Electronegativity

Electronegativity is a measure of the attraction that an atom has for
electrons in a covalent bond.

Two scales of electronegativity are in common use: the Pauling scale
(proposed in 1932) and the Mulliken scale (proposed in 1934). There is also
a Allred-Rochow scale.

Pauling Scale

The Pauling Scale is a widely used, empirical scale of electronegativity
values, originally developed by Linus Pauling in 1932. On this scale, the
most electronegative element (fluorine) is given a value of 4.0, while the
least electronegative element (francium) is given a value of 0.7. The other
elements are assigned electronegativity values in between. The elements in
the second period of the Periodic Table usually have rounded values :

   * Li : 1.0
   * Be : 1.5
   * B : 2.0
   * C : 2.5
   * N : 3.0
   * O : 3.5
   * F : 4.0

Mulliken Scale

On the Mulliken scale, numbers are obtained by averaging ionization
potential and electron affinity. Consequently, the Mulliken
electronegativities are expressed directly in energy units, usually electron volts.

Electronegativity Trends

Each element has a characteristic electronegativity ranging from 0 to 4 on
the Pauling scale. A strongly electronegative element, like fluorine, has an
electronegativity of 4 while weakly electronegative elements, such as
lithium, have values close to 1. Electronegativity trends are to higher
values for elements in the top right of the periodic table. Bonds between
atoms with a large electronegativity difference (greater than or equal to
2.0 on the Pauling scale) are usually considered to be ionic, while values
between 2.0 and 0.4 are considered polar covalent. Values below 0.4 are
considered non-polar covalent bonds.

Electronegativity decreases down the periodic table and increases across, as
shown below. Additionally, atomic radius decreases across but ionization
energy increase.

        -> Atomic radius decreases -> Ionization energy increases ->
                       Electronegativity increases ->
 Group   1  2   3   4   5  6   7   8   9  10  11 12  13  14  15 16  17  18
 Period

   1     H                                                              He
        2.1                                                             Ê

   2    Li  Be                                        B  C   N   O   F  Ne
        1.0 1.5                                      2.0 2.53.0 3.5 4.0 Ê

   3    Na  Mg                                       Al  Si  P   S  Cl  Ar
        0.9 1.2                                      1.5 1.82.1 2.5 3.0 Ê

   4     K  Ca  Sc Ti   V  Cr  Mn Fe  Co  Ni  Cu Zn  Ga  Ge  As Se  Br  Kr
        0.8 1.01.3 1.5 1.6 1.61.5 1.8 1.9 1.81.9 1.6 1.6 1.82.0 2.4 2.8 Ê

   5    Rb  Sr  Y  Zr  Nb  Mo  Tc Ru  Rh  Pd  Ag Cd  In  Sn  Sb Te   I  Xe
        0.8 1.01.2 1.4 1.6 1.81.9 2.2 2.2 2.21.9 1.7 1.7 1.81.9 2.1 2.5 Ê

   6    Cs  Ba  Lu Hf  Ta  W   Re Os  Ir  Pt  Au Hg  Tl  Pb  Bi Po  At  Rn
        0.7 0.9    1.3 1.5 1.71.9 2.2 2.2 2.22.4 1.9 1.8 1.91.9 2.0 2.2 Ê

   7    Fr  Ra  Lr Rf  Db  Sg  Bh Hs  Mt  Ds Uuu Uub Uut UuqUup Uuh Uus Uuo
        0.7 0.9 Ê   Ê   Ê  Ê   Ê   Ê   Ê  Ê   Ê   Ê   Ê  Ê   Ê   Ê   Ê  Ê

         Periodic table of Electronegitivity using the Pauling scale