The electronegativity it is a relative periodic property that concerns the ability of an atom to attract electron density from its molecular environment. It is the tendency of an atom to attract electrons when it is attached to a molecule. This is reflected in the behavior of many compounds and in how they interact intermolecularly with each other..
Not all elements attract electrons from adjacent atoms to the same degree. In the case of those that give electron density easily, it is said that they are electropositive, while those that are "covered" with electrons, are electronegative. There are many ways to explain and observe this property (or concept).
For example, in electrostatic potential maps for a molecule (like the one for chlorine dioxide in the image above, ClOtwo) the effect of the different electronegativities for chlorine and oxygen atoms is observed.
The red color indicates the electron-rich regions of the molecule, δ-, and the blue color indicates the electron-poor regions, δ +. Thus, after a series of computational calculations, this type of map can be established; many of them show a direct relationship between the location of electronegative atoms and δ-.
It can also be visualized as follows: within a molecule, the transit of electrons is more likely to occur in the vicinity of the most electronegative atoms. It is for this reason that for ClOtwo the oxygen atoms (the red spheres) are surrounded by a red cloud, while the chlorine atom (the green sphere) are surrounded by a bluish cloud.
The definition of electronegativity depends on the approach given to the phenomenon, there are several scales that consider it from certain aspects. However, all the scales have in common that they are supported by the intrinsic nature of atoms.
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Electronegativity is not a property that can be quantified, nor does it have absolute values. Why? Because the tendency of an atom to attract electron density towards it is not the same in all compounds. In other words: electronegativity varies depending on the molecule.
Yes for the ClO moleculetwo If the Cl atom was exchanged for the N atom, then the tendency of O to attract electrons would also change; it could increase (make the cloud redder) or decrease (lose color). The difference would lie in the new N-O bond formed, to thus have the O-N-O molecule (nitrogen dioxide, NOtwo).
As the electronegativity of an atom is not the same for all its molecular surroundings, it is necessary to define it in terms of other variables. In this way, there are values that serve as a reference and that allow predicting, for example, the type of bond that is formed (ionic or covalent).
The great scientist and winner of two Nobel Prizes, Linus Pauling, proposed in 1932 a quantitative (measurable) form of the electronegative known as the Pauling scale. In it, the electronegativity of two elements, A and B, forming bonds, was related to the extra energy associated with the ionic character of the bond A-B.
How is this? Theoretically, covalent bonds are the most stable, since the distribution of their electrons between two atoms is equitable; that is to say, for molecules A-A and B-B both atoms share in the same way the pair of electrons of the bond. However, if A is more electronegative, then said pair will be more from A than from B.
In that case, A-B is no longer completely covalent, although if their electronegativities do not differ much, it can be said that their bond has a high covalent character. When this happens, the bond undergoes a small instability and acquires extra energy as a product of the electronegativity difference between A and B.
The greater this difference, the greater the energy of the A-B bond, and consequently the greater the ionic character of said bond..
This scale represents the most used in chemistry, and the electronegativities values arose from the assignment of a value of 4 for the fluorine atom. From there they could be calculated that of the other elements.
While the Pauling scale has to do with the energy associated with the bonds, the Robert Mulliken scale is more related to two other periodic properties: ionization energy (EI) and electron affinity (AE).
Thus, an element with high EI and AE values is very electronegative, and therefore will attract electrons from its molecular environment..
Why? Because EI reflects how difficult it is to "rip" an external electron from it, and AE how stable the anion formed in the gas phase is. If both properties have high magnitudes, then the element is "lover" of electrons..
The Mulliken electronegativities are calculated with the following formula:
ΧM = ½ (EI + AE)
That is, χM equals the average value of EI and AE.
However, unlike the Pauling scale which depends on which atoms form bonds, it is related to properties of the valence state (with its most stable electronic configurations).
Both scales generate similar electronegativity values for the elements and are approximately related to the following reconversion:
ΧP = 1.35 (ΧM)1/2 - 1.37
Both XM as XP they are dimensionless values; that is, they lack units.
There are other electronegativity scales, such as the Sanderson and Allen scales. However, the one that follows the first two is the Allred and Rochow scale (χAR). This time it is based on the effective nuclear charge that an electron experiences on the surface of atoms. Therefore, it is directly related to the attractive strength of the core and the screen effect..
Regardless of the scales or the values you have, the electronegativity increases from right to left for a period, and from bottom to top in the groups. Thus, it increases towards the upper right diagonal (not counting helium) until it meets fluorine.
In the image above you can see what has just been said. In the periodic table, the Pauling electronegativities are expressed as a function of the colors of the cells. Since fluorine is the most electronegative, it has a more prominent purple color, while the least electronegative (or electropositive) darker colors..
Likewise, it can be observed that the heads of groups (H, Be, B, C, etc.) have lighter colors, and that as one descends through the group, the other elements darken. What is this about? The answer again is both in the properties EI, AE, Zef (effective nuclear charge) and in the atomic radius.
The individual atoms have a real nuclear charge Z and the external electrons suffer an effective nuclear charge as a result of the shielding effect..
As it moves over a period, Zef increases in such a way that the atom contracts; that is, the atomic radii are reduced over a period.
This has the consequence that, at the moment of bonding one atom with another, the electrons will “flow” towards the atom with the highest Zef. Also, this gives an ionic character to the bond if there is a marked tendency for electrons to go towards an atom. When this is not the case, then we speak of a predominantly covalent bond.
For this reason the electronegativity varies according to the atomic radii, Zef, which in turn are closely related to EI and AE. Everything is a chain.
What is electronegativity for? In principle to determine if a binary compound is covalent or ionic. When the electronegativity difference is very high (at a rate of 1.7 units or more) the compound is said to be ionic. Likewise, it is useful to discern in a structure which regions will possibly be richer in electrons..
From here, what mechanism or reaction the compound may undergo can be predicted. In electron-poor regions, δ +, negatively charged species may act in a certain way; and in electron-rich regions, their atoms can interact in very specific ways with other molecules (dipole-dipole interactions).
What are the electronegativity values for chlorine, oxygen, sodium, and fluorine atoms? After fluorine, who is the most electronegative? Using the periodic table, it is observed that sodium has a dark purple color, while the colors for oxygen and chlorine are visually very similar..
Its electronegativity values for the Pauling, Mulliken, and Allred-Rochow scales are:
Na (0.93, 1.21, 1.01).
Or (3.44, 3.22, 3.50).
Cl (3.16, 3.54, 2.83).
F (3.98, 4.43, 4.10).
Note that with the numerical values a difference is observed between the negativities of oxygen and chlorine.
According to the Mulliken scale, chlorine is more electronegative than oxygen, contrary to the Pauling and Allred-Rochow scales. The electronegativity difference between the two elements is even more evident using the Allred-Rochow scale. And finally, fluorine regardless of the scale chosen is the most electronegative..
Therefore, where there is an F atom in a molecule it means that the bond will have a high ionic character..
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