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Unformatted text preview: 11/29/2010 1 Bonding and Molecular Structure: Fundamental Concepts Valence Electrons Electrons in an atom are divided into two groups: Valence (involved in reactions) and Core Main Group Elements (A groups Main Group Elements (A groups ): Valence electrons = Outer s and p electrons The number of valence electrons = group number Transition Elements Transition Elements : Valence electrons = ns and (n-1)d orbitals (for highest n number in the atom) Lewis Electron Dot Symbols: Represent the valence electrons in an atom Useful for examining bonding Four pairs of electrons around the atomic symbol constitute an octet (8 valence electrons = filled s and p orbitals) Stable configuration associated with noble gases (He has 2 electrons due to no 1p sublevel) Chemical Bond Formation Chemical Bond Formation Ionic and Covalent Ionic and Covalent Ionic: Ionic: When one or more valence electrons are transferred from one atom to another creating positive and negative ions to form noble gas configuration. The formation of an ionic compound can be broken down into a multi- step process involving some or all of the following:- H o f of the ionic substance- Enthalpy of sublimation (solid to gas) (from enthalpy of formation)- Dissociation of molecules into atoms (bond dissociation energy) (from enthalpy of formation)- Ionization of atoms to cations (ionization energy)- Ionization of atoms to anions (electron affinity)- Assembly of ions into a crystal (lattice energy) Can you identify which processes are endothermic and which are exothermic? The energy associated with the formation of the ion pair is given by (X + (g) + Y- (g) XY (g) ) E ion pair = C(N)[(n + e)(n- e) / d] (in kJ/mol) Where n is the number of positive and negative charges on an ion, e is the charge on an electron, C is a constant, d is the distance between the ion centers and N is Avogadros number. When the number of ions surrounding other ions in the crystal are taken into account and the fact that ionic crystals typically exist as solids, the lattice energy of the ionic compound can be determined. For NaCl the ion pair energy is -498kJ/mol and the lattice energy is -787kJ/mol 11/29/2010 2 Determining the Enthalpy of Formation of Sodium Chloride Using the Born-Haber Cycle Standard enthalpy of formation Lattice Energy Why does the lattice energy become more negative as you go from K to Li and from I to F? Think of ion sizes and inter-atomic distances. The steps of the Born-Haber cycle can be used to make predictions as to why compounds such as NaCl 2 and NaNe do not exist. If the overall process isnt exothermic (negative enthalpy), the reaction is not likely to happen. Example: Calculate the molar enthalpy of formation, H o f , of solid sodium iodide using the Born-Haber cycle. The required data can be found in Appendices F (A-19) and L (A-27) and in Table 9.3 (page 379) Answer: H o f for Na (g) = +107.3kJ/mol (From solid sodium) H o f for I (g) = +106.8kJ/mol (From solid iodine)...
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