Chapter 9 Chemical Bonding I Lewis Theory Georgia Gwinnett College Chem 1212K Fall 2013 Chapter 9 1 Preliminaries Our knowledge of atomic structure electron configurations and periodic properties gives a foundation for understanding bonding Electrons can be divided into Core electrons e in a filled shell Valence electrons e in an unfilled shell outermost electrons Valence electrons participate in bonding through Sharing of e by atoms covalent bonding Transfer of e from one atom to another ionic bonding Nature tends to prefer states of lower energy Atoms come together to form bonds to lower their energy Chapter 9 2 Octet Rule When atoms bond they tend to gain lose or share electrons to result in eight valence electrons ns2np6 noble gas configuration many exceptions H Li Be B attain an electron configuration like He He has two valence electrons a duet expanded octets for elements in period 3 or below using empty valence d orbitals 3 Chapter 9 Lewis Symbols of Atoms aka electron dot structures Use symbol of the element to represent the nucleus and inner electrons Use dots around the symbol to represent valence electrons Pair first two electrons for the s orbital Put one electron on each open side for p electrons Then pair the rest of the p electrons 4 Chapter 9 Practice Write the Lewis symbol for arsenic As is in column 5A therefore it has 5 valence electrons 5 Chapter 9 Ionic Bonding Ionic Bonding Ionic substances are formed when an atom that loses electrons relatively easily reacts with an atom that has a high affinity for electrons Ionic bonding is a result of the electrostatic attraction between the oppositely charged ions Chapter 9 6 Lewis Theory and Ionic Bonding Lewis symbols can be used to represent the transfer of electrons from metal atom to nonmetal atom resulting in ions that are attracted to each other and therefore bond Lithium Fluoride 7 Chapter 9 Example What is formula of ionic salt of Ba and I Chapter 9 8 Example What is formula of ionic salt of O and Al Chapter 9 9 Lattice Energy The energy accompanying the transfer of electrons from a metal to a nonmetal cannot by itself account for the stability of ionic compounds The extra stability that accompanies the formation of the crystal lattice is measured as the lattice energy The lattice energy is the energy released when the solid crystal forms from separate ions in the gas state q1 q2 E k r 10 Crystal Lattice Lattice Energy vs Ion Size 11 Trends in Lattice Energy Ion Charge The force of attraction between oppositely charged particles is directly proportional to the product of the charges Larger charge means the ions are more strongly attracted larger charge stronger attraction stronger attraction larger lattice energy Of the two factors ion charge is generally more important Lattice Energy 910 kJ mol Lattice Energy 3414 kJ mol 12 Example Order the following ionic compounds in order of increasing magnitude of lattice energy CaO KBr KCl SrO First examine the ion charges and order by sum of the charges Then examine the ion sizes of each group and order by radius larger smaller 13 Switching Gears Covalent Bonding Chapter 9 14 Covalent Bonding Covalent Bonding Non metals bonding with non metals None of the atoms are willing to completely give up their electrons Atoms try to achieve complete octets by sharing electrons Example Water Vocabulary Bonding pairs and lone pairs Chapter 9 15 Single Double and Triple Bonds Write Lewis Structures for F2 O2 and N2 Chapter 9 16 The effect of an electric field on hydrogen fluoride molecules Polar covalent bonds arrows and deltas Dipole Moments Bonded atoms of different elements share electrons unequally E g HF The F atom carries a slightly negative electric charge and the H atom a slightly positive charge of equal magnitude Aligns itself in an electric field Any molecule that has an uneven distribution of charges is called a polar molecule Posses a dipole moment 17 Electronegativity Electronegativity is the ability of an atom in a molecule to attract shared electrons to itself Greater electronegativity greater ability to attract electrons Related to other concepts we ve covered Electron Affinity Ionization Energy Chapter 9 18 Electronegativity Trends Chapter 9 19 Classifying Bonds Covalent H2 Cl2 N2 Polar Covalent HF H2O Ionic Bond LiF NaCl Chapter 9 20 Ionic Character of Covalent Bonds Chapter 9 21 Now the hard part Many heteronuclear molecules aren t so easy to figure out what the Lewis structures are Chapter 9 22 Determining Lewis Structures Ex NF3 1 2 3 4 5 Determine the Central Atom Least electronegative except H Count how many electrons we have If polyatomic ion Add electron for each negative charge subtract an electron for each positive charge Form a single bond between all connected atoms Assign remaining electrons as lone pairs to fill octets Start with most electronegative atoms If any atoms do not have a full octet change lone pairs to bonding pairs Chapter 9 23 Determining Lewis Structures Ex NO31 2 3 4 5 Determine the Central Atom Least electronegative except H Count how many electrons we have If polyatomic ion Add electron for each negative charge subtract an electron for each positive charge Form a single bond between all connected atoms Assign remaining electrons as lone pairs to fill octets Start with most electronegative atoms If any atoms do not have a full octet change lone pairs to bonding pairs Chapter 9 24 Determining Lewis Structures Ex CH4O 1 2 3 4 5 Determine the Central Atom Least electronegative except H Count how many electrons we have If polyatomic ion Add electron for each negative charge subtract an electron for each positive charge Form a single bond between all connected atoms Assign remaining electrons as lone pairs to fill octets Start with most electronegative atoms If any atoms do not have a full octet change lone pairs to bonding pairs Chapter 9 25 Resonance Delocalized Electrons Draw the Lewis structure for Ozone O3 Multiple valid Lewis structures Resonance Resonance structures have same arrangement of atoms but different locations of bonding and lone pair electrons Actual structure is a resonance hybrid Electrons are said to be delocalized Resonance Example NO3 Chapter 9 27 Formal Charge The charge an atom would have if the electrons were shared equally Formal charge is total number of valence electrons minus the number of electrons it owns in the molecule An atom in a molecule owns all of its lone pairs and half of its bonding electrons