Lewis Theory of Bonding PDF

Chapter 4.1 Lewis Theory of Bonding Atoms / ions are stable if they have a noble gas like electron structure (a stable octet of electrons). Electrons are most stable when they are paired. Atoms form chemical bonds to achieve a stable octet. In Ionic Bonding: There is an...

Chapter 4.1 Lewis Theory of Bonding Atoms / ions are stable if they have a noble gas like electron structure (a stable octet of electrons). Electrons are most stable when they are paired. Atoms form chemical bonds to achieve a stable octet. In Ionic Bonding: There is an electrostatic attraction between positive and negative ions. Na + Cl → [ Na ] + [ Cl ] → [Na][ Cl ] In Covalent Bonding: valence electrons are shared between atomic nuclei. Cl + Cl → Cl Cl → Cl - Cl Ionic bonding: between metals and non-metals. Covalent bonding: between non-metal atoms. Rules for drawing Lewis Structures: 1) Arrange atoms around the central atom (usually the first in the formula) 2) Count out the valence e- needed (negative ions: add electrons, positive ions: subtract electrons) 3) Place a bonding pair of e- between the central atom and each surrounding atom (octet rules) 4) Complete the octet of surrounding atoms. Extra e- go on the central atom as lone pairs. 5) Check to see if the central atom has an octet, make a double or triple bond if necessary. 6) Draw the structure; enclose polyatomic ions in square brackets. What to do if the Octet Rule Fails: Sometimes following the rules we have previously used you will find that there are too few electrons to complete the octet, or there are electrons left over after all the octets have been filled. We have two rules to handle these problems. 1) When there are not enough electrons to give every atom an octet, multiple bonds must be created 2) When electrons are left over, they are always placed on the central atom in pairs  Electron-Dot Structures of Polyatomic Molecules Draw an electron-dot structure for H2O. Step 1: 2(1) + 6 = 8 valence electrons H H Step 2: O H Step 3: O H bonding pair of electrons lone pair of electrons  Draw an electron-dot structure for CCl4. Step 1: 4 + 4(7) = 32 valence electrons Step 2: Cl Step 3: Cl Cl C Cl Cl C Cl Cl Cl  Draw an Electron-dot Structure for H3O1+. Step 1: 3(1) + 6 - 1 = 8 valence electrons 1+ H H Step 2: H O H Step 4: H O H  Draw an electron-dot structure for CH2O. Step 1: 4 + 2(1) + 6 = 12 valence electrons Step 2: O O Step 4: H C H H C H Step 3: O O H C H H C H  Draw an electron-dot structure for ICl3. Step 1: 7 + 3(7) = 28 valence electrons Cl Step 2: Cl Cl Step 4: Cl I Cl Cl I Step 3: Cl Cl Cl I  Draw an electron-dot structure for SF6. Step 1: 6 + 6(7) = 48 valence electrons F F Step 2: Step 3: F F F F S S F F F F F F  Electron-Dot Structures and Resonance Draw an electron-dot structure for O3. Step 1: 3(6) = 18 valence electrons Step 2: O O O Step 4: O O O Step 3: O O O Step 5: O O O Electron-Dot Structures and Resonance Move a lone pair from this oxygen? Step 4: O O O Or, move a lone pair from this oxygen? O O O O O O Resonance  Formal Charge: Selecting the Best Resonance Structure An atom “owns” all of its nonbonding electrons and half of its bonding electrons. Formal charge is the charge an atom would have if the bonding electrons were shared equally. Formal charge of atom = # valence e− - (# unshared electrons + number of bonds) B For OC For OA O O O # valence e− = 6 # valence e− =6 A C # nonbonding e− = 6 # nonbonding e− =4 For OB # bonds = 1 # bonds = 2 # valence e− = 6 Formal charge = −1 Formal charge = 0 # nonbonding e− = 2 # bonds = 3 Formal charge = +1 Writing Lewis Structures for Octet-Rule Exceptions PROBLEM: Write Lewis structures for (a) SClF5 (b) H3PO4 (draw two resonance forms and select the more important); (c) BFCl2. PLAN: Draw the Lewis structures and examine exceptions to the octet rule. Note (a) and (b) contain a 3rd Period element and can have an expanded valence shell and (c) contains B which often has an unsatisfied octet. SOLUTION: (a) SClF5 has an expanded valence shell. F (b) H3PO4 has two resonance forms and formal F F charges indicate the more important form. S F F −1 0 Cl 0 +1 0 0 O O 0 H O P O H0 0 H O P O H0 (c) BFCl2 will have only 1 O 0 O 0 Lewis structure. 0 0 H H 0 F 0 lower formal charges B Cl Cl more stable  When several Lewis structures are possible, those with the smallest formal charges are the most stable, least energetic and are preferred. Use formal charges to predict the preferred Lewis structure for the following: a) SO2 b) HClO3 c) H3PO4 Writing Resonance Structures PROBLEM: Write resonance structures for the nitrate ion, NO3−. PLAN: After going through the steps, see if other structures can be drawn in which the electrons can be delocalized over more than two atoms. SOLUTION: Nitrate has 1(5) + 3(6) + 1 = 24 valence e− O O O N does not have an octet; a pair of e− N N N will move in to form O O O O O O a double bond. O O O N N N O O O O O O N2 NO3- CO2 SO3 H2O NH4+ SO42- ClO2- NH3 O2 F2 Homework: Page 230 2 – 4 Page 229 10 a – d, 11 Page 227 1, 3, 4