Chemistry 1A Chapter 9 Page 1 Chapter 9 Chemical Bonding I Lewis Theory Homework Read Chapter 9 Work out sample practice exercises Suggested Chapter 9 Problems 11 37 39 43 45 47 53 55 63 65 71 73 75 77 83 87 91 97 101 Check for the MasteringChemistry com assignment and complete before due date Chemical Bonding How atoms are connected together and the three dimensional shapes of molecules are very important Many chemicals need to have the right shape to fit into the correct receptor or react the expected way Finding the correct fit will allow manmade drugs to do a certain job Artificial sweeteners have a shape that fits our receptors on the tongue to fool our brain into believing we taste something sweet The puzzle picture is showing the AIDS drug Indinavar as the missing piece depicting the protein HIV protease Pharmaceutical companies designed molecules that would disable HIV protease by sticking to the molecule s active site protease inhibitors To design such a molecule researchers used bonding theories to simulate the shape of potential drug molecules and how they would interact with the protease molecule Why do chemical bonds form Chemical bonds form because they lower the potential energy between the particles that compose atoms A chemical bond forms when the potential energy of the bonded atoms is less than the potential energy of the separate atoms You need to consider the following interactions 1 nucleus to nucleus repulsions 2 electron to electron repulsions 3 nucleus to electron attractions Chemistry 1A Chapter 9 Page 2 Lewis dot structures A UC Berkeley chemistry professor in 1916 Gilbert Lewis developed a method to teach his beginning chemistry students how to understand chemical bonding that represents valence electrons with dots for main group elements Lewis structures allow us to predict many properties of molecules such as molecular stability shape size polarity Types of Bonding Types of Atoms Types of Bond Bond Characteristics Metal to Nonmetal Ionic Electrons Transfer Nonmetal to Nonmetal Covalent Electrons Shared Metals to Metals Metallic Sea of Electrons Ionic bonds occur between a cation metal and an anion nonmetal Ions are held together by electrostatic attraction opposite charges attracting each other These attractions are quite strong and increase with increasing charges and decreasing ionic sizes Ionic compounds must overcome large lattice energies to separate to melt or boil Ionic compounds have high melting points due to a vast three dimensional network of attractions between ions Covalent bonds occur when electrons are shared between nonmetal atoms The length of a bond increases as the bond order decreases triple double single The amount of energy to break a chemical covalent bond in an isolated gas molecule is called the bond dissociation energy BE or D The strength of the bond energy increases with increasing bond order single double triple bonds Approximate Bond Energy values 10 can be found in reference sources Note Bond energy values assume starting and ending with gas substances One can use these energies to find approximate enthalpies of reaction Hreaction BEreactants BEproducts Covalent compounds have low melting points the strong molecular bonds do not break apart when melting or boiling Covalent compounds are made of discrete molecules held together by weak intermolecular attractions Chemistry 1A Chapter 9 Page 3 Metallic bonds The low ionization energy of metals allows them to lose electrons easily The simplest theory of metallic bonding involves the metal atoms releasing valence electrons to be shared by all atoms ions in the metal Metal cations are surrounded by a sea of electrons The electrons are delocalized throughout the metal structure Bonding results from attraction of the cations for the delocalized electrons Metallic solids conduct electricity well As temperature increases the electrical conductivity of metals decreases Metallic solids conduct heat well Metallic solids reflect light Metallic solids are malleable and ductile Metals generally have high melting points and boiling points all but Hg are solids at room temperature Melting points of metal generally increase left to right across period Na 97 72 C Mg 650 C Al 660 32 C Melting points of metals generally decrease down column Li 180 54 C Na 97 72 C K 63 38 C Chemistry 1A Chapter 9 Page 4 Electronegativity EN is the ability of an atom to attract bonding electrons to itself Metals attract weakly while nonmetals attract electrons more strongly especially as they become smaller and more nonmetallic A way to remember stronger to weaker electronegativities is the term FONClBrISCH EN increases across period left to right and Decreases down group top to bottom Often noble gas atoms are not assigned values Larger the difference in EN more polar the bond Negative end toward more EN atom Most compounds are between the two extremes of ionic and pure covalent bonds sharing electrons unequally polar covalent bonds 1 If difference in electronegativity EN between bonded atoms is 0 the bond is pure covalent Share electrons equally such as O2 2 EN is 0 1 to 0 4 the bond is nonpolar covalent Share electrons nearly equal Number ranges are approximate C H hydrocarbons are considered nonpolar covalent while H2S is polar covalent 3 EN is 0 5 to 1 9 the bond is polar covalent Unequally share electrons Number ranges are approximate a metal nonmetal is considered ionic such as MgBr2 whose EN 1 3 4 EN atoms is larger than or equal to 2 0 the bond is ionic Electrons are transferred as in NaCl Chemistry 1A Chapter 9 Page 5 Note Bond Polarity is between two atoms this is not the same as identifying whether a molecule is polar or nonpolar For polarity of the overall substance we need 3D information found in the VSEPR and VB theories in Chapter 10 Ionic Bonding Lewis theory predicts the attractions between ions are strong ionic compounds should have high melting points and boiling points because breaking down the crystal should require a lot of energy Stronger the attraction Higher the melting point Reality High melting points and boiling points MP 300 C Solids at room temperature Relatively hard compared to most molecular solids Brittle When struck they shatter Do not conduct electricity ions are trapped in place in the solid Ionic compounds conduct electricity in the liquid state or when dissolved in water Chemistry 1A Chapter 9 Page 6 Crystal Lattice Energy or Lattice Energy Crystal Lattice Energy is