3Lewis SymbolsLewis symbols show the valence electrons as dots arranged around the atomic symbol.hydrogen:sodium:chlorine:HNaCl· ·
4The Octet RuleAtoms tend to gain, lose, or share electrons until they have eight valence electrons.
5Ionic and Molecular Compounds Formation of sodium chloride:Cl· ·Na+Cl· ·Na+Formation of hydrogen chloride:Cl· ·Cl· ·HH+A metal and a nonmetal transfer electrons to form an ionic compound. Two nonmetals share electrons to form a molecular compound.
6Ionic CompoundsIonic compounds consist of a lattice of positive and negative ions.NaCl:
7Ionic BondsAn ionic bond is simply the electrostatic attraction between opposite charges.Ions with charges Q1 and Q2:Q2Q1ddQE21The potential energy is given by:
9The Lattice EnergyThe lattice energy is the enthalpy change required to separate one mole of an ionic compound into its ions:NaCl(s) ® Na+(g) + Cl-(g) DH = 788 kJLattice energy increases with increasing ionic charges and decreasing ionic sizes.KCl(s) ® K+(g) + Cl-(g) DH = 701 kJMgCl2(s) ® Mg+2(g) + 2 Cl(g) DH = 3795 kJ
10The Born-Haber CycleThe Lattice energy can be calculated with Hess’s law and the following steps:Na+(g) + e- + Cl(g)HI1(Na)E(Cl)Na+(g) + Cl-(g)LatticeenergyNa(g) + Cl(g)DHf°(Cl,g)Na(g) + ½Cl2(g)DHf°(Na,g)Na(s) + ½Cl2(g)-DHf°(NaCl,s)NaCl(s)
11Estimating Lattice Energy Arrange with increasing lattice energy:KClNaFMgOKBrNaCldQE21701 kJ910 kJK+Cl3795 kJdK+Br671 kJ788 kJd
12Molecular Compounds The simplest molecule is H2: Increased electron density draws nuclei togetherThe pair of shared electrons constitutes a covalent bond.
13Lewis StructuresCovalent bonding in a molecule is repre-sented by a Lewis structure.A valid Lewis structure should have an octet for each atom except hydrogen.H+H2:® H Hor H HCl· ·+® Cl Cl· ·BondingelectronsCl2:or Cl Cl· ·Nonbonding electrons
14Lewis Structures Draw Lewis structures for: or H F HF: H F H O H · ·or H F· ·HF:H F· ·· ·· ·H O H· ·or H O H· ·H2O:H N HH· ·or H N HH· ·NH3:H C HH· ·or H C HHCH4:
15Double and Triple Bonds Atoms can share four electrons to form a double bond or six electrons to form a triple bond.=O O· ·O2:· ·N NN2:The number of electron pairs is thebond order.
16ElectronegativityPolarity refers to a separation of positive and negative charge. In a nonpolar bond, the bonding electrons are shared equally:H2,Cl2:In a polar bond, electrons are shared unequally because of the difference in Zeff.HCl:
17ElectronegativityElectronegativity refers to the ability of an atom in a molecule to attract shared electrons.The Pauling scale of electro negativity:
18Bond Polarity H Cl A polar bond can be pictured using partial charges: +H Cl = 0.92.13.0ElectronegativityDifferenceBond TypeNonpolarPolar2.0 Ionic
19Drawing Lewis Structures Sum the valence electrons from all atoms. Add one for each negative charge and subtract one for each positive charge.Draw a skeleton structure with atoms attached by single bonds.Complete the octets of atoms bound to the central atom.Place extra electrons on the central atom.If the central atom doesn’t have an octet, try forming multiple bonds.
20Drawing Lewis Structures · ·Cl C ClO· ·· ·COCl224 ve’s· ·· ·· ·· ·· ·· ·· ·· ·HOCl14 ve’sH O Cl· ·· ·· ·· ·O Cl OO· ·· ·· ·· ·ClO326 ve’s· ·· ·· ·· ·· ·H C O HH· ·CH3OH14 ve’s· ·