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Chapter 9

Chapter 9.docx

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Department
Chemistry
Course
CHEM 1F92
Professor
Lydia W.L.Chen
Semester
Fall

Description
Chapter 9: Chemical Bonding 9.1 – Lewis Dot Symbols • Gilbert Lewis stated that atoms combine with one another to achieve a more stable electron configuration that is isoelectronic with that of a noble gas. • Lewis dot symbols consist of the symbol of an element surrounded by dots, representing the valence electrons in one atom of the element. • We cannot draw Lewis dot structures for the transition metals, lanthanides, and actinides since they have inner shells that are not filled. o For example, since the 4s orbital fills before the 3d orbitals do, the fourth shell begins to fill before the inner, third shell is filled. • The unpaired dots in the Lewis dot structures represent the maximum number of bonds that can form. 9.2 – The Ionic Bond • An ionic bond is the electrostatic force that holds ions together in an ionic compound. o Usually, they involve a Group 1A/2Ametal cation and an oxide or halide cation. • When LiF forms, Li atoms lose an electron (from outer 2s orbital) to F, forming Li . Consequently, the F atom, which just gained the electron (in 2p orbital), forms F . The attraction of these ions forms LiF. • In ionic compounds involving oxygen, the O sp2its into individual O atoms and then gains electrons to form anions. + • Two Li atom loses an electron from its 2s orbital to the 2p orbital of a single O atom. This forms 2 Li and 1 O .- 9.3 – Lattice Energy of Ionic Compounds • Ionization energy and electron affinity do NOT define the stability of an ionic compound since they depend on the compound being in gaseous state, and all ionic compounds at 1 atm and 25 ºC are solid. • Since many anions interact with a single cation (and vice versa) in the 3D network of an ionic compound, the stability of n ionic compound is related to many interactions, not just one interaction. • Lattice energy is the amount of energy required to separate one mole of a solid ionic compound into its gaseous ions. • Lattice energy determines the stability of the ionic compound (↑ Lattice Energy = ↑ Stability). • Lattice Energy can be calculated using Coulomb’s law OR the Born-Haber Cycle • Coulomb’s law can be used to calculate the potential energy (E) between two ions in an ionic compound. Since one ion is (+) and the other one is (-), E < 0 and the formation of the bond between 2 ions is an exothermic process. Hence, separating an ionic compound into ions (lattice energy) is endothermic (-E). • The Born-Haber cycle assumes that the formation of an ionic compound takes place in a series of steps, and each step is involves ionization energies/electron affinities/other properties. An example of the Born- Haber cycle for the formation of LiF is shown in the PPT slide. Sublimation is the conversion of a solid to a gas (endothermic process). Dissociation is the splitting of F 2 (g) into F (g) (endothermic process). It is 150.6 kJ/mol. Since only ½ mol of F 2re converted to F, the value is 0.5(150.6 kJ) = 75.3 kJ. The ionization of Li to Li + requires energy and is endothermic. The electron affinity of F is 328 kJ. However, since enthalpy is the negative of electron affinity, the energy change is – (electron affinity) or -328 kJ. The energy released when bonding the gaseous Li and F into solid LiF (∆H º5 can be calculate from the other values. Since this value is the energy to for LiF , (s) energy to break LiF , or t(s)lattice energy is -∆H º is 1015 kJ. • For a compound to be stable, the lattice energy must be greater than the sum of the ionization energies and electron affinities involved in the formation of ions. o The lattice energy of LiF is(s)eater than the net energy required to form Li (ionization energy) and the F (electron affinity). 9.4 – The Covalent Bond • The attraction of the nucleus to the electrons of both atoms involved in the covalent bond helps to maintain the bond. • The Lewis structures use dots for lone pairs. Bonded pairs can be shown using dots for the electrons, or lines representing the bonds. • Octet Rule states that all atoms other than H form covalent bonds so that they have 8 valence electrons. o The octet rule only works best for atoms of period 2 with 2s and 2p orbitals in their outer shell, containing a total of 8 valence electrons. • H needs to obtain a He valence electron configuration (2 valence electrons) when it forms a covalent bond. • Other than CO, there are no C compounds that have lone pairs on the C atom. • Bond length is the distance between the nuclei of both atoms involved in a covalent bond. o Triple bonds are shorter than double bonds and double bonds are shorter than single bonds. Triple bonds are more stable than double bonds and double bonds are more stable than single bonds. • The atoms in covalent compounds are held together by intramolecular forces and the individual molecular units in covalent compound are held together by intermolecular forces. o Since the intermolecular forces are much weaker than the intramolecular forces, covalent compounds exist as liquids, gases, or as low melting solids. • There are strong electrostatic forces between the ions in ionic compounds. Hence, ionic compounds are usually solids at room temperature and they have high melting and boiling points. They are also soluble and are strong electrolytes. • However, molecular compound do not usually dissolve in water.
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