons 3. H wants 2 electrons ii. Third Period elements can exceed the octet rule by using empty d orbitals 1. Satisfy octet first and then fil d orbitals d. Resonance i. For some molecules, there is more than one possible Lewis Structure 1. Many times, the double bond can be moved, and in this case an average structure should be drawn 2. Represent this possibility through dotted lines e. VSEPR Model i. Explanation 1. This model is based on how electrons have a tendency to push away from one another ii. Predicting Molecular Geometry 1. Steps a. Draw the Lewis structure according to the rules you know b. Determine how many electron pairs are around the central atom. Double, triple bonds etc. count as one pair of electrons when counting. c. Arrange the electron pairs in a shape around the atom d. Determine the Electron geometry i. Linear- 2 bond pairs around the central atom ii. Trigonal Planar- 3 bond pairs around the central atom iii. Tetrahedral- 4 bond pairs around the central atom e. Determine the Molecular geometry i. Each compound also has a molecular shape based on the pull of its electrons ii. Shapes 1. Linear a. Both electron and molecular geometry b. Bond angle is 180 degrees 2. Trigonal Planar a. Both electron and molecular geometry b. Bond angle 120 degrees 3. Bent a. Linear geometry is trigonal planar b. Molecular geometry is bent c. Often due to a lone pair on the central atom d. Bond angle <120 degrees 4. Tetrahedral a. Both Linear and Molecular geometry b. Bond angle 109.5 degrees 5. Trigonal Pyramidal a. Linear geometry is tetrahedral b. Molecular geometry is trigonal pyramidal c. Bond angle <109.5 degrees 6. Bent a. Linear geometry is tetrahedral b. Molecular geometry is trigonal pyramidal c. Bond angle < 109.5 degrees f. Polarity i. Polarity is based on the shift of electrons between molecules based on electronegativity of the atoms 1. When the electrons favor one side briefly, giving it a negative charge and the other a positive, it is known as a dipole moment. 2. Any time a molecule has a lone pair of electrons around the central atom, it is polar ii. Electronegativity difference determine what is polar, nonpolar or ionic. 1. Ionic bond a. Has an electronegativity difference between 2.0 and 4.0 b. An electron is completely moved from one atom to another 2. Polar Covalent Bond a. Has an electronegativity difference between 0.4 and 1.9 b. Has a slight shift of electrons to one side, often a dipole movement 3. Nonpolar Covalent Bond a. Has an electronegativity difference between 0.0 and 0.4 b. Electrons have equal or no pull from both atoms iii. The shape of a molecule also must be considered when determining of the ENTIRE molecule is polar or not
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