Chapter 8

1
Chapter 8 Basic Concepts of Chemical Bonding
2
Types of Bonds

• Chemical bond attractive force holding two or
  more atoms together.
• Covalent bond results from sharing electrons
  between the atoms. Usually found between
  nonmetals.
• Ionic bond results from the transfer of electrons
  from a metal to a nonmetal.
• Metallic bond attractive force holding pure
  metals together.

3
Lewis Symbols

• As a pictorial understanding of where the
  electrons are in an atom, we represent the
  electrons as dots around the symbol for the
  element.
• The number of electrons available for bonding are
  indicated by unpaired dots.
• These symbols are called Lewis symbols.
• We generally place the electrons one four sides
  of a square around the element symbol.

4
Lewis Symbols
Note Atoms in same group have same Lewis symbol
(Ex Li and Na) Add electrons to each side of
the square, then add another electron to
finally give 4 pairs (an octet) of electrons
5
The Octet Rule

• All noble gases except He has an s2p6
  configuration.
• Octet rule atoms tend to gain, lose, or share
  electrons until they are surrounded by 8 valence
  electrons (4 electron pairs).
• Caution there are many exceptions to the octet
  rule

6
Ionic Bonding
Consider the reaction between sodium and
chlorine Na(s) ½Cl2(g) ? NaCl(s) DHºf -410.9
kJ
The reaction is violently exothermic, so we infer
that the NaCl is more stable than its constituent
elements. Why? Na has lost an electron to
become Na (now has a Ne core configuration). Ch
lorine has gained the electron to become Cl- (now
has an Ar core configuration.
7
Ionic Bonding and the Structure of Sodium Chloride
NaCl forms a very regular structure in which each
Na ion is surrounded by 6 Cl- ions and each Cl-
ion is surrounded by six Na ions. Note that
the ions are packed as closely as possible. it
is not easy to find a molecular formula to
describe the ionic lattice.
8
Energetics of Ionic Bond Formation

• The formation of Na(g) and Cl-(g) involves
• breaking up the bond between the two chlorine
  atoms,
• forming Na(l) from Na(s), and
• taking an electron away from sodium
• adding an electron to a Cl atom
• Although the electron affinity of Cl is slightly
  negative , the rest are all very endothermic, so
  why is the formation of NaCl(s) exothermic?

9
Lattice energy

• The reaction NaCl(s) ? Na(g) Cl-(g) is
  endothermic (?H 788 kJ/mol).
• The reverse of this reaction, the formation of a
  crystal lattice from the ions in the gas, phase
  is exothermic
• Na(g) Cl-(g) ? NaCl(s) ?H -788 kJ/mol
• Lattice energy the energy required to completely
  separate an ionic solid into its gaseous ions.