Chapter 1 — IntroductionCHEM 101CHEM 101Fall 2013Fall 2013Dr. Michael Stollenz8. Bonding and Molecular Structure2•A chemical bond between two atoms is an arrangement of valence electrons resulting in a net attractive force between these two atoms.•In this chapter, we distinguish between ionic bonds and covalent bonds.Chemical Bonds: Ionic BondsChemical Bonds: Ionic Bonds8. Bonding and Molecular Structure8. Bonding and Molecular Structure•Ionic bonds result from the transfer of an electron (or electrons) from one atom to another. •The transfer results in each attaining an octet or noble gas electron configuration.Chemical Bonds: Ionic BondsChemical Bonds: Ionic Bonds8. Bonding and Molecular Structure8. Bonding and Molecular StructureNa + Cl3s13s23p5→→→→ Na+Cl−−−−e–2s22p6 = [Ne] 3s23p6 = [Ar]•If one or more electron pairs are shared between two atoms, then the chemical bond is covalent.•Example: The electron pair bond between the two atoms of an H2 molecule is represented by a pair of dots or a line (Lewis structure).Chemical Bonds: Covalent BondsChemical Bonds: Covalent Bonds8. Bonding and Molecular Structure8. Bonding and Molecular StructureH–HHH-atom: H2molecule:HH►C1Covalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures8. Bonding and Molecular Structure8. Bonding and Molecular Structure•Electrons are distinguished between core and valence electrons.•Example: B 1s22s22p1•Core = [He] , valence = 2s22p1• Example: Br [Ar] 3d104s24p5• Core = [Ar] 3d10, valence = 4s24p58. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis StructuresChapter 1 — Introduction•How to determine a Lewis structure of molecules and polyatomic ions in up to 5 steps:1) Determination of the central atom: This is usually the atom with the lowest electron affinity.2) Determine the total number of valence electrons in the molecule or ion. 8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures2) Determination of the total number of valence electrons in the molecule or ion. ►In a neutral molecule, this number will be the sum of the valence electrons for each atom.►For an anion, we add the number of electrons equal to the negative charge.►For a cation, we subtract the number of electrons equal to the positive charge.8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures3) We place one pair of electrons between each pair of bonded atoms to form single bonds. 4) We use the remaining pairs to form more multiple bonds or lone pairs to complete the electron octet of each atom.8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures4) We use the remaining pairs to form more multiple bonds or lone pairs to complete the electron octet of each atom.5) If the central atom has less than eight electrons at this point, we change one or more of the lone pairs on the terminal atoms into a bonding pair between the central and terminal atom to form a multiple bond.Hydrogen atoms will never have lone pairs or 8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures5) If the central atom has less than eight electrons at this point, we change one or more of the lone pairs on the terminal atoms into a bonding pair between the central and terminal atom to form a multiple bond.•Hydrogen atoms will never have lone pairs or multiple bonds!8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures•Example: Hydrogen Compounds8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis StructuresChapter 1 — Introduction•Example: Determine the Lewis structure of ammonia1) Determination of the central atom: The central atom is usually the atom of lowest affinity for electrons and never hydrogen.Therefore, N is the central atom.8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures•Example: Determine the Lewis structure of ammonia2) Determination of the total number of valence electrons in the molecule.H = 1 and N = 5Total = (3 × 1) + 5 = 8 electrons / 4 pairs8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures•Example: Determine the Lewis structure of ammonia3) We place one pair of electrons between each pair of bonded atoms to form single bonds. 8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis StructuresHHHN•Example: Determine the Lewis structure of ammonia4) We use the remaining pairs to form more multiple bonds or lone pairs to complete the electron octet of each atom.►3 bond pairs and 1 lone pair8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis StructuresH••HHN►C2•Molecules or ions with the same number of valence electrons and similar Lewis structures are isoelectronic:8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis Structures•Molecules or ions with the same number of valence electrons and similar Lewis structures are isoelectronic:8. Bonding and Molecular Structure8. Bonding and Molecular StructureCovalent Bonds and Lewis StructuresCovalent Bonds and Lewis StructuresChapter 1 — Introduction•The formal charge is the charge that would reside on an atom in a molecule or polyatomic ion if we assume that all bonding electrons are shared equally. •It is calculated based on the Lewis structure of the molecule or ion:Formal Charge = NVE − [LPE + ½ BE] 8. Bonding and Molecular Structure8. Bonding and Molecular StructureAtom Formal Charges Atom Formal Charges •It is calculated based on the Lewis structure of the molecule or ion:Formal Charge = NVE − [LPE + ½ BE] •NVE = the number of valence electrons in the uncombined atom (which is equal to its group number in the periodic table).•LPE