Chapter 9 Ebbing and Gammon Chemical Bonding and an Introduction to Lewis Structures Why do atoms form chemical bonds 1 The energy of the bonded atoms must be than the energy of the nonbonded atoms 2 What causes this energy change Types of Chemical bonds 1 Ionic bonds 2 Covalent bonds 3 Metallic bonds Predicting Bond Type 1 Nonmetal to nonmetal bonds are always 2 Metals to polyatomic anions always 3 Polyatomic cations to polyatomic anions always 4 The bonding within polyatomic ions is always 5 Metal to metal are always 6 Metal to nonmetals a using metal oxidation states b using electronegativity values Using metal oxidation states to predict bond type 1 If the oxidation state of the metal is 4 or higher the bond is always covalent If the oxidation state of the metal is less than four the bond may be either ionic or covalent and you will need to use electronegativities to predict the bond type For example VO ionic V2O3 ionic VO2 covalent CuO covalent Cu2O covalent Using electronegativities to predict bond type The following chart seems to provide the best way to predict bond type for metal nonmetal compounds when the metal oxidation state is below 4 4 Lower Electronegativity 3 5 E N 1 7 3 E N 2 2 2 5 Covalent 2 1 5 1 Ionic Metallic 0 5 0 5 1 0 1 5 2 0 2 5 3 0 Higher Electronegativity 2 3 5 4 0 Summary of bond type chart If the electronegativity of the higher electronegative element is greater than 2 2 and if the electonegativity of the other element is greater than 1 7 the bond is most likely covalent If the electronegativity of the higher electronegative element is greater than 2 2 and if the electonegativity of the other element is less than 1 7 the bond is most likely ionic If the highest electronegativity is less than 2 2 the bond is most likely metallic Practice Predict the bond types in the following Cu IF5 C12H22O11 Ag3N CrO3 KCl SCl2 Au NO3 3 MnO41 CaS PbCr2O7 XeF4 Lewis Structures Lewis structures a k a Lewis electron dot formulas are a simple way of visualizing the role of valence electrons in chemical bond formation Lewis electron dot formulas for elements Examples K Ba N Cl Ti Lewis electron dot formulas for ions Examples K Ba2 Cl1 N3 Ti3 Lewis electron dot formulas for binary ionic compounds 3 Examples compounds between potassium and oxygen and between barium and nitrogen Lewis Formulas for Covalent Compounds Basic rules for drawing Lewis structures for binary compounds and polyatomic ions which follow the octet rule What is the so called octet rule 1 Count total valence electrons in the compound or ion 2 Connect each of the extra atoms to the central atom with a single bond 3 Complete the octets of all atoms except hydrogen which gets only two electrons 4 Count the electrons used Use double and triple bonds to reduce the electron count if needed Examples HCl H2O CI4 NH4 SO3 N2 NO31 SCN 1 Atomic Radii radii expressed in pm 4 Average Covalent Bond Lengths picometers pm 5 6 Average Bond Energies In kJ mol 7 Relationship Between Bond Order Bond Length and Bond Energy Electronegativity Definition Electronegativity is a measure of a bonded atoms attraction for the shared electrons in the bond 8 The concept of electronegativity is very important in any discussion of chemical bonding Each element has been assigned an electronegativity value based on atomic size electron affinity and bond energies The most common electronegativity scale was developed by Linus Pauling more than 50 years ago Other scales have been developed over the years but the Pauling scale remains the most popular On the Pauling scale fluorine the most electronegative element is assigned a value of 4 Polar Bonds Remember Electronegativity is defined as a measure of an atom s attraction for the electrons in a bond Imagine two atoms A and B of equal electronegativities sharing a pair of electrons A B Since the E N are equal what is the distribution of the electron density in the bond Now imagine that we can step up the E N of A and step down the E N of B What will begin to happen to the electron density in the bond as the E N difference increases Examples of Polar and Nonpolar Bonds Predict if the following bonds are polar or nonpolar If the bond is polar indicate the direction of the polarity 9 Cl Cl Br Cl H Cl O Cl C H C O F F C S Covalent Lewis Structures Part II Remember the basic rules for drawing Lewis structures of covalent compounds and ions 1 Count total valence electrons in the compound or ion 2 Connect each of the extra atoms to the central atom with a single bond 3 Complete the octets of all atoms except hydrogen which gets only two electrons 4 Count the electrons used Use double and triple bonds to reduce the electron count if needed Compounds and polyatomic ions whose central atoms do not follow the octet rule 10 We are now going to consider substances whose central atom may have more or less than an octet of valence electrons We will also consider substances that can have more than one reasonable Lewis structure We will use the method of formal charge to help us address the question of whether one of the structures is best or if the structures are all equally probable For example draw the three possible Lewis structures for BeCl2 Formal Charge Formal Charges are a way to help us evaluate the stability of Lewis structures when more than one structure is possible Formal charges are calculated for each atom in the compound ion FC VE NBE BE Where VE number of valence electrons NBE number of nonbonding BE number of bonding electrons on the for the atom electrons on the atom atom Apply formal charge rules to BeCl2 Lewis structures General Rules 1 The best structure will be the one with the lowest set of formal charges 11 2 Any negative formal charges should be on the most electronegative elements 12 13 General Guidelines for Lewis Structures a H always has 2 electrons b C N O F almost always have octets c Most reduced octets occur with Boron or metals as the central atom d Expanded octets occur with both metals and nonmetals as the central atom e In all cases of either reduced or expanded the octet octets it is the central atom that does not have Practice with non octet Lewis Structures Draw Lewis structures for the following covalent substances a SF4 b XeF2 14 c BeCl2 d I3e ClF3 f XeF4 Resonance Structures Multiple possible Lewis structures for the same substance are said to be resonance forms if the connectivity in each structure is the same and only the location of one or more electron pairs has