Chemical Bonding Chemical bonds are classified into two types: Ionic bonding results from electrostatic attractions among ions, which are formed by the transfer of one or more electrons from one atom to another. Covalent bonding results from sharing one or more electron pairs between two atoms.
Chemical bonds are the attractive forces that hold atoms together in compounds. The electrons involved in bonding are usually those in the outermost (valence) shell.
(aqueous) Physical Properties of Ionic and Covalent Compounds Ionic Covalent Melting Pt High Low Solubility Soluble Insoluble (polar solvents) Solubility Insoluble Soluble (nonpolar solvents) Conductivity High Low (molten) Conductivity High Low (aqueous)
The Lewis dot representation (Lewis dot formulas, Lewis dot structures) convenient bookkeeping method for valence electrons electrons that are transferred or involved in chemical bonding Table 7-1 pg. 272
metals react with nonmetals to form ionic compounds 2Li(s) + F2(g) 2LiF(s) 1s 2s 2p Li F becomes Li+ [He] F- [Ne] Table 7-2 pg. 277
Groups Gen. Form. Example IA + VIIA MX NaF IIA + VIIA MX2 BaCl2 IIIA + VIIA MX3 AlF3 IA + VIA M2X Na2O IIA + VIA MX BaO IIIA + VIA M2X3 Al2S3 IA + VA M3X Na3N IIA + VA M3X2 Mg3P2 IIIA + VA MX AlN H forms ionic cpds. with IA + IIA metals LiH, KH, CaH2, BaH2,, etc. other H cpds are covalent
Covalent Bonding .. Covalent bonding results from sharing one or more electron pairs between two atoms. Covalent Bonding occurs when two atoms with similar electronegativities form a bond share 2 electrons - single covalent bond share 4 electrons - double covalent bond share 6 electrons - triple covalent bond H-H O=O NN .. .. .. = .. .. .. ..
Octet Rule and Lewis Formulas for Molecules octet rule - in most of their compounds, the representative elements achieve noble gas configuration S=N-A S = # of shared valence e- exceptions will be discussed later N= # of valence e- needed to acheive octet except H (2 e-), Be (4 e-) and B (6 e-) A= # of valence e- available H2O N= (2*2) + (8*1) = 12 A = (2*1) + (6*1) = 8 S = 4
Octet Rule and Lewis Formulas for Molecules octet rule - in most of their compounds, the representative elements achieve noble gas configuration S=N-A S = # of shared valence e- exceptions will be discussed later N= # of valence e- needed to acheive octet except H (2 e-), Be (4 e-) and B (6 e-) A= # of valence e- available H2O N= (2*2) + (8*1) = 12 A = (2*1) + (6*1) = 8 S = 4 .. .. .. .. O H H
Writing Lewis Structures 1) Symmetrical Skeleton least electronegative element is central - CS2 S C S oxygen does not bond to oxygen (exept in peroxide and superoxides) ternary acids the H is usually bonded to the oxygen H O N O exception H3PO3 and H3PO2 2) Calculate N, A and S SO42- N = 5*8 = 40 A = 6*5 + 2 = 32 S = 40-32 = 8 3) Place the S (shared) electrons between atoms use double and triple bonds only when necessary 4) Place additional electrons into the skeleton as lone pairs to fill the octet of every atom except H (2 e-), Be (4 e-) and B (6 e-) 2- O H O S O H O O S O H2SO4 SO42- 2- O O S O
CO2 Step 1:
CO2 Step 1: Step 2: N = 3*8 = 24 A = 2*6 + 4 = 16 S = 24-16 = 8
CO2 Step 1: Step 2: N = 3*8 = 24 A = 2*6 + 4 = 16 S = 24-16 = 8
CO2 Step 1: Step 2: N = 3*8 = 24 A = 2*6 + 4 = 16 S = 24-16 = 8
CO2 Resonance Structures Step 1: Step 2: N = 3*8 = 24 A = 2*6 + 4 = 16 S = 24-16 = 8 Step 3: Step 4: Resonance Structures
Resonance Structure of CO32-
Resonance Structure of CO32- N= 4*8 = 32 A=1*4 + 3*6 + 2 = 24 S=8 C O O
Resonance Structure of CO32- N= 4*8 = 32 A=1*4 + 3*6 + 2 = 24 S=8 C O O
Resonance Structure of CO32- N= 4*8 = 32 A=1*4 + 3*6 + 2 = 24 S=8 C O O 2-
Limitations to the Octet Rule Electron Deficient BeCl3 N=4+(2*8)=20 A=2*7 + 2 =16 S=4 BF3 N=6+(3*8)=30 A=3*7 + 3 =24 S=6 Cl Be Cl Radicals
Expanded Valence Shell (hypervalent) PF5 F N=6*8=48 A=5+(5*7)=40 S=8 S has to be equal to 10 F F P F F SF4 N=5*8=40 A=6+(4*7)=34 S=6 S has to be equal to 8 F F S F F