1.0 Introduction

  Periodic Table

Prout’s Hypothesis

The atomic weight of all elements are simple multiple of atomic weight of hydrogen. Prout gave this hypothesis on the basis of Dalton’s theory and the atomic weights of some elements known at that time. But this hypothesis could not last longer because there are some atomic weights which are fractional and are not whole number.

Dobereiner’s Triads

According to Dobereiner when elements of same properties are kept in the increasing order of their atomic weights, the atomic weight of middle element is equal to the mean of the remaining two elements. Such a group of elements is called as Dobereiner’s Triads.

Dobereiner could arrange only a few elements as triads and there are few such elements present as a triad, whose atomic weights are approximately equal.

Therefore, this hypothesis was not acceptable for all the elements.

Newland’s Rule of Octave

As in music, every eighth node is same as the first node. If the elements are arranged in the increasing order of atomic weights, on starting with an element, the first element will exhibit similarities with the eighth element.

It is clear from the above table that sodium is the eighth element from lithium, whose properties resemble that of lithium.
This type of classification was limited up to only 20 elements.

Lothar Meyer’s Volume Curve

• The graphs of atomic volume against atomic weights are known as Lothar Meyer volume curves.
• The alkali metals have highest atomic volumes.
• Alkaline earth metals $(Be, Mg, Ca, Sr, Ba \:etc.)$ which are relatively a little less electropositive. Occupy positions on the descending part of the curve.
• Halogens and the noble gases (except helium) occupy positions on the ascending part of the curve.
• Transition elements have very small volumes and therefore these are present at the bottoms of the curve.

Mendeleef’s Periodic Law

• According to Mendeleef’s periodic law, the physical and chemical properties of elements are periodic functions of their atomic weights.
• Mendeleef’s Periodic Law
• Periodic table is based on atomic weight.
• In the periodic table, the horizontal lines are called periods and the vertical lines are called groups.
• The periodic table consists of seven periods and nine groups. (Note: The earlier periodic table had only eight groups. The noble gases were added later in the zero group because these were not discovered when Mendeleef put forward his periodic table.
• All groups (except $VIII$ and zero groups) are divided into subgroups A and B).
• 2, 8, 18 and 32 are called magic numbers.
• Merits of Mendeleef’s Periodic Table
• Classification of elements then known, was done for the first time and the elements having similar properties were kept in the same group.
• It encouraged research and led to discovery of newer elements.
• Mendeleef’s had even predicted the properties of many elements not discovered at that time. This helped in the discovery of these elements. For example, Mendeleef’s predicted the properties of the following elements.

a. Eka-boron: This was later called as Scandium $(Sc)$
b. Eka-aluminium: This was later called as Gallium $(Ga)$
c. Eka-silicon: This was later called Germanium $(Ge)$

• Position of Hydrogen: Hydrogen resembles alkali metals and halogens in its properties. Hence its position was not sure.
• Position of Isotopes: The isotopes have different atomic weights and the periodic table is based on atomic weights. Therefore, isotopes should get different places in the periodic table on the basis of atomic weights.
• The periodic table is not fully based on increasing order of atomic weights.
• It is not proper to place together the elements having different properties, such as coinage metals $(Cu, Ag\: and|; Au)$ with alkali metals; $Zn,Cd \:and \:Hg$ with alkaline earth metals and metal like $Mn$ with halogens. Similarly, $Pt$ and $Au$ having similar properties have been placed in different groups.
• There is no indication whether lanthanides and actinides are associated with group $IIIA$ or group $IIIB$.
• Position of Isobars: These elements have different groups when mass remains same.
• Lot of stress was given to valency of elements.