Types of Bonding : Ionic Compounds. Types of Bonding : Ionic Compounds

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1 Types of Bonding : Ionic Compounds Ionic bonding involves the complete TRANSFER of electrons from one atom to another. Usually observed when a metal bonds to a nonmetal Types of Bonding : Ionic Compounds Ionic bonding involves the complete TRANSFER of electrons from one atom to another. Usually observed when a metal bonds to a nonmetal. Metals have low ionization energy, making it relatively easy to remove electrons from them Nonmetals have high electron affinities, making it advantageous to add electrons to these atoms The oppositely charged ions are then attracted to each other, resulting in an ionic bond

2 Types of Bonding: Ionic Compounds Ionic compounds tend to be hard, rigid, and brittle, with high melting points. Types of Bonding: Ionic Compounds Ionic compounds tend to be hard, rigid, and brittle, with high melting points. Ionic compounds do not conduct electricity in the solid state. In the solid state, the ions are fixed in place in the lattice and do not move. Ionic compounds conduct electricity when melted or dissolved. In the liquid state or in solution, the ions are free to move and carry a current.

3 Types of Bonding: Covalent Compounds Covalent bonding involves the SHARING of electrons Usually observed when a nonmetal bonds to a nonmetal. Nonmetal atoms have relatively high ionization energies, so it is difficult to remove electrons from them When nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons Potential energy lowest when the electron is between the nuclei, holding the atoms together by attracting nuclei of both atoms Types of Bonding: Metals Metallic bonding involves electron POOLING Occurs when a metal bonds to another metal. The relatively low ionization energy of metals allows them to lose electrons easily Metallic bonding involves the metal atoms releasing their valence electrons to be shared as a pool by all the atoms/ions in the metal Organized metal cations islands in a sea of electrons Electrons delocalized throughout the metal structure Bonding results from attraction of cation for the delocalized electrons Explains many of the properties of metals

4 Types of Bonding: Metals Types of Atoms metals to nonmetals nonmetals to nonmetals metals to metals Type of Bond Ionic Covalent Metallic Bond Characteristic electrons transferred electrons shared electrons pooled

5 Lewis Dot Symbols: Elements and Ions Lewis dot structures of elements Use the symbol of element to represent nucleus and inner electrons Use a dot to represent each valence electron in the atom Na Ca In Sn N O F H Xe Lewis Dot Symbols: Octet Rule Atoms bond because it results in a more stable electron configuration. more stable = lower potential energy Atoms bond together by either transferring or sharing electrons Usually this results in all atoms obtaining an outer shell with eight electrons octet rule there are some exceptions to this rule!!

6 Lewis Dot Symbols: Octet Rule When atoms bond, they tend to gain, lose, or share electrons to result in eight valence electrons noble gas configuration - ns 2 np 6 Many exceptions H, Li, Be, B attain an electron configuration like He Helium = two valence electrons, a duet Lithium loses its one valence electron Hydrogen shares or gains one electron commonly loses its one electron to become H + Beryllium loses two electrons to become Be 2+ commonly shares its 2 electrons in covalent bonds, resulting in 4 valence electrons Boron loses three electrons to become B 3+ commonly shares its 3 electrons in covalent bonds, resulting in 6 valence electrons expanded octets for elements in Period 3 or below using empty valence d orbitals Basically, only C, N, O, F, and Ne MSUT follow the octet rule Lewis Dot Symbols: Elements and Ions Cations have Lewis symbols without valence electrons lost in the cation formation Anions have Lewis symbols with eight valence electrons electrons gained in the formation of the anion

7 Lewis Dot Symbols and Other Electron Stuff Electron configurations Li 1s 2 2s 1 + F 1s 2 2s 2 2p 5 Li + 1s 2 + F 1s 2 2s 2 2p 6 Orbital diagrams Li Li + + 1s 2s 2p 1s 2s 2p F F - 1s 2s 2p 1s 2s 2p Lewis electron-dot symbols Energetics of Ionic Bonding The ionization energy of the metal is endothermic Na (s) Na + (g) + 1 e H = +496 kj/mol The electron affinity of the nonmetal is exothermic ½ Cl 2(g) + 1 e Cl (g) H = 349 kj/mol Generally, the ionization energy of the metal is larger than the electron affinity of the nonmetal, therefore the formation of the ionic compound should be endothermic Yet the heat of formation of most ionic compounds is exothermic and generally large. Why? Na (s) + ½ Cl 2(g) NaCl (s) H f = 411 kj/mol

