Created by C. Ippolito Dec 2006 Chemical Bonding Objectives: 1.describe the nature of a chemical bond and its relationship to valence electrons 2.compare ionic and covalent bonding 3.use Lewis Dot Diagrams to represent ionic and covalent compounds 4.describe the relationship between molecular polarity and bond polarity 5.explain the nature and effects of metallic bonding, hydrogen bonding, and van der Waals forces 6.compare the structure and properties of polar and nonpolar molecules 7.compare the four classes of solids: ionic, molecular, metallic, and network

Created by C. Ippolito Dec 2006 Chemical Bonding Chemical Bond –force of attraction that holds atoms together to form compounds Types of Bonds –Ionic Bond –Ionic Bond – transfer electrons – electronegativity  >1.67 –Covalent Bond –Covalent Bond – share electrons – electronegativity  <1.67 –Metallic Bond –Metallic Bond – free moving electrons in “cloud” –Network Bond –Network Bond – extended covalent bonds

Created by C. Ippolito Dec 2006 Ionic Bonds electrons transferred from one element to another to complete their valence shells –Cation –Cation – positive ion loses electron – metal –Anion –Anion – negative ion gains electron – nonmetal Stable Octet –atoms gain stability by completing their valence shell –this is done when shell is filled with eight electrons Ionic Compound (salt) –formed from electrovalent attraction of cation for anion

Created by C. Ippolito Dec 2006 Lewis Dot Diagrams used to represent valence shells of atoms –kernel –kernel – nucleus and all inner energy levels represented by the element’s symbol –valence electrons represented by dots surrounding the symbol determined by the Group or Family in periodic table

Created by C. Ippolito Dec 2006 Dot Diagrams of Atoms Group 1 Group 2 Group 13 Group 14 Group 15 Group 16 Group 17 Group 18

Created by C. Ippolito Dec 2006 Dot Diagrams of Ions Group 1 Group 2 Group 13 Group 14 – no ion Group 15 Group 16 Group 17 Group 18 – no ion

Created by C. Ippolito Dec 2006 Covalent Bond atoms share electrons to complete valence shell –molecular substance – atoms held together by covalent bonds Nonpolar Covalent Bond –electrons are shared equally – electronegativity  <1.3 Polar Covalent Bond –electrons are shared unequally –electronegativity 1.3<  <1.67 DipoleDipole –molecule has a slightly positive side (δ + ) –molecule has a slightly negative side (δ - )

Created by C. Ippolito Dec 2006 Nonpolar Covalent Bond Representations Molecular Formula Dot Diagram Structural Formula

Created by C. Ippolito Dec 2006 Multiple Covalent Bonds Single Bond –1 electron pair shared Double Bond –2 electron pairs shared Triple Bond –3 electron pairs shared

Created by C. Ippolito Dec 2006 Hybridization rearrangement of electrons within the valence orbitals during chemical reactions –sp hybridization in carbon atom 1s 2 2s 2 2p 2 configuration is changed in bonding one 2s electron is “elevated” to “equalize” all 2 orbitals –1s 2 2sp x 1 sp y 1 sp z 1

Created by C. Ippolito Dec 2006 Resonance explains structures that experimentally do not conform to dot diagrams –electrons are seen to move from one bond to another bond and back again –they “resonate”resonate

Created by C. Ippolito Dec 2006 VSEPR Modeling Valence Shell Electron Pair Repulsion –used to explain shapes of molecules –bonded atom’s electron clouds balance forces of repulsion and attraction –this “spaces” them apart as far as they possible can be from each other

Created by C. Ippolito Dec 2006 Molecular Shapes Tetrahedral Shape –bond angle o –CH 4 Trigonal Pyramidal Shape –bond angle 107 o –NH 3

Created by C. Ippolito Dec 2006 Molecular Shapes Trigonal Planar Shape –bond angle 120 o –SO 3 Bent Shape –bond angle 105 o –H2O–H2O Linear Shape –bond angle 180 o

Created by C. Ippolito Dec 2006 Hydrogen Bonding special attraction between dipoles and compounds with hydrogen –hydrogen atom with (δ + ) slight positive charge forms weak bond with highly electronegative atoms in other molecules that have (δ - ) slight negative charge special properties of water –ice less dense than water –high heat of vaporization (cooling property) holds DNA molecule halves together allowing replication

Created by C. Ippolito Dec 2006 Metallic Bond metal atoms in solid and liquid state –“positive ions” in a “sea” of free flowing electrons

Created by C. Ippolito Dec 2006 Network Solids covalent crystals –atoms continuously bonded in a definite pattern –“one giant” molecule very hard very high melting points poor conductors

Created by C. Ippolito Dec 2006 Ionic Crystals ions packed in a regular pattern –each anion is surrounded by cations –each cation surrounded by anions high melting point nonconductive as solid conductive as liquid or dissolved –because “lattice” structure is broken