Classification of Elements

CLASSIFICATION OF ELEMENTS

 CLASSIFICATION OF ELEMENTS AND PERIODICITY

WHY DO WE NEED TO CLASSIFY ELEMENTS?

It is difficult to study each and every element individually and to know its properties and uses. Therefore, they have been classified into groups on the basis of their similarities in properties.

CLASSIFICATION OF ELEMENTS

Classification is done on the basis of similarities in properties so that a systematic study can be made about them.

Dobereiner’s Triads

·         Dobereiner arranged a group of three elements with similar properties in the order of increasing atomic masses and called it a triad.

·         Dobereiner could identify only the following three triads from the elements known at that time.

 

·        He showed that the atomic mass of the middle element is approximately the arithmetic mean of the other two. 

Limitation – He was failed to arrange all known element in the form of triad. And identify only 3 triad from the known element at that time.

Newlands’ Octaves

  • He arranged the elements according to the increasing order of atomic mass.
  • He found that every eighth element had properties similar to that of the first. He compared this to the octaves found in music. Therefore, he called it the ‘Law of Octaves’.

Limitations:

·        It was found that the Law of Octaves was applicable only up to calcium, as after calcium every eighth element did not possess properties similar to that of the first.

·        It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future.

·        Cobalt and nickel are in the same slot and these are placed in the same column as fluorine, chlorine, and bromine which have very different properties than these elements. 

·        Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements.

Mendeleev’s Periodic Table

  • According to Mendeleev's periodic table the properties of elements are periodic functions of their atomic masses.
  • Only 63 elements are known at that time.

Achievements of Mendeleev’s Periodic Table

  • Mendeleev left proper gaps for some elements that had not been discovered at that time. 
  • He names these as Eka to the name of the preceding element.
  • For Example Eka aluminum, Eka Boron, Eka silicon  have a similar properties with Gallium, scandium, and Germanium which were discovered later . 
  • Noble gases were discovered late because they are inert and have a low concentrations in the atmosphere. Mendeleev put noble gases in separate groups that did not disturb the original Mendeleev table.

Limitations of Mendeleev’s Periodic Table

·        Hydrogen resembles both, the alkali metals (IA) and the halogens (VIIA) in properties, so, Mendeleev could not justify its position.

·        Atomic weight of isotopes differ, but, they were not placed in different positions in Mendeleev’s periodic table.

·        Cobalt (Co) has higher atomic weights but was placed before Nickel (Ni) in the periodic table.

·        Platinum (Pt) and Gold (Au) have similar properties but were placed in different groups.

·        He could not explain the cause of periodicity among the elements.