Development Of Modern Periodic Table - Practice Questions & MCQ

Introduction:

Periodic table helps us to undergo a systematic study of the various elements found in nature, Without which it would have been impossible for us to study all the elements. By classifying the elements into various groups and periods a comparative study of the elements and their compounds can be done. It also helps us to analyze the periodic trend in various properties such as ionization potential, electron affinity, electronegativity, etc.

Development of Periodic Table: 

1. PROUT'S HYPOTHESIS 

He simply assumed that all the elements are made up of hydrogen, so we can say that

Atomic weight of elements = n  (Atomic weight of one hydrogen atom) 

Atomic weight of H = 1 , where n = number of hydrogen atom = 1, 2, 3,.. 

Drawback or Limitation:

1.  Ex. Every element can not be formed by Hydrogen. 

2. The atomic weights of all elements were not found as whole numbers. 

Ex. Chlorine (atomic weight 35.5) and Strontium (atomic weight 87.5)

2. DOBEREINER TRIAD RULE

J.W. Dorbereiner pointed out that within a group of three elements having similar chemical and physical properties, the atomic weight of the middle element is the mean of the other two. Some examples of such triads are given below. He also pointed out the triad - iron, cobalt and nickel in which the atomic weights of the elements are almost the same.

Some representative triads of Dobereiner

 Other examples. (K, Rb, Cs), (P, As, Sb) (H, F, Cl) (Sc, Y, La).

Though it was the first successful attempt to rationalize the problem, it could not be generalised or extended.

Drawback or Limitation: All the known elements could not be arranged as triads.

3. NEWLAND'S OCTET LAW

John Alexander Reina newland in England made the first attempt to correlate the chemical properties of the elements with their atomic weight. According to him -

1. If the elements are arranged in order to their increasing atomic weights, every eighth element had similar properties to first one like the first and eighth note in music. For example

2. Inert gases were not discovered till then.

3. All the elements could not be classified on this basis.