Presentation on theme: "AP Notes Chapter 8 Bonding and Molecular Structure: Fundamental Concepts Valence e- and Bonding Covalent Ionic Resonance & Exceptions to Octet Rule Bond."— Presentation transcript:
1AP Notes Chapter 8Bonding and Molecular Structure: Fundamental ConceptsValence e- and BondingCovalentIonicResonance & Exceptions to Octet RuleBond Energy & LengthStructure, Shape & Polarity of Compounds
2What is a Bond? A force that holds atoms together. Why? We will look at it in terms of energy.Bond energy the energy required to break a bond.Why are compounds formed?Because it gives the system the lowest energy.
3Covalent compounds?The electrons in each atom are attracted to the nucleus of the other.The electrons repel each other,The nuclei repel each other.The reach a distance with the lowest possible energy.The distance between is the bond length.
23ElectronegativityThe ability of an electron to attract shared electrons to itself.Pauling methodImaginary molecule HXExpected H-X energy = H-H energy + X-X energyD = (H-X) actual - (H-X)expected
24Electronegativity D is known for almost every element Gives us relative electronegativities of all elements.Tends to increase left to right.decreases as you go down a group.Noble gases aren’t discussed.Difference in electronegativity between atoms tells us how polar.
25ElectronegativitydifferenceBondTypeZeroCovalentCovalent CharacterdecreasesIonic Character increasesPolarCovalentIntermediateIonicLarge
26Dipole MomentsA molecule with a center of negative charge and a center of positive charge is dipolar (two poles),or has a dipole moment.Center of charge doesn’t have to be on an atom.Will line up in the presence of an electric field.
67ResonanceSometimes there is more than one valid structure for an molecule or ion.NO3-Use double arrows to indicate it is the “average” of the structures.It doesn’t switch between them.NO2-Localized electron model is based on pairs of electrons, doesn’t deal with odd numbers.
69FORMAL CHARGEthe charge assigned to an atom in a molecule or polyatomic ionFC atom = Family# - [LPE + ½(BE)]Sum FC’s atoms = ion chargeCloser sum FC’s is to zero more stable
70Formal ChargeFor molecules and polyatomic ions that exceed the octet there are several different structures.Use charges on atoms to help decide which.Trying to use the oxidation numbers to put charges on atoms in molecules doesn’t work.
71Formal ChargeThe difference between the number of valence electrons on the free atom and that assigned in the molecule.We count half the electrons in each bond as “belonging” to the atom.SO4-2Molecules try to achieve as low a formal charge as possible.Negative formal charges should be on electronegative elements.
72Assignment of e- 1. Lone pairs belong entirely to atom in question 2. Shared e- are divided equally between the two sharing atoms
73The sum of the formal charges of all atoms in a species must equal the overall charge on the species.
74A useful equation (happy-have) / 2 = bonds POCl3 P is central atom SO S is central atomSO3-2 S is central atomPO P is central atomSCl2 S is central atom
75Exceptions to the octet BH3Be and B often do not achieve octetHave less than and octet, for electron deficient molecules.SF6Third row and larger elements can exceed the octetUse 3d orbitals?I3-
76Exceptions to the octet When we must exceed the octet, extra electrons go on central atom.ClF3XeO3ICl4-BeCl2
77If nonequivalent Lewis structures exist, the one(s) that best describe the bonding in the species has...
78FAVORED LEWIS STRUCTURES 1. formal charges closest to zero2. negative formal chargeis on the most electronegative atom
79EXAMPLESCarbon dioxideThiocyanate ionSulfate ion
102LE = Lattice EnergyWhere: k = proportionality constant dependent on structure of solid and on electron configuration of the ionsWhere: Q1 & Q2 = charges on the ionsWhere: r = the shortest distancebetween the centers of the cation and anion