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1.2 chemical bonding_1 - UNIVERSITY OF ALBERTA INTRODUCTORY...

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1 UNIVERSITY OF ALBERTA INTRODUCTORY UNIVERSITY CHEMISTRY I (CHEM 101) CHEMICAL BONDINGS I: BASIC CONCEPTS You are expected to know the nomenclature and formulas of compounds. Separate lecture notes on “Names and formulas of compounds” and related homework are uploaded. Please find time to review and get acquainted to it as this will be included in your midterm exam 2 and the final. Introduction Only a few elements exist as single atoms – the noble gases All other elements form bonds Why do atoms form bonds? Bond formation lowers the potential energy for both atoms - the driving force for bond formation Lewis Theory Chemical bonding involves the valence electrons of an atom. Therefore, the chemistry of an element is determined by the number of valence electrons the orbital(s) they occupy. For examples: Nitrogen N 1s 2 2s 2 2p 3 Phosphorus P 1s 2 2s 2 2p 6 3s 2 3p 3 o Both have five valence electrons and so they have similar chemical properties . o They both form similar type of bonds with other elements. When atoms form bonds, they acquire a filled outer shell of eight (or two for hydrogen) electrons . ( Octet Rule ) *Note: Most elements obey the Octet Rule, but there are exceptions especially those higher than Period 2. In order to achieve an octet electron configuration, atoms may: transfer (lose, gain) electrons – Ionic bond. share electrons – Covalent bond Lewis Electron-Dot Symbols and Lewis Structures The Lewis symbol is useful for counting the valence electrons in forming chemical bonds. For example: Group 1A • Na Group 2A • Mg • Valence electrons
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2 Group 3A • Al • Group 4A • Si • Ionic bond formation The Lewis structure can be used to show ionic bond formation: Li + F + F Li - Draw the Lewis Structure for the formation of Na 2 O Ionic bond is usually formed between metal (cation) and non-metal (anion) No single ionic pair exists in solid state. It forms a giant 3-dimensional crystal lattice Properties of ionic compounds: o High melting and boiling point solids (For NaCl, m.p. = 801 o C) o Their liquid or aqueous solution conducts electricity. Because ions are presen The strong electrostatic force between the ions A and B forms the ionic bond. Thus the Coulomb’s Law applies. charge A x charge B Electrostatic energy α Distance charge A x charge B Electrostatic energy (lattice energy) α cation radius + anion radius
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3 Which of the following pair of ionic compounds do you expect to have higher lattice energy (higher melting point)? NaCl or NaI NaCl or MgO MgO or BaO Covalent bond formation Covalent bond is formed between two non-metals. Each atom can achieve an octet structure by sharing their electrons. Cl Cl + Cl Cl H H + H H Covalent bond formation in H 2 Potential energy between two hydrogen atoms Fig 11-1 p 424 Covalent bond formation Each nucleus attracts other atom’s electron Atoms too far apart to attract each other Strong repulsion between nuclei increases the potential energy Bond energy Internuclear distance (bond length) For H 2 Bond energy = 432 kJ / mol Bond length = 74 pm
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