Electronic Structure and Anders Jöns Ångström ( ) Bonding 1 Å = 10 picometers = 0.1 nanometers = 10-4 microns = 10-8 centimeters Molecular

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1 Chapters 1 & 2 ~ 0.1 nm General Chemistry Review Electronic Structure and Bonding Anders Jöns Ångström ( ) 1 Å = 10 picometers = 0.1 nanometers = 10-4 microns = 10-8 centimeters Molecular Representations 1 nm = 10 Å An atom vs. a nucleus ~10,000 x larger Nucleus = 1/10,000 of the atom Question 1.1 What is the electronic configuration of carbon? A) 1s 2 2s 2 2p x2 B) 1s 2 2s 2 2p 1 x 2p y1 2p z0 C) 1s 2 2s 2 2p x1 2p y1 2p z1 D) 1s 2 1p 1 x 1p y1 2s 2 Electron Configurations Noble Gases and The Rule of Eight When two nonmetals react to form a covalent bond: They share electrons to achieve a Noble gas electron configuration. When a nonmetal and a metal react to form an ionic compound: Valence electrons of the metal are lost and the nonmetal gains these electrons. G.N. Lewis Photo Bancroft Library, University of California/LBNL Image Library Footnote: G.N. Lewis, despite his insight and contributions to chemistry, was never awarded the Nobel prize. 1

2 Ionic Compounds Ionic compounds are formed when electron(s) are transferred. Electrons go from less electronegative element to the more electronegative forming ionic bonds. Covalent Compounds Share electrons. 1 pair = 1 bond. Octet rule ( duet for hydrogen) Lewis structures: Notice the charges: In one case they balance, can you name the compound? In the other they do not, can you name the polyatomic ion? More about formal charge to come. Worksheet 1: Bonds, Formulas, Structures & Shapes Covalent Bonding Simple Lewis Structures Question 1.2 For simple Lewis structures: 1. Draw the individual atoms using dots to represent the valence electrons. 2. Put the atoms together so they share PAIRS of electrons to make complete octets. Take NH 3, for example: Select the correct Lewis structure for methyl fluoride (CH 3 F). A) B) C) D) Practice with SKILLBUILDER 1.3. Important Bond Numbers (Neutral Atoms / Normal electron distribution / Free electron pairs one bond two bonds not shown) H F Cl Br I O Question 1.3 What is the best Lewis structure for formaldehyde (H 2 CO)? A) B) three bonds N four bonds C C) D) 2

3 Question 1.4 Which of the following contains a triple bond? A) SO 2 B) HCN C) C 2 H 4 D) NH 3 Formal Charge Formal charge is the charge of an atom in a Lewis structure which has a different than normal distribution of electrons. Klein: 2.4, 2.9 Important Bond Numbers (Neutral Atoms / Normal electron distribution) Important Bond Numbers (Neutral Atoms / Normal electron distribution) one bond H F Cl Br I two bonds O three bonds N four bonds C Formal Charge Formal charge = number of valence electrons (number of lone pair electrons +1/2 number of bonding electrons) HNO 3 Nitric Acid Equals the number of valence electrons (Group Number of the free atom) minus [the number of unshared valence electrons in the molecule + 1/2 the number of shared valence electrons in the molecule]. Moving/Adding/Subtracting atoms and electrons. 3

4 Complete the following table. It summarizes the formal charge on a ( central ) atom for the most important species in organic chemistry. Formal Charge = # of valence e - s - [1/2(# of bonding e - s ) + # of non-bonding e - s ] =? =? Worksheet #1 & Worksheet #4 Question 1.5 What is the formal charge of the carbon atom in the Lewis structure? A) -1 B) 0 C) +1 D) +2 C Question 1.6 What is the formal charge of the oxygen atom in the Lewis structure? A) -1 B) 0 C) +1 D) +2 Resonance Resonance is a very important intellectual concept that was introduced by Linus Pauling in 1928 to explain experimental observations. Resonance Klein: Eg. SO 2 Bond order 1.5 Bond length > double bond; Bond length < single bond TUTORIAL 4

5 Resonance Two or more Lewis structures may be legitimately written for certain compounds (or ions) that have double bonds and/or free pairs of non-bonded electrons It is a mental exercise in pushing or moving electrons. Rules of Resonance Step 1: The atoms must stay in the same position. Atom connectivity is the same in all resonance structures. Only electrons move. NON-Example: The Lewis formulas below are not resonance forms. A hydrogen atom has changed position. Step 2: Each contributing structure must have the same total number of electrons and the same net charge. Example: All structures have 18 electrons and a net charge of 0. Step 3: Calculate formal charges for each atom in each structure. Example: None of the atoms possess a formal charge in this Lewis structure. Step 4: Calculate formal charges for the second and third structures. Example: These structures have formal charges. NOTE: They are less favorable Lewis structures. Pushing Electrons same atomic positions differ in electron positions only 5

