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Chemistry > Classification of Elements and Periodicity in Properties > Electronegativity and Oxidation State
Classification of Elements and Periodicity in Properties

Electronegativity and Oxidation State

Electronegativity (χ) is a tendency of an atom to attract the shared pair of electrons of a covalent bond to itself. For measuring electronegativity Pauling scale is used since the concept is discovered by Linus Pauling. On the periodic table, Fluorine with the value of 4.0 is the most electronegative element, whereas francium and cesium with the value of 0.7 is the least electronegative element. In this article, we will discuss applications of electronegativity, non-polar and polar bonds, and oxidation number.

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What is Electronegativity?

Electronegativity of an element is the tendency of its atoms to attract the shared pair of electrons towards itself in a covalent bond. The electronegativity of any given element depends upon the following factors:

  • State of hybridization: An sp-hybridized carbon is more electronegative than sp2 hybridized carbon. This is, in turn, more electronegative than an sp3 hybridized carbon.
  • Oxidation state of the element: The electronegativity of an element increases with the oxidation state of the element.
  • Nature of the substituents attached to the atom: The carbon atom in CF3I acquires greater positive charge than in CH3I . The electronegativity values for the elements increases along a period from left to right and decreases down a group. As we move along the period from left to right, nuclear charge increases and atomic radius decreases. However, when we move down the group, atomic radius as well as cleaning effect increases.

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List of Electronegative Elements

Element Electronegativity Atomic Number
Fluorine 3.98 9
Oxygen 3.44 8
Chlorine 3.16 17
Nitrogen 3.04 7
Krypton 2.96 36
Cesium 2.66 55
Barium 2.6 56
Sulfur 2.58 16
Carbon 2.55 6

 

Application of Electronegativity

Let us now look at the various applications of electronegativity.

1) Metallic and Non-metallic character

As the electronegativity increases, the non-metallic character increases. As the electronegativity decreases, the metallic character increases. Fluorine with the highest electronegativity of 4 is the most non-metallic element and caesium with the lowest electronegativity of  0.7 is the most metallic element.

Electronegativity

2) Polar and Non-polar Bonds

A bond between two similar atoms is non-polar. This is because the shared pair of electrons is equally attracted by the two atoms as the electronegativity of the atoms is the same. For eg, H2, Cl2, O2, N2

When the electronegativity of the two atoms forming a bond is different, the shared pair of electrons is attracted more towards the more electronegative element atom. Therefore, the more electronegative atom acquires a partial negative charge and the less electronegative atoms acquire a partial positive charge. As a result, two poles are developed and the molecule is said to be Polar. For eg, HF, HCl, HBr, HI

Higher the difference in electronegativity of the two binding atoms, the more is the dipole moment of the molecule. This is all about electronegativity. Hence, we move on to the next section about oxidation number.

Oxidation Number

Oxidation number is the number of electrons atom gain or loses in a chemical compound to form a chemical bond with another atom. It is the charge an atom appears to have when all other atoms are removed from it as ions. The loss or gain of an electron is also known as the degree of oxidation. The combination of the lose and gain of an electron is known as Redox reaction. Oxidation number is also known as Oxidation state, which can be zero, positive or negative. The oxidation number of atoms in O2, O3, P4, and S8 is zero.

Important Points for Determining Oxidation Number

  • The algebraic sum of the oxidation numbers of all the atoms in an uncharged (neutral) compound is zero. In an ion, the algebraic sum is equal to the charge on the ion.
  • All the elements in the elementary state have oxidation number zero, e.g., He, Cl2, S8, Petc.
  • As fluorine is the most electronegative element, it always has an oxidation number of  -1 in all of its compounds.
  •  In compounds containing oxygen, the oxidation number of oxygen is – 2 except in peroxides (-1) such as Na2O2, in OF2 and in O2 F2 (+2 and +1 respectively).
  • In all compounds. except for ionic metallic hydrides, the oxidation number of hydrogen is +1. Metal hydrides like NaH, MgH2, CaH2, LiH, etc have the oxidation number of hydrogen as -1.
  • The oxidation number of alkali metals is +1 and for alkaline earth metals is + 2.
  • The oxidation number of metal in amalgams is zero. This finds many practical applications.
  • In case of a coordinate bond, it gives +2 value of oxidation number to the less electronegative atom and -2 values to the more electronegative atom when the coordinate bond is directed formless electronegative atom to more electronegative atom.
  • If the coordinate bond is directed from more electronegative to the less electronegative atom then its contribution is zero for both the atoms.
  • For p-block elements [Except F and O], the highest oxidation number is equal to their group number and lowest oxidation number is equal to the group number minus eight.
  • In transition elements, the lowest oxidation number is equal to the number of ns electrons and highest oxidation number is equal to the number of ‘ns’ and (n – l)d unpaired electrons.

A Solved Example for You

Q: What is the oxidation number of Mn is KMnO4 ?

SolutionLet the oxidation number of Mn in KMnO4 be x. We know that, the Oxidation number of K = +1 and of O = –2

(Oxidation number K) + (Oxidation number of  Mn) + 4(Oxidation number of O) = 0

(+1) + (x) + 4(-2) = 0 or x = +7

Therefore, the oxidation number of Mn = +7

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