Chapter 9 Chemical Bonding - sciences.ucf.edu

1 Chapter 9 Chemical BondingChapter Goals Understand the difference between ionic and covalent bonds. Draw Lewis electron dot structures for small molecules and ions. Use the valence shell electron-pair repulsion theory (VSEPR) to predict the shapes of simple molecules and ions and to understand the structure of more complex molecules. Use electronegativity to predict the charge distribution in molecules and ions and to define the polarity of bonds. Predict the polarity of molecules Understand the properties of covalent bonds and their influence on molecular : Valence Electrons * Attractive forces that hold atoms together incompounds are called Chemical bonds.

2 * The electrons involved in Bonding are usually those in the outermost (valence) shell. The (inner) core electrons are not involved in Chemical electrons:For the main groups (representative) elements they are the outer most s and pelectrons. The # of valence electrons is equal to the group number.* For the transition elements they are the nsand (n 1) Dot Symbols for Atoms* Lewis dot formulas or Lewis dot symbols are a convenient bookkeeping method for tracking valence electrons are those electrons that are transferred or involved in Chemical Bonding . They are chemically considered to have 4 sides. Two dots per Be B C N O F.

3 Lewis Dot Symbols for AtomsElements that are in the same periodic group have the same Lewis dot & & & of BondsWhen a Chemical reaction occurs, the valence electrons of the atoms are reorganized so that net attractive forces Chemical bonds occur between bonds are classified into two types:Ionic bondingresults from electrostatic attractions between ions, which are formed by the transferof one or more electrons from one atom to bondingresults from sharingone or more electron pairs between two of Ionic and covalent CompoundsMelting point comparisonIonic compounds are usually solids with high melting pointsTypically > 400oCCovalent compounds are gases, liquids, or solids with low melting pointsTypically < 300oCSolubility in polar solvents (such as water)

4 Ionic compounds are generally solubleCovalent compounds are generally insolubleComparison of Ionic and covalent CompoundsSolubility in nonpolar solventsIonic compounds are generally insolubleCovalent compounds are generally solubleConductivity in molten solids and liquidsIonic compounds generally conduct electricityThey contain mobile ionsCovalent compounds generally do not conduct electricityComparison of Ionic and covalent CompoundsConductivity in aqueous solutionsIonic compounds generally conduct electricityThey contain mobile ionsCovalent compounds are poor conductors of electricityFormation of CompoundsIonic compounds are formed between elements with large differences in electronegativityOften a metal and a nonmetalCovalent compounds are formed between elements with similar electronegativitiesUsually two or more nonmetalsIonic BondingFormation of Ionic CompoundsAn ionis an atom or a group of atoms possessing a net electrical come in two basic types.

5 Positive (+) ions or cationsThese atoms have lost 1 or more ( ) ions or anionsThese atoms have gained 1 or more applies to binarycompoundsFormation of Ionic CompoundsIonic bonds are formed by the attraction of cations for anions usually to form , metals react with nonmetals to form ionic formation of NaCl is one example of an ionic compound of Ionic CompoundsReaction of Group IA Metals with Group VIIA Nonmetals gas solidye llo w silver F Li 2 nonmetalVIIA me ta lIA 2(g)(s) Formation of Ionic CompoundsReaction of Group IA Metals with Group VIIA Nonmetalspoint me lting C842an with gas solidsolid whiteyellow s ilve r LiF 2 F Li 2 nometalVIIA me ta lIA o(s)2(g)(s)

6 Formation of Ionic CompoundsReaction of Group IA Metals with Group VIIA NonmetalsThe underlying reason for the formation of LiFlies in the electron configurations of Li and loses one electronF gains one electronThese atoms form ions with these + same configuration as [He] 1s2F same configuration as [Ne]Formation of Ionic CompoundsReaction of Group IA Metals with Group VIIAN onmetals*We can also use Lewis dot formulas to represent the neutral atoms and the ions they form.*electrons are transferred until the metal loses all its valence electrons and the nonmetal has an octet(Octet Rule)Li + +F[ ].

7 Formation of Ionic CompoundsFor the reaction of IA metals with VIA nonmetals, a good example is the reaction of lithium with reaction equation is: -2s22(g)(s)OLi O2/1Li 2 Formation of Ionic CompoundsDraw the electronic configurations for Li, O, and their appropriate Li [He] Li+ O [He] O2- octet ruleDraw the Lewis dot formula representation of this reaction. Formation of Ionic CompoundsDraw the electronic representation of the Ca and N 4p4s4p Ca [Ar] Ca2+ 2s2p2s2p N [He] N3- Other IIA and VA elements behave similarly. Symbolically, this reaction can be represented as:3 M(s)+ 2 X(g) M32+ X23-M can be the IIA elements Mg to can be the VA elements N to +.

8 []Ca2+3 Formation of Ionic CompoundsSimple Binary Ionic Compounds TableReacting GroupsCompound General FormulaExampleIA + VIIAMXNaFIIA + VIIAMX2 BaCl2 IIIA + VIIAMX3 AlF3IA + VIAM2 XNa2 OIIA + VIAMXBaOIIIA + VIAM2X3Al2S3 Ion Attraction and Lattice EnergyIonic Bond electrostatic attraction between oppositely charged ions non-directional strength directly proportional to charges of ions and inversely proportional to distance between ion centersCoulombic Ion Attraction and Lattice EnergyE QcQadQc QaE = C is negativebecause of chargesd1E = ionic bond strength C (a constant) = 4 0Qc = charge on cation (is positive)Qa = charge on anion (is negative)d = distance between centers of ionsE QcQadNaCl Na+Cl CaCl2Ca2+Cl CaS Ca2+S 2Al2S3Al3+S2 Product of charges 1 6 2 4 Qcx Qais the heaviest factor.

9 Product is much more negative for Al2S3. The least negative product is NaCl sE QcQadF Na+Na+Na+Na+Cl Br I shortest ddlongest ddNaF has the most negative E and NaI the least nm nm nm nm E Energy ( Hlattice)the energy of formation of 1 mol of solid crystalline ionic compound when ions in the gas phase combine. It is always several compounds, the one with themost negative Hlatticeis said to have the highest lattice energy (absolute value), and highest TfusionIt is also said to be the most +(g)+ 2F (g) MgF2(s) Hlattice= 2910 kJ/molfor NaF, Hlattice= 911 kJ/molfor KF, Hlattice= 815 kJ/molWHY?

10 Charge of Mg2+(2) > Na+= K+, # of F (1) d, radius of Mg2+< Na+< K+(see chapt. 8)Lattice EnergySmall ions with high ionic charges have more negative lattice energies (higher lattice energies). Large ions with small ionic charges have less negative lattice this information, plus the periodicity rules fromChapter 8, to arrange these compounds in order ofdecreasing values of lattice energy KCl, Al2O3, CaOIonic radii: HlatticeK+Cl > Ca2+O2 > 2Al3+3O2 least negative most negative| Hlattice|(absolute values)Al2O3> CaO > KCl Lattice EnergyArrange the following compounds in order of decreasing valueof lattice energy (least negative tomost) CaO, MgO, SrO, and BaOThe anion (O2 ) is common to the four oxides.