Periodicity in Valency and Oxidation States

What do the Terms ‘Oxidation State’ and ‘Valency’ Mean?

The oxidation state of an atom is the number of electrons lost or gained by it. The valency of an element is a measure of its combining power and can be defined as the number of electrons that must be lost or gained by an atom to obtain a stable electron configuration.

Oxidation State and valency are one of the most fundamental properties of elements and can be studied with the help of electron configurations. Electrons which are found in the outermost shell are generally known as valence electrons and the number of valence electrons determines the valency (or valence) of an atom. The valencies of the elements belonging to the s-block and the p-block of the periodic table are generally calculated as the number of valence electron or eight minus the number of valence electrons. For the d-block and f-block elements, valency is determined not only on the basis of valence electrons but also on d and f orbital electrons. However, the general valencies of these d and f block elements are 2 and 3.

The general oxidation state of the elements of the periodic table is illustrated in the chart provided below.

Oxidation State and Valency Chart

Oxidation State and Valency Chart

Periodic Trends in the Oxidation States of Elements

 Variation Of Oxidation State Along a Period

While moving left to right across a period, the number of valence electrons of elements increases and varies between 1 to 8. But the valency of elements, when combined with H or O first, increases from 1 to 4 and then it reduces to zero. Consider two compounds containing oxygen Na2O and F2O. In F2O, the electronegativity of F is more than oxygen. Hence, each of F atoms will attract one electron from oxygen i.e. F will show -1 oxidation state and O will show +2 oxidation state. Whereas, in the case of Na2O, oxygen is highly electronegative than sodium atom. So oxygen will attract two electrons from each sodium atom showing -2 oxidation state and Na will have +1 oxidation state. The oxidation state of the element represents the charge possessed by an atom due to loss or gain of electrons (due to the electronegativity difference between the combining atoms) in the molecule.

Variation Of Oxidation State within a Group

As we move down in a group the number of the valence electron does not change. Hence, all the elements of one group have the same valency.

Guidelines For Assigning Oxidation States

  • Oxidation states of elements like O2, S8, H2, P4, Fe etc is zero.
  • Oxygen has an oxidation state of -2. But in its peroxides like Na2O2  and H2O2, it has -1 as its oxidation state
  • Similarly, hydrogen has +1. But in Metal Hydrides, such as NaH, LiH etc, it has -1
  • Some elements have the same oxidation states as in their compounds such as
    • Halogens have -1 except the time they form a compound with one another or Oxygen.
    • Alkali Metals such as Na, K, Rb, -Li, Cs; have +1
    • And Alkali Earth Metals have +2 such as Mg, Ca, Ba, -Be, Sr etc

To learn more about the periodicity in the properties of elements the trends in the oxidation states of elements in the periodic table, download BYJU’S – The Learning App.

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