8 Crystal Lattice The extra energy that is released comes from the formation of a structure in which every cation is surrounded by anions, and vice versa This structure is called a crystal lattice held together by the electrostatic attraction of the cations for all the surrounding anions maximizes the attractions between cations and anions, leading to the most stable arrangement Electrostatic attraction is nondirectional!! no direct anion cation pair There is no ionic molecule the chemical formula is an empirical formula, simply giving the ratio of ions based on charge balance Formula Unit Crystal Lattice The extra stability that accompanies the formation of the crystal lattice is measured as the lattice energy Lattice energy is the energy released when the solid crystal forms from separate ions in the gas state always exothermic Lattice energy depends directly on size of charges and inversely on distance between ions

9 Lattice Equations Lattice equations are different than formation equations!! formation reaction: 1 mole of any product from elements in standard states lattice equations: 1 mole of ionic compound from ions in gaseous state Trends in Lattice Energy: Ion Size Lattice energy is a measure of the strength of an ionic bond. The force of attraction between charged particles is inversely proportional to the distance between them Larger ions mean the center of positive charge (nucleus of the cation) is farther away from the negative charge (electrons of the anion) larger ion = weaker attraction = smaller lattice energy electrostatic potential energy charge "A" charge "B" distance cation charge anion charge electrostatic potential energy H cation radius + anion radius lattice

10 Trends in Lattice Energy: Ion Size Trends in Lattice Energy: Ion Size

11 Trends in Lattice Energy: Ion Charge The force of attraction between oppositely charged particles is directly proportional to the product of the charges Larger charge means the ions are more strongly attracted larger charge = stronger attraction stronger attraction = larger lattice energy Lattice Energy = 910 kj/mol Of the two factors, ion charge is generally more important Lattice Energy = 3414 kj/mol Trends in Lattice Energy Order the following ionic compounds in order of increasing magnitude of lattice energy: CaO, KBr, KCl, SrO

12 Trends in Lattice Energy Order the following ionic compounds in order of increasing magnitude of lattice energy: MgS, NaBr, LiBr, SrS Lattice Energy: The Born Haber Cycle The Born Haber Cycle is a hypothetical series of reactions representing the formation of an ionic compound from its constituent elements Reactions are chosen so that the change in enthalpy of each is known except for the last one, which is the lattice energy Hess s Law returns!! H f(salt) = H f(metal atom, g) + H f(nonmetal atom, g) + H f(cation, g) + H f(anion, g) + H (crystal lattice) H (crystal lattice) = Lattice Energy for metal: atom (g) cation (g), H f = 1 st ionization energy for nonmetal: atom (g) anion (g), H f = electron affinity

13 Lattice Energy: The Born Haber Cycle heat of formation, Na (g) bond energy Cl Cl OR heat of formation Cl (g) 1 st ionization energy Na (g) electron affinity Cl (g) lattice energy NaCl (s) heat of formation NaCl (s) Lattice Energy: The Born Haber Cycle Na (s) Na (g) 108 kj ½ Cl 2(g) Cl (g) ½ (244 kj) Na (g) Na + (g) 496 kj Cl (g) Cl (g) 349 kj Na + (g) + Cl (g) NaCl (s) H (NaCl lattice ) Na (s) + ½ Cl 2(g) NaCl (s) 411 kj NaCl lattice = 411 kj 108 kj 122 kj 496 kj kj NaCl lattice = 788 kj

14 Lattice Energy: The Born Haber Cycle Calculate the lattice energy of MgCl 2 : formation reaction: H f lattice equation: H lattice

