Chemical_Bonding_-_Lecture_1

Chemical_Bonding_-_Lecture_1 - Chapter Chapter 8 Chemical...

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Unformatted text preview: Chapter Chapter 8 Chemical Bonding Atomic Atomic properties & chemical bonds Types Types of Bonding: Metal nonMetal to non-metal (Ionic bonding) NonNon-metal to non-metal (Covalent bonding) nonMetal Metal to metal (Metallic bonding) The The Ionic Bond The Ionic Bonding Model Lattice Energy Lattice energy (∆H0Lattice ) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions. Electrostatic energy ∝ ∆H0 Lattice ∝ Cation charge x Anion charge rcation + ranion Therefore, Lattice energy (∆H0Lattice ) increases as Charge of the cation and anion increases and/or as radii of cation and anion decreases. The Ionic Bonding Model Lattice Energy Some trends in Lattice energies: The The Valance Electrons When When atoms interact to form chemical bonds, only the outer (valance) electrons take part. Therefore, (valance) we need a tool for keeping track of valence electrons, e.g., The Lewis dot symbol .. Na . + :Cl : . NaCl 1 v. e 7 v. e’s When When these two elements combine to form a compound 2 Na (s) + Cl2 (g) → NaCl (s) NaCl [Ne]3s1 [Ne]3s23p7 [Ne]3s What’s What’s Happening? NaCl Na . + :Cl : .. . eNa .(g) → Na+ (g) + e- (ionizes, loses e-) an an electron configuration of [Ne] .. :Cl : . (g) (g) + e- → Cl- (g) an an electron configuration of [Ar] [Ar] In In the crystal lattice, we have Na+ and Cl- ions; strong electrostatic attractions between the positive ions and negative cations keep the NaCl unit together. The The NaCl Crystal The The Covalent Bond As two hydrogen atoms approach each other, constructive interference between the two valence orbitals leads to a new wave, a new orbital containing both electrons. This new orbital is called a covalent bond. The The Covalent Bonding Model Covalent Covalent Bond Formation The Bond length • Atoms in bonds vibrate back and forth. • The bond acts like a spring. The average distance between the atoms is the bond length. The The Covalent Bonding Model Properties Properties of Covalent Bonds Covalent Covalent bond formed balances attractive attractive and repulsive forces between between bonded atoms Bond Bond energy (BE): energy required to overcome net attraction between bonded atoms (break bond). The The Covalent Bonding Model Bond Bond length Relates Relates to radii of bonded atoms Linus Linus Pauling 1901– 1901–94 American chemist One One of the few recipients of of two Nobel prizes. His paper “The Nature of the Chemical Bond and the Structure of Molecules and Crystals” won him the 1954 Nobel Prize in chemistry. Won the peace prize in 1962. Electronegativity Electronegativity Pauling Scale: relative relative attraction of an atom for electrons, electrons, its own and those of other atoms atoms same same trends as ionization energy, increases from lower left corner to the upper right corner Electronegativity Electronegativity Pauling reasoned that the dissociation energy of a purely covalent bond A-B {D(A-B)}should be the mean mean of the dissociation energies for the homonuclear bonds A-A{D(A-A)}, and B-B {D(B-B)}: D(A-B),theory = ½ (D(A-A) + D(B-B)) Electronegativity Electronegativity Any additional energy must be caused by electrostatic attraction between A and B (attributed to ionic character in a bond). If ∆’(A-B) is is the ionic resonance energy in kJ/mol (∆(A-B) is in eV) then: ∆’(A-B) = D(A-B),experimental - D(A-B),theory Electronegativity The ionic character, ∆’, must be related to the difference in the electronegativities of A and B. Pauling calculated this difference as follows: XA – XB = 0.102 (∆’(A-B) Electronegativities of A & B ½ ) Electronegativity Electronegativity XA – XB < 0.5 0.5< XA – XB < 2 XA – X B > 2 Slightly polar Polar Ionic Bonding Between Different Atoms Electronegativity Electronegativity and Bond Polarity POLAR POLAR covalent bonds are formed when atoms of different EN share e’s. Eg. Eg. HF, polarity designated by: electron rich region δ+ δ- electron poor region H F or H F H F +δ -δ F On H Off - + - + Dipole Dipole Moments and Ionic Character It is useful to have some measure of ionic character when considering a series of molecules such as H2 (covalent) HF (polar covalent) K+F- (ionic) The dipole The dipole moment of a molecule (µ) can be determined by by spectroscopic measurements. For a diatomic molecule AX: +Q -Q A R X µ = QR (R = distance in m) Dipole Dipole Moments and Ionic Character Character +Q -Q A R X µ = QR the the SI unit for µ is the Coulomb meter (Cm). the Debye the Debye is commonly used: 1 D = 3.336 x 10-30 Cm. Cm. Dipole Moments Example Consider Consider the polar covalent molecule HF. If HF were HF HF totally ionic (H+F-), we would expect: (H µ (Calc) = ( 1 electronic charge ) ( 1 HF distance) = (1.60 x 10-19 C) (9.17 x 10-11) = 1.47 x 10-29 Cm = 4.40 D However the measured µ for HF is only 1.83 D !! Using this result we can express the percentage ionic character in HF as: δ+ δ- δ = 1. 83 = 0.42 (42%) H-F 4. 40 ...
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This note was uploaded on 02/07/2011 for the course CHEM 202 taught by Professor Mazenelghoul during the Summer '07 term at American University of Beirut.

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