Classification of Elements and Periodicity in Properties

Classification of Elements was necessary since many elements were being discovered in the 19th century and the study of these elements individually was proving difficult.

There were many attempts at classifying elements including ‘Dobereiner’s Triads’ and ‘Newland’s Octaves’.

Dobereiner’s Triads

German chemist Johann Wolfgang Dobereiner attempted to classify elements with similar properties into groups of three elements each. These groups were called ‘triads’. Dobereiner suggested that in these triads, the atomic mass of the element in the middle would be more or less equal to the mean of the atomic masses of the other two elements in the triad.

An example of such a triad would be one containing lithium, sodium, and potassium. The atomic mass of lithium 6.94 and that of potassium is 39.10. The element in the middle of this triad, sodium, has an atomic mass of 22.99 which is more or less equal to the mean of the atomic masses of lithium and potassium (which is 23.02).

The Limitations of Dobereiner’s Triads are :

  • All the elements known at that time couldn’t be classified into triads.
  • Only four triads were mentioned – (Li,Na,K + Ca,Sr,Ba + Cl,Br,I + S,Se,Te).

Newland’s Octaves

English scientist John Newlands arranged the 56 known elements in increasing order of atomic mass in the year 1866. He observed a trend wherein every eighth element exhibited properties similar to the first. This similarity in the properties of every eighth element can be illustrated as follows.

Classification of Elements and Periodicity in Properties

Newland’s Law of Octaves states that when the elements are arranged in increasing order of atomic mass, the periodicity in properties of two elements which have an interval of seven elements in between them would be similar.

Limitations of Newland’s octaves are:

  • It was only up till calcium that the classification of elements is done via Newland’s Octaves.
  • The discovery of noble gases added to the limitations of this method since they couldn’t be included in this arrangement without disturbing it completely.

Mendeleev’s Periodic Table

Russian chemist Dmitri Ivanovich Mendeleev put forth his periodic table in 1869. He observed that the properties of elements, both physical and chemical, were periodically related to the atomic mass of the elements.

The Periodic Law (also referred to as Mendeleev’s Law), states that the chemical properties of elements are a periodic function of their atomic weights.

The advantages of Mendeleev’s Periodic table are:

  • The inclusion of these newly discovered elements did not disturb the periodic table. Examples include germanium, gallium, and scandium.
  • It was used to correct the wrong atomic weights in use at that time.
  • A variance from the atomic weight order was provided by Mendeleev’s table.

The limitations of Mendeleev’s Periodic table are:

  • Hydrogen’s position was in the group of alkali metals but hydrogen also exhibited halogen like qualities.
  • Isotopes were positioned differently since this type of classification of elements was done by considering the atomic weight of the element. Therefore – protium, deuterium, and tritium would occupy varying positions in Mendeleev’s table.
  • An anomalous positioning of a few elements showed that the atomic masses did not increase regularly from one element to the next. An example of this would be the placement of cobalt (atomic mass of 58.9) before nickel (atomic mass of 58.7).

These methods were the foundation on which the modern periodic table was built. However, the greatest contributor to the modern periodic table was Dmitri Mendeleev. Mendeleev is also known as the Father of the Modern Periodic Table. The modern periodic law is also called Mendeleev’s Law to honour him.

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