15 Lattice Energy: The Born Haber Cycle Calculate the lattice energy of MgCl 2 : H f = 641 kj/mol H lattice =? H atomization = kj/mol H IE 1 = +738 kj/mol H IE 2 = kj/mol H Cl Cl bond = +244 kj/mol H EA 1 = 698 kj/mol Mg 2+ (g) + 2 Cl (g) + 2 e heat of formation Mg (g) H = kj Mg 2+ (g) + Cl (g) + Cl (g) + 1 e heat of formation Cl (g) H = +122 kj Mg 2+ (g) + 2 Cl (g) 1 st ionization magnesium H = +738 kj 2 nd ionization magnesium H = kj 1 st electron affinity chlorine H = 349 kj Mg + (g) + 2 Cl (g) + 1 e Mg (g) + 2 Cl (g) Lattice energy MgCl 2(s) H = 2522 kj Mg (g) + ½ Cl 2(g) + Cl (g) heat of formation MgCl 2(s) H = 641 kj Mg (s) + Cl 2(g) Mg (g) + Cl 2(g) MgCl 2(s)

16 Types of Bonding: Covalent Compounds Covalent bonding involves the SHARING of electrons Usually observed when a nonmetal bonds to a nonmetal. Nonmetal atoms have relatively high ionization energies, so it is difficult to remove electrons from them When nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons Potential energy lowest when the electrons are between the nuclei, holding the atoms together by attracting nuclei of both atoms Covalent Bonds

17 Covalent Bonds Atoms share electrons to achieve a full outer level of electrons. The shared electrons are called a shared pair or bonding pair. The shared pair is represented as a pair of dots or a line: An outer-level electron pair that is not involved in bonding is called a lone pair, nonbonding pair, or unshared pair. Covalent Bonds The bond order is the number of electron pairs being shared by a given pair of atoms. A single bond consists of one bonding pair and has a bond order of 1 A double bond consists of two bonding pair and has a bond order of 2 A triple bond consists of three bonding pair and has a bond order of 3 The bond energy (BE) is the energy needed to overcome the attraction between the nuclei and the shared electrons. The stronger the bond the higher the bond energy. The bond length is the distance between the nuclei of the bonded atoms.

18 Covalent Bonds For a given pair of atoms, a higher bond order results in a shorter bond length and higher bond energy. Between any two atoms, more bonds = shorter bonds Between any two atoms, more bonds = larger bond energy Covalent Bonds For a given pair of atoms, a higher bond order results in a shorter bond length and higher bond energy. Between any two atoms, more bonds = shorter bonds Between any two atoms, more bonds = larger bond energy Bond length increases down a group in the periodic table and decreases across the period. Bond energy shows the opposite trend. Internuclear distance (bond length) Covalent radius Internuclear distance (bond length) Covalent radius 72 pm 133 pm

19 Covalent Bonds: H rxn The heat released or absorbed during a chemical change is due to differences between the bond energies of reactants and products. Bond breaking is endothermic, H (breaking) = Bond making is exothermic, H (making) = º rxn = Covalent Bonds: H rxn º rxn = Hº reactant bonds broken Hº product bonds formed 4(C H bond = 413 kj) 2(O=O bond = 498 kj) 2(C=O bond = 799 kj) 4(O H bond = 467 kj) º rxn =

20 Electronegativity and Polarity A covalent bond in which the shared electron pair is not shared equally, but remains closer to one atom than the other, is a polar covalent bond. If X and Y share bonding e - equally: If X and Y do NOT share bonding e - equally: Unequal sharing of bonding e - leads to polar covalent bonds Electronegativity and Polarity A covalent bond in which the shared electron pair is not shared equally, but remains closer to one atom than the other, is a polar covalent bond. The ability of an atom in a covalent bond to attract the BONDING electrons towards itself is called its electronegativity.

21 Electronegativity and Polarity A covalent bond in which the shared electron pair is not shared equally, but remains closer to one atom than the other, is a polar covalent bond. The ability of an atom in a covalent bond to attract the BONDING electrons towards itself is called its electronegativity. Unequal sharing of electrons causes the more electronegative atom of the bond to be partially negative and the less electronegative atom to be partially positive. Electronegativity and Polarity If electronegativity difference is: electronegativity < 0.5 the bond is considered to be pure covalent C C C S = = = = > EN 0.5 the bond is considered to be polar covalent = = = = 1.7 EN 2.0 the bond is considered to be ionic Al F Ca O Na Cl Rb N = = = = 2.2

22 Electronegativity and Polarity The lowercase Greek letter delta,, is used to indicate a polar bond. The MORE EN element has extra e, so it is negative and is indicated by the symbol. The LESS EN element is short of e, so it is positive and is indicated by the symbol +. H Cl Electronegativity and Polarity Give delta notation and polarity arrows for the following: