Ionic Bonding Test Scale
Covalent Bonding
Covalent Bonding Review A sharing of electrons Polarity of bond refers to how equally electrons are shared Bonding Pair/s: The shared electrons Non-Bonding Pair/s: The electrons that do not participate in bonding
Lewis Dot Diagrams Element symbol X Insert S-sublevel electrons first
Lewis Dot Formulas F F Non-Bonding Pair Bonding Pair
Different Formulas Molecular Formula Structural Formula Not the same as Empirical Formula Structural Formula A dash (-) is used to represent bonding pair No non-bonding electrons shown Lewis Dot Formula Use dash to represent bonding pairs All non-bonding electrons shown
When writing Lewis Structures: ALL valence electrons must be shown for ALL atoms present Atoms that are bonded share one or more pairs of electrons All atoms must have 8 electrons* *Exception Hydrogen (2 electrons) *Exceptions to Octet Rule
Steps for writing Lewis Dot Formulas Find the total number of valence electrons Draw a dash between each atom Generally least electronegative atom goes in center Add electron dots until every atom has enough electrons (H=2; Everything else =8) Count up electrons If you have the same # as you calculated in step 1 you are finished If you have more you have a multiple bond If you have an ion, put brackets around the structure and denote the charge
Cl2 Cl Cl Cl Cl Valence Electrons = 7 + 7 = 14 e- 2. Add a dash between each atom (single bond) 3. Give every atom full valence electrons 4. Count up Electrons Cl Cl Cl Cl
What happens if you have to many Electrons? Compound has a multiple bond Single bond – 1 bonding pair Double bond – 2 bonding pairs Triple bond – 3 bonding pairs C, N, O, and S may have multiple bonds
C2H4 C C C C C C H H H H H H H H H H H H H H H Valence Electrons = 4+4+1+1+1+1= 12 Add a single bond between each atom: Give every atom full valence: Count up electrons: 14 12 add double bond and recount H H H H C C H H H H H C C H H H H C C H H
Lewis Structure Practice CH3I SO4-2 CO2H
SO2 O S O O S O O S O O S O # V.E. = 6+6+6= 18 Single Bonds between atoms and fill valence Add a double bond But ... We can write two different structures. Which one is right? O S O O S O O S O O S O
Bond Energies and Bond Length What do we know about bond energy and strength of a bond? Bond energy = energy required to break a bond Higher bond energy Higher bond strength We can also relate Bond energy/strength and Bond length Bond Length (pm) Energy (kJ/mol) C-C 154 348 C=C 134 614 CC 120 839
Resonance X-Ray diffraction can be use to determine bond lengths For SO2 we would expect to observe different bond lengths But… Both bonds have the same length? Resonance is the bonding in molecules or ions that cannot be correctly represented by one Lewis structure.
Resonance and SO2 O S O O S O O S O O S O NOT the correct structure NOT the correct structure Correct structure is an average of the 2 Correct Lewis Structure is represented like this: O S O O S O
Resonance Practice: Draw the Lewis Structure for 03 (include all resonance structures)
Exceptions to Octet Rule Some Group II and Group III that bond covalently can have less than 8 electrons (Be, B) Example BF3 BeCl2
Exceptions to the Octet Rule Some nonmetals may have more than 8 electrons (expanded octet) Only row 3 and greater nonmetals SF6 PCl5
Oxidation Numbers Oxidation numbers also called oxidation states, indicate the general distribution of electrons among the bonded atoms in a molecular compound or polyatomic ion Have no physical meaning Arbitrary But… can be very useful in naming, writing formulas and balancing chemical equations
Assigning Oxidation Numbers An atom in its elemental state has an oxidation number of zero The oxidation number of a monoatomic ion is considered to be equal to the charge on the ion. The oxidation of Fluorine in a compound is always -1 Group 1 elements (except H) in compounds are assigned oxidation numbers of +1 Group 2 elements are assigned an oxidation number of +2
Assigning Oxidation Numbers 6. The elements of group 17 are assigned a charge of -1, when combined with less electronegative atoms 7. Oxygen is assigned an oxidation number of -2 except: When paired with fluorine OF2 (oxygen +2) In peroxides O2-2 (each oxygen is -1) In superoxide O2- (each oxygen is -1/2) 8. For Hydrogen: If in a compound with more electronegative elements Charge is +1 In compounds with metals, (less electronegative) Charge is -1
Assigning Oxidation Numbers 9.The sum of oxidation numbers for neutral compounds must equal 0.The sum of oxidation numbers of polyatomic ions must equal the ions overall charge.
Assigning Oxidation Numbers UF6 H2SO4 ClO3-