Understanding Electronegativity of the Nucleophilic Atom

Electronegativity and electron affinity both are correlated with respect to the tendencies toward accepting electrons. Electron affinity refers to the property of a gaseous and neutral atom in an isolated state. It refers to the tendency of a gaseous and neutral atom to accept an electron. It is a quantitative term measured in terms of energy (E). The SI unit for a change in this energy ( E) is Joule per mole (J mol-1).

X(g) + e– → X–(g)

Electronegativity, on the other hand, is the tendency of an atom or molecule to attract the shared pair of electrons towards itself.  It is applicable to both atoms and molecules. Unlike electron affinity, it is a qualitative and dimensionless measure. However, it can be measured with the help of other factors like bond enthalpy, ionisation energy, etc. 

More on Electronegativity:

As electronegativity is considered in a bonded state, hence it is not directly measurable. Although a number of numerical scales of electronegativity, such as the Pauling scale, Mulliken-Jaffe scale and Allred Rochow scales. Yet, the Pauling scale is most widely used.

Here are some of the formulae to determine the electronegativity of a given element.

1. Mulliken scale: Electronegativity= IE1+IE22

           Here, IE1= First ionisation enthalpy and IE2= Second ionisation enthalpy

1. Pauling scale: Electronegativity= Mulliken scale2.8

According to the Pauling scale, F (fluorine) has been arbitrarily assigned a value of 4.0, the highest on this scale. It means that F is the most electronegative element of the periodic table. Cs (caesium) and Fr (francium), on the other hand, are the least electronegative elements of the periodic table, each having an arbitrary value of 0.7. 

The electronegativity of a given element is not a constant value. It varies according to the other element with which it is bonded. Simply put, electronegativity gives an estimated value to predict the type of bond existing between two bonded atoms.

Electronegativity of Hydrogen:

A special mention needs to be given to H (hydrogen), which is the only gaseous element of the periodic table to have an electronegativity value, that of 2.1. The other gases on the table belong to the noble gas family. The electronegativities of the noblest gases are zero because, being inert in nature, they naturally do not react with other elements to form compounds. As electronegativity is used to relatively measure the tendency of attracting an electron to an atom, hence except for H, no other gas reacts under ideal conditions.

What Is A Nucleophile?

A nucleophile is an attacking reagent which is an electron-rich species and nucleus-seeking in nature. It contains an atom having unshared electron lone pairs. Nucleophiles act as Lewis bases due to their tendency to share electrons. Examples include:

Neutral nucleophile: NH3, H2O, etc.

Negatively charged nucleophile: OH–, SH–, CN–, F–, etc.

How is nucleophilic strength correlated with electronegativity?

Electronegativity and the nucleophilic strength of a given nucleophile are inversely proportional to each other. The more the electrophilic strength of an atom or molecule, the lesser would be its electronegativity and vice versa. 

The reason behind this inverse relationship is that since a highly electronegative atom would tend to attract electrons towards itself more strongly, it would be reluctant to share the electron pairs with another element, thereby becoming a poor nucleophile.

For a given element in a periodic table, electronegativity increases across the period and nucleophilic strength decrease. Electronegativity decreases down the group, and nucleophilic strength increases.

Conclusion:

In this article, we saw an overview of electron affinity and a detailed description of electronegativity. Electron affinity refers to the property of a gaseous and neutral atom in an isolated state, whereas electronegativity is the tendency of an atom or molecule to attract the shared pair of electrons towards itself. Electron affinity is a quantitative value defined in terms of energy. Electronegativity, on the other hand, is being relatively measured with numerical scales, in which the Pauling scale is most widely used. After discussing the formulae used for measuring the electronegativity of a given element, we talked about H being electronegative, despite being a gas. We also discussed nucleophiles and how it is correlated with electronegativity.