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Basic Concepts of Chemical Bonding Chapter 8

What’s Ahead (p 297) Three types of bonds Lewis symbols Ionic bonding Octet rule Lattice energy in ionic compounds

Three Types of Chemical Bonds Ionic bond Transfer of electrons Between metal and nonmetal ions Covalent bond Sharing of pairs of electrons Between nonmetal atoms Metallic bond Bonding electrons relatively free to move

Valence electrons: outer shell electrons of an atom; valence electrons participate in chemical bonding. Group # of valence e- e- configuration 1A 1 ns1 2A 2 ns2 3A 3 ns2np1 4A 4 ns2np2 5A 5 ns2np3 6A 6 ns2np4 7A 7 ns2np5

Show only valence electrons! Table 8.1 Show only valence electrons!

Ionic Bonding Na (s) + ½ Cl2 (g) → NaCl (s) ΔHfo = −410.9 kJ

The Crystal Structure of Sodium Chloride Fig 8.3

The Ionic Bond - - - Li+ F Li + F 1s22s1 1s22s22p5 [He] 1s2 1s22s22p6 [Ne] Li Li+ + e- e- + F - LiF(s) F - Li+ + Li+ Octet Rule: Atoms tend to gain, lose, or share electrons until surrounded by 8 valence electrons

Electrostatic (Lattice) Energy Lattice energy (E): the energy required to completely separate one mole of a solid ionic compound into gaseous ions Q+ is the charge on the cation Eel = k Q+Q- d Q- is the charge on the anion d is the distance between the ions Eqn. [8.4] Lattice energy (E): increases as Q increases and/or increases as d decreases

Electron Configurations of Cations and Anions of Representative Elements Metal atoms lose electrons so that cation has a noble-gas outer electron configuration Nonmetal atoms gain electrons so that anion has a noble-gas outer electron configuration

Cations and Anions Of Representative Elements +1 +2 +3 -3 -2 -1

Electron Configurations of Cations of Transition Metals When a cation is formed from an atom of a transition metal, electrons are always removed first from the ns orbital and then from the (n – 1)d orbitals. “First in, first out” Fe: [Ar]4s23d6 Mn: [Ar]4s23d5 Fe2+: [Ar]4s03d6 or [Ar]3d6 Mn2+: [Ar]4s03d5 or [Ar]3d5 Fe3+: [Ar]4s03d5 or [Ar]3d5

Three Types of Chemical Bonds Ionic bond Transfer of electrons Between metal and nonmetal ions Metallic bond Bonding electrons relatively free to move Covalent bond Sharing of electrons Between nonmetal atoms

Cross Section of a Metallic Crystal Electron-Sea Model Metallic Crystals Lattice points occupied by metal atoms Held together by metallic bonds Soft to hard, low to high melting point Good conductors of heat and electricity Cross Section of a Metallic Crystal nucleus & core e- mobile “sea” of valence e-

Covalent Bonding Objectives: Characteristic properties Lewis structures Multiple bonds Bond polarity and electronegativity

Covalent Bonding Atoms share pairs of electrons Several electrostatic interactions in these bonds: Attractions between electrons and nuclei Repulsions between electrons Repulsions between nuclei

Change in electron density as two hydrogen atoms approach each other

Covalent Bond Why should two atoms share electrons? 7e- 7e- 8e- 8e- F + F Lewis structure of F2 lone pairs F single covalent bond single covalent bond F

Lewis structure of water single covalent bonds H + O + H O H or Double bond – two atoms share two pairs of electrons O C or O C double bonds Triple bond – two atoms share three pairs of electrons N or N triple bond