What is Electronegativity?
The tendency of an atom in a molecule to attract the shared pair of electrons towards itself is known as electronegativity.
It is a dimensionless property because it is only a tendency. It basically indicates the net result of the tendencies of atoms in different elements to attract the bond-forming electron pairs. We measure electronegativity on several scales. The most commonly used scale was designed by Linus Pauling.
According to this scale, fluorine is the most electronegative element with a value of 4.0 and cesium is the least electronegative element with a value of 0.7.
Periodic Trends in the Electronegativities of Elements
As we move across a period from left to right the nuclear charge increases and the atomic size decreases, therefore the value of electronegativity increases across a period in the modern periodic table. For example, the electronegativity trend across period 3 in the periodic table is depicted below.
There is an increase in the atomic number as we move down the group in the modern periodic table. The nuclear charge also increases but the effect of the increase in nuclear charge is overcome by the addition of one shell. Hence, the value of electronegativity decreases as we move down the group. For example, in the halogen group as we move down the group from fluorine to astatine the electronegativity value decreases and it is shown in the diagram below.
It is a general observation that metals show a lower value of electronegativity as compared to the non-metals. Therefore, metals are electropositive and non-metals are electronegative in nature. The elements in period two differ in properties from their respective group elements due to the small size and higher value of electronegativity.
The elements in the second period show resemblance to the elements of the next group in period three. This happens due to a small difference in their electronegativities. This leads to the formation of a diagonal relationship.
Most and Least Electronegative Elements
Fluorine is the most electronegative element on the periodic table. Its electronegativity value is 3.98. Cesium is the least electronegative element. Its electronegativity value is 0.79. Electro positivity is the exact opposite of electronegativity, therefore, we can say that Cesium is the most electropositive element.
Those elements requiring only a few electrons to complete their valence shells, and having the least quantity of inner electron shells between the positive nucleus and the valence electrons, are the most electronegative. The most electronegative of all elements is fluorine. Its electronegativity is 4.0. Metals have electronegativities less than 2.0. The least electronegative elements are cesium (Cs) and francium (Fr), with electronegativity values of 0.7.
Fluorine is the most electronegative element and cesium is the least electronegative element.
Impact of Electronegativity on Covalent Bonding
The strength of a covalent bond is highly dependent on the electronegativities of the two bonded atoms (especially the difference in the electronegativities of the bonded atoms). Homonuclear diatomic molecules feature relatively ‘pure’ covalent bonds since the electronegativities of the bonded atoms are the same (resulting in the bonded pair of electrons being almost equidistant from the two bonded nuclei). Examples of such covalent bonds can be seen in H2 molecules, Cl2 molecules, and O2 molecules.
On the other hand, the covalent bonds between two species of varying electronegativities tend to become polarized. This occurs because the more electronegative atom pulls the bond pair of electrons closer to itself, developing a partially negative charge in the process (which is usually denoted by the symbol 𝛿-). At the same time, the more electropositive atom develops a partial positive charge (denoted by 𝛿+). These partial charges are responsible for the polarity of the chemical bond.
Bonds Between Highly Electronegative and Highly Electropositive Atoms
In the covalent bonds featuring a large difference in the electronegativities of the bonded atoms, it is uncommon for the more electronegative atom to gain complete control over the bond pair of electrons, resulting in the formation of two ions. Here, the more electronegative atom forms an anion and the more electropositive atom becomes a cation.
It is important to understand that all covalent bonds between dissimilar species have some ionic character. Similarly, all ionic bonds have some covalent character as well. The ionic character of the covalent bond is determined by the difference in electronegativity. When the electronegativities of the bonded species are not very different, the bond will be more covalent than ionic. However, when there is a large enough difference in the electronegativities of the bonded atoms, the bond becomes polar enough to be considered more ionic than covalent.
Electronegativity is a chemical property that describes the power of an atom in a molecule to attract shared-pair electrons towards itself. There is a large difference in electronegativity for atoms from the left- and right-hand sides of the periodic table. Electronegativity is an important quantity in determining the nature of bonds between elements and will be considered as the main factor in chemical bonding.
The periodic table of elements with the electronegativity table is given below.
Factors Affecting Electronegativity
1. Size of an Atom:
A greater atomic size will result in less value of electronegativity, this happens because electrons being far away from the nucleus will experience a lesser force of attraction.
2. Nuclear Charge:
A greater value of nuclear charge will result in a greater value of electronegativity. This happens because an increase in nuclear charge causes electron attraction with greater force.
3. Effect of Substituent:
The electronegativity of an atom depends upon the nature of the substituent attached to that atom. For example, the carbon atom in CF3I acquires a greater positive charge than CH3I. Therefore, C-atom in CF3I is more electronegative than in CH3I. The difference in electronegativity of an atom caused by substituents results in different chemical behaviour of that atom.
Frequently Asked Questions – FAQs
Which is the best definition of electronegativity?
Electronegativity is a function of an atom’s ability to attract an electrons binding pair. The most frequently used is the Pauling scale. Fluorine is assigned a value of 4.0, and values that are the least electronegative at 0.7 range down to cesium and francium.
What is high electronegativity?
Electronegativity decrease as it moves from top to bottom and increases over time from left to right. The most electronegative element is, therefore, fluorine, while francium is one of the least electronegative elements.
What is the electronegativity difference?
The degree to which an atom attracts electrons in a chemical bond is described by electronegativity. If the difference in electronegativity is greater than 1.7, the character of the bond will be ionic. If the difference in electronegativity is between 0.4 and 1.7, the character of the bond is polar covalent.
What is the difference between electron affinity and electronegativity?
The difference between the two is that electronegativity is a chemical property that shows how well an atom can attract electrons to itself as the amount of energy released when an electron is added to a neutral atom.
Is electronegativity a relative quantity?
Electronegativity is an example of an atom’s ability to attract electrons. It is proportional to the difference between the potential for ionization of an atom and its attraction to the electron.
How does electronegative vary along the period?
Electronegativity increases as we move left to the right in the period because as we move across the period, the effective nuclear charge increases and the atomic size decreases. Therefore, the tendency to attract shared pairs of electrons increases, thereby increasing electronegativity.
How does electronegative vary in a group?
Electronegativity decreases as we move down the group because as we move down the group, the atomic size increases and the effective nuclear charge decreases. Therefore, the tendency to attract shared pairs of electrons decreases, thereby decreasing electronegativity.
Name the most electronegative element and least electronegative element in the periodic table?
Fluorine is the most electronegative element, and caesium is the least electronegative element in the periodic table.
How does the electronegativity of an element affect its bonding?
The electronegativity of an element affects the bonding of an element. Elements with high electronegativity tend to form ionic bonds with other elements.
How does the size of an atom affect its electronegativity?
The size of an atom affects the electronegativity of an element. A greater atomic size corresponds to less value of electronegativity. Electrons being far away from the nucleus will experience a lesser force of attraction, therefore less electronegativity.