Lecture Presentation Chapter 8 Basic Concepts of Chemical Bonding 2012 Pearson Education Inc John D Bookstaver St Charles Community College Cottleville MO Chemical Bonds Three basic types of bonds Ionic Electrostatic attraction between ions Covalent Metallic Sharing of electrons Metal atoms bonded to several other atoms Basic Concepts of Chemical Bonding 2012 Pearson Education Inc Lewis Symbols G N Lewis pioneered the use of chemical symbols surrounded with dots to symbolize the valence electrons around an atom When forming compounds atoms tend to add or subtract electrons until they are surrounded by eight valence electrons the octet rule Basic Concepts of Chemical Bonding 2012 Pearson Education Inc Ionic Bonding 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Energetics of Ionic Bonding As we saw in the last chapter it takes 496 kJ mol to remove electrons from sodium 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Energetics of Ionic Bonding We get 349 kJ mol back by giving electrons to chlorine 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Energetics of Ionic Bonding But these numbers don t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Energetics of Ionic Bonding There must be a third piece to the puzzle What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Lattice Energy This third piece of the puzzle is the lattice energy The energy required to completely separate a mole of a solid ionic compound into its gaseous ions The energy associated with electrostatic interactions is governed by Coulomb s law Eel Q1Q2 d 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Lattice Energy Lattice energy then increases with the charge on the ions It also increases with decreasing size of ions 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Energetics of Ionic Bonding By accounting for all three energies ionization energy electron affinity and lattice energy we can get a good idea of the energetics involved in such a process 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Covalent Bonding In covalent bonds atoms share electrons There are several electrostatic interactions in these bonds Attractions between electrons and nuclei Repulsions between electrons Repulsions between nuclei 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Polar Covalent Bonds Though atoms often form compounds by sharing electrons the electrons are not always shared equally Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does Therefore the fluorine end of the molecule has more electron density than the hydrogen end Basic Concepts of Chemical Bonding 2012 Pearson Education Inc Electronegativity Electronegativity is the ability of atoms in a molecule to attract electrons to themselves On the periodic chart electronegativity increases as you go from left to right across a row from the bottom to the top of a column 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Polar Covalent Bonds When two atoms share electrons unequally a bond dipole results The dipole moment produced by two equal but opposite charges separated by a distance r is calculated Qr It is measured in debyes D 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Polar Covalent Bonds The greater the difference in electronegativity the more polar is the bond 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Lewis Structures Lewis structures are representations of molecules showing all electrons bonding and nonbonding 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Writing Lewis Structures PCl3 1 Find the sum of valence electrons of all atoms in the polyatomic ion or molecule If it is an anion add one electron for each negative charge If it is a cation subtract one electron for each positive charge Basic Concepts of Chemical Bonding Keep track of the electrons 5 3 7 26 2012 Pearson Education Inc Writing Lewis Structures 2 The central atom is the least electronegative element that isn t hydrogen Connect the outer atoms to it by single bonds Keep track of the electrons 26 6 20 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Writing Lewis Structures 3 Fill the octets of the outer atoms Keep track of the electrons 26 6 20 20 18 2 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Writing Lewis Structures 4 Fill the octet of the central atom Keep track of the electrons 26 6 20 20 18 2 2 2 0 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Writing Lewis Structures 5 If you run out of electrons before the central atom has an octet form multiple bonds until it does 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Writing Lewis Structures Then assign formal charges For each atom count the electrons in lone pairs and half the electrons it shares with other atoms Subtract that from the number of valence electrons for that atom the difference is its formal charge 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Writing Lewis Structures The best Lewis structure is the one with the fewest charges puts a negative charge on the most electronegative atom 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Resonance This is the Lewis structure we would draw for ozone O3 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Resonance But this is at odds with the true observed structure of ozone in which both O O bonds are the same length both outer oxygens have a charge of 1 2 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Resonance One Lewis structure cannot accurately depict a molecule like ozone We use multiple structures resonance structures to describe the molecule 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Resonance Just as green is a synthesis of blue and yellow ozone is a synthesis of these two resonance structures 2012 Pearson Education Inc Basic Concepts of Chemical Bonding Resonance In truth the electrons that form the second C O bond in the double bonds below do not always sit between that C and that O but rather can move among the two oxygens and the carbon They are not localized they are delocalized 2012 Pearson Education Inc Basic Concepts