Forming Chemical Bonds
Chemical Bonds Three basic types of bonds 2012 Pearson Education, Inc. Ionic Electrostatic attraction between ions. Covalent Sharing of electrons. Metallic Metal atoms bonded to several other atoms. Basic Concepts of Chemical Bonding
Types of Chemical Bonds Why do atoms form chemical bonds? so that the system can achieve the lowest possible potential energy Example covalent bonding in H 2 H H
Potential energy 0 Distance of separation between atoms
Ionic Bonding
Ionic compounds especially prevalent in compounds formed between group 1A and 2A elements with group 6A and 7A elements. between Elements with the biggest difference in electronegativity
Ionic and Molecular Compounds Ionic compounds are usually formed between metals and nonmetals. Molecular compounds are usually formed between two nonmetals.
Ions When electrons are removed from or added to a neutral atom or molecule, a charged particle called an ion is formed.
Positively charged ions are called cations Negatively charged ions are called anions
Atoms vs Ions Na: 11 protons; 11 electrons Cl: 17 protons; 17 electrons + Na + : 11 protons; 10 electrons Cl : 17 protons; 18 electrons
Lewis Dot Symbols A Lewis dot symbol consists of the symbol of an element and one dot for each valence electron in an atom of the element.
Lewis Dot Symbols 1A 2A 3A 4A 5A 6A 7A 8A H He Li Na K Rb Cs Be Mg Ca Sr Ba B Al Ga In Tl C Si Ge Sn Pb N P As Sb Bi O S Se Te Po F Cl Br I At Ne Ar Kr Xe Rn Fr Ra
Formation of LiF Li(s) + 1/2 F 2 (g) LiF(s).... Li. +. F : Li + : F : -.... 1s 2 2s 1 1s 2 2s 2 2p 5 1s 2 1s 2 2s 2 2p 6
Solid structure of LiF Crystal lattice the ions are packed together to maximize (+) (-) attractions and minimize (+) (+) and (-) (-) repulsions
Coulomb s Law Energy of electrostatic attraction is directly proportional to product of charge and inversely proportional to distance Coulombic = k attraction between two ions Q 1 Q 2 r k = (2.31 x 10-19 J nm) therefore, strong lattices are favored when the ions have a high charge to size ratio
Coulomb s Law Energy of electrostatic attraction is directly proportional to product of charge and inversely proportional to distance Coulombic = k Q 1 Q 2 attraction between r two ions highly charged small ion size therefore, strong lattices are favored when the ions have a high charge to size ratio
Coulomb s Law Example: sodium-chlorides energy of interaction is equal to Coulombic attraction between two ions (Na + )(Cl - ) = (2.31 x 10-19 J nm) ( ) r
Coulomb s Law Example: sodium-chlorides energy of interaction is equal to Coulombic attraction between two ions (+1)(-1) = (2.31 x 10-19 J nm) ( ) r
Coulomb s Law Example: sodium-chlorides energy of interaction is equal to Coulombic attraction between two ions = - 8.37 x 10-19 J The negative sign indicates an attractive force i.e. the ion pair has a lower potential energy (+1)(-1) = (2.31 x 10-19 J nm) ( ) 0.276 nm
Lattice Energy of Ionic Compounds
Energetics of Ionic Bonding As we saw in the last chapter, it takes 496 kj/mol to remove electrons from sodium. 2012 Pearson Education, Inc. Basic Concepts of Chemical Bonding
Energetics of Ionic Bonding We get 349 kj/mol back by giving electrons to chlorine. 2012 Pearson Education, Inc. Basic Concepts of Chemical Bonding
Energetics of Ionic Bonding Na +496 more energy required to remove the electron than is released 2012 Pearson Education, Inc. Basic Concepts of Chemical Bonding
Energetics of Ionic Bonding the numbers don t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic! 2012 Pearson Education, Inc. Basic Concepts of Chemical Bonding
Energetics of Ionic Bonding There must be a third piece to the puzzle. What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion. Na + Cl - 2012 Pearson Education, Inc. Basic Concepts of Chemical Bonding
Lattice Energy This third piece of the puzzle is the lattice energy: The energy required to completely separate a mole of a solid ionic compound into its gaseous ions. The energy associated with electrostatic interactions is governed by Coulomb s law: E el = κ Q 1 Q 2 d Basic Concepts of Chemical Bonding 2012 Pearson Education, Inc.
Lattice energy the energy released when an ionic solid forms from its ions negative sign (-)
Lattice Energy Lattice energy, then, increases with the charge on the ions. It also increases with decreasing size of ions. 2012 Pearson Education, Inc. Basic Concepts of Chemical Bonding
Lattice energies of some ionic compounds compound lattice energy (kj/mol compound lattice energy (kj/mol KI -632 KF -808 KBr -671 AgCl -910 RbF -774 NaF -910 NaI -681 LiF -1030 NaBr -732 SrCl2-2142 NaCl -769 MgO -3795 K + I - Mg 2+ O 2- smaller L.E. = larger L.E. = larger ions smaller ions
F(g) Li + (g) +75 kj 333 kj F (g) +520 kj 1012 kj +155 kj Li(g) Li(s) + 1/2F 2 (g) 594 kj LiF(s)
Formation of ionic compounds requires the lattice energy to be sufficiently large to overcome ionization energy of the element that forms the cation. balance between energy input (ionization energies) and stability gained from formation of the solid
Formation of ionic compounds The main impetus for the formation of an ionic compound rather than a covalent compound results from the strong mutual attraction among the ions
Example bonding in MgCl 2 is ionic; bonds in AlCl 3 are polar covalent Ionization energies ( I.E.) Mg(g) Mg 2+ (g) + 2e - ΔH = I.E. 1 +I.E. 2 = 753 +1435 = +2180 kj/mol Al (g) Al 3+ (g) + 3e - ΔH = I.E. 1 + I.E. 2 + I.E. 3 = 580 +1815 + 2740 = +5135 kj/mol
Example bonds in AlCl 3 are polar covalent energy input (ionization energies) out weighs stability gained from formation of an ionic solid Al (g) Al 3+ (g) + 3e - ΔH = I.E. 1 + I.E. 2 + I.E. 3 = 580 +1815 + 2740 = +5135 kj/mol
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Ionic compounds Any compound when melted that conducts electricity is considered ionic Example : NaCl (s) NaCl ( l) NaCl ( s) is made up of Na + and Cl - ions