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# EN by jizhen1947

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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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