6 Why use Resonance Structures? Delocalization of electrons and charges between two or more atoms helps explain energetic stability and chemical reactivity. Electrons in a single Lewis structure are insufficient to show electron delocalization. Ozone (O 3 ) Resonance Example Lewis structure of ozone shows one double bond and one single bond A composite of all resonance forms more accurately depicts electron distribution. (HYBRID) NOTE: Resonance forms are not always evenly weighted. Some forms are better than others. Ozone (O 3 ) Resonance Example Lewis structure of ozone shows one double bond and one single bond Resonance Example Ozone (O 3 ) Electrostatic potential map shows both end carbons are equivalent with respect to negative charge. Middle carbon is positive. Question 1.7 Which resonance structure contributes more to the hybrid? A) B) VSEPR Klein:

7 VSEPR Model Valence Shell Electron Pair Repulsion VSEPR Model The molecular structure of a given atom is determined principally by minimizing electron pair (bonded &free) repulsions through maximizing separations. Some examples of minimizing interactions. Predicting a VSEPR Structure Chem Draw Lewis structure. 2. Put pairs as far apart as possible. 3. Determine positions of atoms from the way electron pairs are shared. 4. Determine the name of molecular structure from positions of the atoms. Molecular Geometry Summary Lewis Structures / VSEPR / Molecular Models Computer Generated Models Ball and stick models of ammonia, water and methane. Practice: SKILLBUILDER 1.8. Worksheet 1: Bonds, Formulas, Structures & Shapes 7

8 Covalent Bond Polarity Molecular Polarity Dipole Moment Covalent Compounds Equal sharing of electrons: nonpolar covalent bond, same electronegativity (e.g., H 2 ) Unequal sharing of electrons between atoms of different electronegativities: polar covalent bond (e.g., HF) Klein: 1.11 Polar Covalent Bonds Electrons tend to shift away from lower electronegativity atoms to higher electronegativity atoms. The greater the difference in electronegativity, the more polar the bond. Practice: SKILLBUILDER 1.5 Question 1.8 Which of the following bonds is the most polar? A) B) C) D) 8

9 Bond Dipole & Dipole Moment Dipole moments are experimentally measured. Polar bonds have dipole moments. dipole moment: D = µ = e x d (e) : magnitude of the charge on the atom (d) : distance between the two charges Question 1.9 Which of the following bonds have the greatest dipole moment (µ)? A) B) 1 debye = esu cm An electrostatic unit of charge (esu) is a unit of charge. One electron has a charge of 4.80 x esu. C) D) Bond Polarity A molecule, such as HF, that has a center of positive charge and a center of negative charge is polar, and has a dipole moment. The partial charge is represented by δ and the polarity with a vector arrow. Question 1.10 In which of the following is oxygen the positive end of the bond dipole? A) O-F B) O-N C) O-S D) O-H Question 1.11 In which of the compounds below is the δ+ for H the greatest? A) CH 4 B) NH 3 C) SiH 4 D) H 2 O Molecular Polarity Electrostatic Potential Maps Models that visually portray polarity and dipoles 9

10 Hydrogen Halides When identical polar bonds point in opposite directions, the effects of their polarities cancel, giving no net dipole moment. When they do not point in opposite directions, there is a net effect and a net molecular dipole moment, designated δ. Molecular Dipole Moment The vector sum of the magnitude and the direction of the individual bond dipole determines the overall dipole moment of a molecule An electrically charged rod attracts a stream of chloroform but has no effect on a stream of carbon tetrachloride. Molecular Polarity Consider the resultant dipole for CH 3 Cl Dipole moment (μ) = charge (e) x distance (d)!e = x esu; d = x 10-8 cm v The amount of charge separation is less than if it were complete charge separation (4.80 x esu). μ = 1.87 x esu cm v Conversion to debye μ = 1.87 D Molecular Polarity Electrostatic Potential Maps Visual depictions of polarity: 10

11 Ammonia and in the Ammonium Ion Water Polarity & Physical Properties Ozone and Water nm Resultant Molecular Dipoles > 0 Solubility: Polar molecules that dissolve or are dissolved in like molecules The Lotus flower Water & dirt repellancy Worksheet 4: Functions, Polarity, Formal Charge Solubility Generally likes dissolves like. Polar compounds with other polar compounds: v If compounds that are mixed are capable of H-bonding and/or strong dipole dipole interactions, then they should dissolve. Nonpolar compounds with other nonpolar compounds: v If a compound has no strong intermolecular attraction with itself, then there are no strong forces which would have to be overcome to have it dissolve with a similar compound. The Lotus Effect Biomimicry The Lotus Effect Biomimicry Wax Lotus petals have non-polar wax on its micrometer-scale rough surface, resulting in water contact angles up to 170 See the middle image above. When it rains, water dissolves any polar materials on the surface and gravity takes care of any insoluble, non-polar dirt on the lotus surface, much like a snow ball. Isotactic polypropylene (i-pp) melted between two glass slides and subsequent crystallization provided a smooth surface. Atomic force microscopy tests indicated that the surface had root mean square (rms) roughness of 10 nm. A) The water drop on the resulting surface had a contact angle of 104 ± 2 B) the water drop on a superhydrophobic i-pp coating surface has a contact angle of 160. Science, 299, (2003), pp , H. Yldrm Erbil, A. Levent Demirel, Yonca Avc, Olcay Mert 11