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, and is especially important 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 in a molecule to attract electrons to itself. This definition was first suggested by Pauling and has not been improved upon. The reverse of electronegativity is termed electropositivity.
Basic Concepts
Neutral atoms of different elements have differing abilities to gain or loose electrons. The magnitude of these properties can be experimentally determined for each element and are the electon affinity and ionization energy of the element. The electron affinity is a measure of the energy released (or gained in some cases) when one electron is added to the atom, and the ionization energy is the energy needed to remove an electron from the atom. Atoms with a greater "pull" on their electrons have a high ionization energy and high electron affinity and tend to form monatomic ions with a negative charge. These tend to be the atoms of the non metal elements. Atoms with a weaker "pull" have a low ionization energy and low electron affinity, form ions with a positive charge, and are the metallic elements.
Since electronegativity is due to the "pull" on electrons it can be seen to be related to electron affinity and ionization energy. In a covalent bond between different atoms the electrons in the bond will be more stable in the presence of the atom that has greater attraction for electrons. This leads to a distortion of the electon cloud and the bonding electrons spend more time close to that atom. The bond is said to be polarized. As might be expected atoms with the greater electron affinity and ionization energy have the greater attraction for the bonding electrons. However in electronegativity we are looking at the atoms in the context of the chemical compound it is in, not the neutral atoms themselves. One consequence is that electronegativity is not a property of the atom itself, though we tend to treat it as such. Rather it depends on the state of the atom in the molecuel and we cannot directly measure the electronegativity of an element. It has to be calculated on a relative scale. There have been several methods for calculating these relative electronegativities.
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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