Chapter 8 Concepts of Chemical Bonding

Chapter 8 Concepts of Chemical Bonding

Concepts of Chemical Bonding Chemical Bonding 2009, Prentice-Hall, Inc. Chemical Bonds Three basic types of bonds Ionic Electrostatic attraction between ions

Covalent Sharing of electrons Metallic Metal atoms bonded to several other atoms Chemical Bonding 2009, Prentice-Hall, Inc. Ionic Bonding Chemical Bonding

2009, Prentice-Hall, Inc. Energetics of Ionic Bonding As we saw in the last chapter, it takes 495 kJ/mol to remove electrons from sodium. Chemical Bonding 2009, Prentice-Hall, Inc. Energetics of Ionic Bonding

We get 349 kJ/mol back by giving electrons to chlorine. Chemical Bonding 2009, Prentice-Hall, Inc. Energetics of Ionic Bonding But these numbers dont explain why the reaction of sodium metal and

chlorine gas to form sodium chloride is so exothermic! Chemical Bonding 2009, Prentice-Hall, Inc. 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. Chemical Bonding 2009, Prentice-Hall, Inc. 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 Coulombs law: Q1Q2 Eel = d Chemical Bonding 2009, Prentice-Hall, Inc. Lattice Energy Lattice energy, then, increases with the charge on

the ions. It also increases with decreasing size of ions. Chemical Bonding 2009, Prentice-Hall, Inc. Energetics of Ionic Bonding By accounting for all three energies (ionization energy, electron affinity, and

lattice energy), we can get a good idea of the energetics involved in such a process. Chemical Bonding 2009, Prentice-Hall, Inc. Energetics of Ionic Bonding These phenomena also helps explain the octet rule.

Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies. Chemical Bonding 2009, Prentice-Hall, Inc. Covalent Bonding In covalent bonds atoms share electrons. There are several electrostatic interactions in these bonds:

Attractions between electrons and nuclei Repulsions between electrons Repulsions between nuclei Chemical Bonding 2009, Prentice-Hall, Inc. Polar Covalent Bonds Though atoms often form compounds by sharing electrons, the electrons are not always shared equally.

Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does. Therefore, the fluorine end of the molecule has more electron density than the hydrogen end. Chemical Bonding 2009, Prentice-Hall, Inc. Electronegativity Electronegativity is the ability of atoms in a

molecule to attract electrons to themselves. On the periodic chart, electronegativity increases as you go from left to right across a row. from the bottom to the top of a column. Chemical Bonding 2009, Prentice-Hall, Inc.

Polar Covalent Bonds When two atoms share electrons unequally, a bond dipole results. The dipole moment, , produced by two equal but opposite charges separated by a distance, r, is calculated: = Qr It is measured in debyes (D). Chemical Bonding 2009, Prentice-Hall, Inc.

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