CHEM 101 1st Edition Lecture 21Outline of Last Lecture I. Ionization EnergyII. Electron Attachment EnthalpyIII. Trends in Ion SizesIV. Chemical Bonds: Ionic BondsV. Chemical Bonds: Covalent BondsVI. Lewis StructureVII. Atom Formal ChargesOutline of Current Lecture I. Covalent Bonds and Lewis StructuresII. Atom Formal ChargesIII. Bonding and Molecular StructureIV. Molecular ShapesV. Bond Polarity and ElectronegativityVI. Formal Charge and ElectronegativityCurrent Lecture- Covalent Bonds and Lewis Structureso Molecules or ions with the same number of valence electrons and similar Lewis structures are isoelectronic- Atom Formal Chargeso 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 equallyo It’s calculated based on the Lewis structure of the molecule or ion:Formal Charge = Number of Valence Electrons – [Lone Pair Electrons + ½ Bonding Electrons]o Ex: Calculate formal charges of chlorateClO3^(-) valence electrons: :O: Cl: 7 l 3xO: 18:O – CL – O: Charge: -1 1 [26 electrons (13 pairs)]Formal charge: NVE –[LPE + ½ BE] O: 6-[6 + ½ (2)] = -1Cl: 7- [2 + ½ (6)] = 2 3(-1) + 2= -1 ANSWER- Resonance Structureso Resonance structures are used to represent bonds in a molecule or ion when a single Lewis structure fails to describe accurately the actual electronic structureo Resonance structures have identical patterns of bonding and equal energyo The actual structure is a hybrid of the equivalent resonance structure Ex: Ozone (O3); an allotrope of oxygen:O – O (double bond) O: :O (double bond) O – O:O-O single bond: 132 pmO (double bond) O double bond: 121 pmActual bond length: 128 pm Ex: Benzene (C6H6); a hydrocarbon Ex: Calculate formal charges of the nitrate ion and determine the resonance structures :O: :O: :O: ll l l :O - N - O: :O - N (double bond) O: :O (double bond) N - O: Formal Charge: N: 6-[6 + ½ (2)] = -1O: 5-[0 + ½ (8)] = 16-[4 + ½ (4)] = 02(-1) + 0 + 1 = -1Per O: 2(-1) + 0 = -2/3 - Bonding and Molecular Structureo Ex: Sulfite (SO3^(2-)) :O: :O: l l :O – S – O: :O (double bond) S – O: - Exceptions to the Octet Ruleo Ex: Compounds in which an atom has more than 8 valence electronso Ex: Compounds in which an atom has less than 8 valence electronso Ex: Molecules with an odd number of electrons- Molecular Shapeso Lewis structures tell us how atoms are connected in a moleculeo The 3D structure of a molecule is, however, determined by bond angelso 3D molecular structures can be predicted by the Valence Shell Electron-Pair Repulsion (VSEPR) model: Bonds (electron pairs) and electron lone pairs in the valence shell of an element repel each other and seek to be as far apart as possible Ex: The Lewis structure of methane suggests that the H – C – H bond angles are 90°. The molecule would be flat. H H l lH – C – H but in reality… H---C – H l l H H- Bond Polarity and Electronegativityo In covalent bonds of elements the electron bonds are shared equallyo If different atoms are combined with each other and form covalent bonds, the binding electron pairs can be unequally sharedo They can for polar covalent bondso The degree of polarity of a covalent bond depends on the difference of the electronegativity’s of the atoms which form the bondso The electronegativity (x) is the ability of an atom to attract electrons to itself.o Remember: Electronegativity increases from the bottom of the periodic table to the top and from left to righto Typical ΔX value ranges: ΔX=0: totally covalent bond. There is equal sharing of electrons ΔX= 0.1 - 0.4: weakly-polar covalent bond ΔX= 0.4 – 1.7: polar covalent bond ΔX= 1.7: ionic bond. The electrons of the electropositive atom is transferred to the electronegative oneEx: BF3 :F: :O: l ll :F – B – F: :F - B – F: Formal Charge F: 7-[6 + ½ (2)] = 0Formal Charge B: 3-[0 + ½ (8)] = -1Formal Charge : 3-[0 + ½ (6)]= 0- Formal Charge and Electronegativityo Electro neutrality Principle: Electrons in a molecule will be distributed in such a way that the chargesof all atoms are as close to zero as possible If a negative charge is present, it should…o Dipole moment: mu = q d d Partical charges in a molecule are equal to +q and –q These charges are separated by d- Bond Order and Bond Lengtho The bond order is the number of bonding electron pairs shared by two atoms in a moleculeBond order = number of shared pairs in all X-Y bonds number of X-Y links in the
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