Chapter 8

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Chapter 8 Basic Concepts of Chemical Bonding

• Abegim Undieh

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8.1 Chemical Bonds, Lewis Symbols, and the
Octet Rule

• When atoms or ions are strongly attached to one
  another, there is a chemical bond between them
• There are THREE types of chemical bonds ionic,
  covalent, and metallic

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Lewis Symbols

• The electrons involved in chemical bonding are
  called valence electrons
• Valence electrons are in the outer shell of an
  atom
• Valence electrons are symbolized using Lewis
  Symbols
• The Lewis Symbol of an element shows the chemical
  symbol for the element and a dot for each valence
  electron

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Picture of a Lewis Symbol

• Lewis Dot Symbol for Se

http//www.cbu.edu/mcondren/Se-Lewis-dot-structur
e.jpg
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The Octet Rule

• All noble gases (besides He) have 8 valence
  electrons
• Noble gases have the most stable electron
  arrangements
• The Octet Rule Atoms tend to gain, lose, or
  share electrons until they are surrounded by
  eight valence electrons

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8.2 Ionic Bonding

• The bonding of a metal and a non-metal through
  the electrostatic attraction of opposite charges
  is called ionic bonding
• In an ionic bond, the metal transfers electrons
  to the nonmetal
• This occurs when there is a metal of low
  ionization energy and a nonmetal with a high
  electron affinity

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Formula showing a sample Ionic Bond

• Na Cl ? NaCl
• Na Cl ? Na Cl-

http//upload.wikimedia.org/wikipedia/commons/7/75
/Ionic_bonding_animation.gif
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Ionic Bonding (cont.)

• Ionic bonding is a very exothermic process
• The measure of the stability of an ionic bond is
  called the lattice energy
• Lattice energy the energy required to
  completely separate a mole of a solid ionic
  compound into gaseous ions

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Ionic Bonding (cont.)

• Formula for lattice energy
• E kQ1Q2
• D
• Lattice energy increases as the charges on the
  ions increase and as their radii decrease
• Ionic radii do not vary much over a wide range,
  so the magnitude of lattice energy depends mainly
  on the ionic charges

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8.3 Sizes of Ions

• Ionic size determines many properties of ionic
  solids, such as lattice energy, the way it packs
  in a solid, and the properties of the ions in
  solution
• The size of an ion depends on
• its nuclear charge
• the number of electrons it possesses
• the orbitals in which the valence electrons reside

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Sizes of Ions (cont)

• In positive ions, electrons from the outermost
  orbital have been removed
• This decreases the total electron-electron
  repulsion, allowing the atom to pull closer
  together
• Therefore, cations are smaller than their parent
  atoms, and conversely..
• Anions are larger than their parent atoms
• For ions of the same charge, size increases as
  you go down a group in the periodic table

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Sizes of Ions (cont)

• an isoelectronic series is a series of ions that
  have the same number of electrons
• Example
• O2-, F-, Na, Mg2, Al3 all have 10
  electrons arranged in a 1s22s22p6 configuration
  like Ne
• In an isoelectronic series, the radius of the ion
  decreases with increasing nuclear charge

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Ionic Radii
http//www.chem.umass.edu/botch/Chem111F04/Chapte
rs/Ch8/IonicRadii.jpg
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8.4 Covalent Bonding

• In covalent bonding, two nonmetals share
  electrons
• The atoms are held together by the electrostatic
  attraction between the two nuclei

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Multiple Bonds

• The sharing of a pair of electrons produces one
  covalent bond, called a single bond
• In some cases, molecules form multiple bonds in
  order to achieve an octet

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Multiple Bonds

• Two shared electrons produce a double bond
• Three shared electrons produce a triple bond
• the triple bond in N2 as shown by a
  Lewis structure
• As a rule, the distance between bonded atoms
  decreases as the number of shared electron pairs
  increases

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8.5 Bond Polarity and Electronegativity
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8.6 Drawing Lewis Structures
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8.7 Resonance Structures

• In some molecules, the arrangement of atoms
  cannot fully be described by a single Lewis
  structure
• This requires showing multiple equivalent Lewis
  structures, called resonance structures
• The actual structure of the molecule is a blend
  of its resonance structures

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Resonance Structures (cont.)

• Example
• In ozone, both of these resonance structures are
  equally as correct, and the molecule does not
  oscillate between the two

http//www.mikeblaber.org/oldwine/chm1045/notes/Bo
nding/Resonan/IMG00007.GIF
Remember to put brackets around each
resonance structure that you draw, with the
double arrow in-between
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Resonance in Benzene

• Benzene, C6H6, has two equivalent Lewis
  structures, each showing three C-C single bonds
  and three CC double bonds

http//wpcontent.answers.com/wikipedia/commons/thu
mb/7/79/Benz3.svg/300px-Benz3.svg.png
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Resonance in Benzene

• Benzene is often represented as such
• OR
• Millions of organic compounds have structures
  based on Benzene

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8.8 Exceptions to the Octet Rule

• Molecules with an odd number of electrons
• Molecules in which an atom has less than an octet
• Molecules in which an atom has more than an octet

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Odd Number of Electrons

• In some cases, complete pairing of electrons is
  impossible and an octet around each atom cannot
  be achieved
• Examples ClO2, NO, NO2

ClO2 Lewis Diagram
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Less than an Octet

• There are also cases in which an atom has less
  than 8 electrons
• This most frequently occurs in compounds
  containing boron or beryllium
• Example

http//www.up.ac.za/academic/chem/mol_geom/bf32.gi
f
BF3 Lewis structure, showing that B only has 6
electrons surrounding it.
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More than an Octet

• Many molecules/ions have more than an octet of
  electrons
• This only occurs for elements in period 3 and
  beyond

Diagram for PCl5, showing an expanded octet
around P.
http//www.wwnorton.com/college/chemistry/gilbert/
concepts/chapter7/pcl5.gif
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More than an Octet

• The larger the central atom, the larger the
  number of surrounding atoms it can hold
• Expanded shells occur most often when the central
  atom is bonded to the smallest and most
  electronegative atoms, such as F, Cl, O

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8.9 Strengths of Covalent Bonds

• Bond enthalpy the enthalpy change for the
  breaking of a particular bond into a mole of
  gaseous substance
• Bond enthalpy is always a positive quantity
• The strengths of covalent bonds increase with the
  number of shared electron pairs

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Strengths of Covalent Bonds

• The bond length between two bonded atoms is the
  distance between the two nuclei
• The average bond length between two atoms
  decreases as the number of bonds between them
  increases