CHAPTER-3

CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES

 

  ü  Introduction

  ü  Earlier classifications

Ø  Dobereiner's Triads

Ø  Newland’s law of octaves

Ø  Mandeleev’s Periodic Classification

Ø Modern Periodic Table or Moseley’s Periodic Law                                                                   

  ü  Division of Elements into s, p, d, f blocks

  ü  Trends in Physical Properties

Ø  Atomic Radius

Ø  Ionic radius

Ø  Ionization Enthalpy

Ø  Electron gain enthalpy

Ø  Electronegativity or Non- Metallic Characters

Ø  Electropositivity or Metallic Character

Ø  Valency

  ü  Anomalous behaviour of first element of every group

 

 


 

With the discovery of a large number of elements, it became difficult to study the elements individually, so classification of elements was done to make the study easier.

 

ü  Earlier Classification

Ø  Dobereiner's Triads: In triads, the atomic mass of the middle element is approximately the average of the other two elements. This is known as Law of Triads. This classification was applicable to very few elements and so it was rejected. For example:



 

Ø  New Lands Law of Octaves: He arranged the elements in increasing order of their atomic weights and showed that the properties of every eight elements were similar to those of the first one. The relationship is just like the resemblance of first and eighth musical notes. He named this as law of octaves.

But his classification was rejected since the law of octaves was applicable to elements up to calcium. The properties of eighth element become not similar for rest of the element.

 


 

 

Ø Mendeleev's Classification: Dimitri Mendeleev classified the elements in the increasing order of their atomic weights. He founded that the properties of elements repeat after a regular interval. Based on this observation, he proposed a periodic law which states that “The properties of elements are the periodic functions of their atomic weights.” That is, when elements are arranged in the increasing order of their atomic weights, their properties repeat after a regular interval.



 

          Mendeleev arranged elements in horizontal rows called Periods and vertical columns       

        called Groups.

 

When Mendeleev proposed his periodic table, some of the elements were not discovered. He left some vacant places (gaps) for them in the periodic table and predicted some of their properties. For e.g. both Gallium and Germanium were not discovered at that time. He named these elements as Eka-Aluminium and Eka-Silicon respectively and predicted their properties.


F Merits of Mendeleev’s periodic table:

§  It was the first comprehensive classification of elements.

§  He corrected the wrong atomic weights of some elements and placed them in correct position in the periodic table.

§  He left vacant places for undiscovered elements and predicted some of their properties.

§  Elements with similar properties are placed in the same group.

 

F Drawbacks of Mendeleev’s periodic table:

§  Elements with dissimilar properties are found in same group.

§  He could not give an exact position for hydrogen.

§  He could not give exact position for Lanthanoids and Actinoids and also for isotopes.

§  Mendeleev’s periodic table did not strictly obey the increasing order of atomic weights.