ELECTRONEGATIVITY: SCALES, VARIATION, UTILITY, AND EXAMPLES - CHEMISTRY

The electronegativity is a periodic property relative concerning the ability of an atom to attract electron density of 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 easily give up electron density, they are said to be electropositive, while those that "cover" themselves with electrons are electronegative. There are many ways to explain and observe this property (or concept).

Source: Wikipedia Commons.

For example, in the electrostatic potential maps for a molecule (like the one for chlorine dioxide in the image above, ClO ₂) the effect of different electronegativities is observed for the chlorine and oxygen atoms.

The red color indicates the electron-rich regions of the molecule, δ-, and the blue color indicates those that are electron-poor, 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 the 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 ClO ₂ the oxygen atoms (the red spheres) are surrounded by a red cloud, while the chlorine atom (the green sphere) a bluish cloud.

The definition of electronegativity depends on the approach that is 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.

Electronegativity scales

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.

If, for the ClO ₂ molecule, the Cl atom were 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 NO bond formed, thus having the ONO molecule (nitrogen dioxide, NO ₂).

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, we have values that serve as a reference and that allow predicting, for example, the type of bond that is formed (ionic or covalent).

Pauling scale

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 AB.

How is this? Theoretically covalent bonds are the most stable, since the distribution of their electrons between two atoms is equitable; that is, for molecules AA and BB both atoms share the pair of electrons of the bond in the same way. However, if A is more electronegative, then that pair will be more from A than from B.

In that case, AB is no longer completely covalent, although if its electronegativities do not differ much, it can be said that its 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 AB bond, and consequently the greater the ionic character of that 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 calculate that of the other elements.

Mulliken scale

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 is in the gas phase. If both properties have high magnitudes, then the element is "lover" of electrons.

The Mulliken electronegativities are calculated with the following formula:

Χ _M = ½ (EI + AE)

That is, χ _M is equal to 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:

Χ _P = 1.35 (Χ _M) ^1/2 - 1.37

Both X _M and X _P are dimensionless values; that is, they lack units.

AL Allred and E. Rochow scale

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 (χ _AR). This time it is based on the effective nuclear charge that an electron experiences on the surface of the atoms. Therefore, it is directly related to the attractive force of the core and the screen effect.

How does electronegativity vary on the periodic table?

Source: Bartux at nl.wikipedia.

Regardless of the scales or values you have, the electronegativity increases from right to left for a period, and from bottom to top in 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. As 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 the 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 atom in the molecule

The individual atoms have a real nuclear charge Z and the external electrons suffer an effective nuclear charge from 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 it for?

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. It is also useful for discerning in a structure which regions will possibly be richer in electrons.

From here, it can be predicted what mechanism or reaction the compound may undergo. 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).

Examples (chlorine, oxygen, sodium, fluorine)

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.

References

Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition., Pages 30 and 44). Mc Graw Hill.
Jim Clark. (2000). Electronegativity. Taken from: chemguide.co.uk
Anne Marie Helmenstine, Ph.D. (December 11, 2017). Electronegativity Definition and Examples. Taken from: thoughtco.com
Mark E. Tuckerman. (November 5, 2011). Electronegativity scale. Taken from: nyu.edu
Wikipedia. (2018). Electronegativity. Taken from: es.wikipedia.org

ELECTRONEGATIVITY: SCALES, VARIATION, UTILITY, AND EXAMPLES - CHEMISTRY - 2024