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					    Covalent Bonding and Electronegativity
                Chapter 9…       (9.5)

    A covalent bond between two atoms: X–Y

•   In a covalent bond, electrons are shared; sharing is
    uneven, unless X and Y are the same.

•   The Electronegativity of an element is a measure of
    how much the element attracts the shared electrons in a
    chemical bond.

•   Electronegativity increases with increasing Zeff. It follows
    a similar trend to EA (both are involved with how badly an
    element wants to accept electrons), and has an inverse
    relationship to atomic size.

•   So electronegativity increases as you go up a group or
    left to right across a period.
Figure 9.16   The Pauling electronegativity (EN) scale.
Figure 9.17   Electronegativity and atomic size.
                  Electronegativity (cont.)

•   The greater the electronegativity difference (DEN) in a bond, the
    more uneven the electron sharing is, and the bond has greater
    ionic character (remember we said bonding models represent
    extremes; in fact there is a continuum of bonding types)

•   A bond in which electrons are shared unevenly is called a polar
    covalent bond. Polar bonds have both covalent and ionic
    character.

•   e.g. H–H          non polar

            ∂–
        O             polar – more electron density on O, partial negative
                 ∂+
    H        H                               charge (∂–), dipole moment.

    Na+Cl– – Extreme case of polar bond – Electron completely
                                     transferred from Na to Cl
                                    3.0




 Figure 9.18                        2.0
                              DEN

Boundary ranges for
classifying ionic character
of chemical bonds.



                                    0.0
 SAMPLE PROBLEM 9.3          Determining Bond Polarity from EN Values

PROBLEM:     (a) Use a polar arrow to indicate the polarity of each bond:
             N-H, F-N, I-Cl.
             (b) Rank the following bonds in order of increasing polarity:
             H-N, H-O, H-C.

PLAN:     (a) Use Figure 9.16(button at right) to find EN values; the
          arrow should point toward the negative end.
          (b) Polarity increases across a period.

SOLUTION: (a) The EN of N = 3.0, H = 2.1; F = 4.0; I = 2.5, Cl = 3.0

               N-H                     F-N                    I - Cl

             (b) The order of increasing EN is C < N < O; all have an EN
             larger than that of H.
                            H-C < H-N < H-O
            Compound Semiconductors
    •     The Group IVA (14) elements Si and Ge are
          semiconductors; can also make compound
          semiconductors, usually isoelectronic to Group IVA.

    e.g. GaAs, isoelectronic to Ge, has Zinc Blende structure,
        same as diamond structure of Ge, but Eg is different

        Substance             DEN             Band Gap
        Germanium              0              0.66 eV
        GaAs                  0.4             1.42 eV
        ZnSe                  0.8             2.70 eV
        CuBr                  0.9             2.90 eV
•       Band gap correlates with DEN. As DEN increases, bonding
        becomes more ionic, making the electrons harder to move, so
        band gap increases.

				
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posted:9/22/2011
language:English
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