Electronegativity
Electronegativity is one of the fundamental concepts for the understanding of chemical bonding in chemistry. It provides guidelines for qualitatively understanding the difference between ionic bonding and covalent bonding, especially for bonding between different types of atoms. The type of bond formed is largely determined by the difference in electronegativity between the atoms involved. It also allows us to estimate the polarity of a chemical bond and, when taken together with molecular geometry, the polarity of a molecule. Most chemical reactions have to do with polarity in some way, so electronegativity lies at the heart of chemistry. Given its importance, however, it is a difficult topic to pin down and there have been several approaches to deriving electronegativities of the elements. The first definition was suggested by Pauling and has not been improved upon. In this definition electronegativity is defined as the ability of an atom or molecule to attract electrons in the context of a chemical bond. This definition was first suggested by Pauling (see below) and has not been improved upon.
The reverse of electronegativity, the ability of an atom to lose electrons, is known as electropositivity.
Two scales of electronegativity are in common use: the Pauling scale (proposed in 1932) and the Mulliken scale (proposed in 1934). Another proposal is the Allred-Rochow scale.
Pauling scale
The Pauling scale was devised in 1932 by Linus Pauling. On this scale, the most electronegative chemical element (fluorine) is given an electronegativity value of 3.98 (textbooks often state this value to be 4.0); the least electronegative element (francium) has a value of 0.7, and the remaining elements have values in between. On the Pauling scale, hydrogen is arbitrarily assigned a value of 2.1 or 2.2.
'ΔEN' is the difference in electronegativity between two atoms or elements. Bonds between atoms with a large electronegativity difference (greater than or equal to 1.7) are usually considered to be ionic, while values between 1.7 and 0.4 are considered polar covalent. Values below 0.4 are considered non-polar covalent bonds, and electronegativity differences of 0 indicate a completely non-polar covalent bond.
Mulliken scale
The Mulliken scale was proposed by Robert S. Mulliken in 1934. 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. The most strongly electronegative element, fluorine, has an electronegativity of 3.98 while weakly electronegative elements, such as lithium, have values close to 1. The least electronegative element is francium at 0.7. In general, the degree of electronegativity decreases down each group and increases across the periods, as shown below. Across a period, non-metals tend to gain electrons and metals tend to lose them due to the atom striving to achieve a stable octet. Down a group, the nuclear charge has less effect on the outermost shells. Therefore, the most electronegative atoms can be found in the upper, right hand side of the periodic table, and the least electronegative elements can be found at the bottom left. Consequently, in general, atomic radius decreases across the periodic table, but ionization energy increases.
| → 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 2.20 |
He | |||||||||||||||||
| 2 | Li 0.98 |
Be 1.57 |
B 2.04 |
C 2.55 |
N 3.04 |
O 3.44 |
F 3.98 |
Ne | |||||||||||
| 3 | Na 0.93 |
Mg 1.31 |
Al 1.61 |
Si 1.90 |
P 2.19 |
S 2.58 |
Cl 3.16 |
Ar | |||||||||||
| 4 | K 0.82 |
Ca 1.00 |
Sc 1.36 |
Ti 1.54 |
V 1.63 |
Cr 1.66 |
Mn 1.55 |
Fe 1.83 |
Co 1.88 |
Ni 1.91 |
Cu 1.90 |
Zn 1.65 |
Ga 1.81 |
Ge 2.01 |
As 2.18 |
Se 2.55 |
Br 2.96 |
Kr 3.00 | |
| 5 | Rb 0.82 |
Sr 0.95 |
Y 1.22 |
Zr 1.33 |
Nb 1.6 |
Mo 2.16 |
Tc 1.9 |
Ru 2.2 |
Rh 2.28 |
Pd 2.20 |
Ag 1.93 |
Cd 1.69 |
In 1.78 |
Sn 1.96 |
Sb 2.05 |
Te 2.1 |
I 2.66 |
Xe 2.6 | |
| 6 | Cs 0.79 |
Ba 0.89 |
* |
Hf 1.3 |
Ta 1.5 |
W 2.36 |
Re 1.9 |
Os 2.2 |
Ir 2.20 |
Pt 2.28 |
Au 2.54 |
Hg 2.00 |
Tl 1.62 |
Pb 2.33 |
Bi 2.02 |
Po 2.0 |
At 2.2 |
Rn | |
| 7 | Fr 0.7 |
Ra 0.9 |
** |
Rf |
Db |
Sg |
Bh |
Hs |
Mt |
Ds |
Rg |
Uub |
Uut |
Uuq |
Uup |
Uuh |
Uus |
Uuo | |
| Lanthanides | * |
La 1.1 |
Ce 1.12 |
Pr 1.13 |
Nd 1.14 |
Pm 1.13 |
Sm 1.17 |
Eu 1.2 |
Gd 1.2 |
Tb 1.1 |
Dy 1.22 |
Ho 1.23 |
Er 1.24 |
Tm 1.25 |
Yb 1.1 |
Lu 1.27 | |||
| Actinides | ** |
Ac 1.1 |
Th 1.3 |
Pa 1.5 |
U 1.38 |
Np 1.36 |
Pu 1.28 |
Am 1.13 |
Cm 1.28 |
Bk 1.3 |
Cf 1.3 |
Es 1.3 |
Fm 1.3 |
Md 1.3 |
No 1.3 |
Lr | |||
Electronegativity and Oxidation Number
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