TAMILNADU BOARD CLASS XII CHEMISTRY

CHAPTER 1 :   PERIODIC PROPERTIES & p-BLOCK  ELEMENTS

PERIODIC CLASSIFICATION:
      
The classification of data is one of  most important stages in 
scientific method. The first step is to perform experiments and 
record observations . This process gives large volume of data 
and makes it mandatory to arrange the growing volume of data 
in some systematic order.   Classification facilitates the next 
stage of development of a theory to explain them and also it 
acts as a research tool to study the apparent anomalies and 
deficiencies.  These stages one can observe in the development 
of classification of elements which culminated in designing  
Mendeleef’s ( Mendeleev's) Periodic Table  and the Modern Long 
form of Periodic Table.  

GROUP PERIODIC TABLE AS PROPOSED BY MENDELEEF IN 1871  

As science grew as mentioned above, several new matter called elements came into being with interesting physical and chemical properties  (DATA).  It became necessary to arrange them in order to study and understand their inter relationship better. 

In 1829, Debereiner proposed his law of triads.  In 1865 Newland proposed his law of octaves.  Then almost simultaneously in 1869 Mendeleef's proposed his  periodic table and Lother Meyer’s  his atomic volume curve . 

Mendeleef’s  table gave us a full view  of the elements that were known that day unlike the  graph given by Lother Meyer, Mendeleef's periodic table gained wide acceptance throughout the world.

For sometime Mendeleef’s periodic law was accepted.  As the deficiencies and defects came to light the necessity of Modern periodic Law become imminent.  
     
Then IUPAC(International Union of  pure and Applied Chemists) developed the long term of periodic table and its characteristics.  It has four blocks of elements arranged systematically and helps us to study them.     

This chapter is divided into two parts. The first part ( Unit-1.1 of the text book) deals with periodic properties and the trends shown by elements in the modern form of the periodic table. The second part deals with the chemistry of the p-block elements, as represented by the nitrogen family.

The Nitrogen family consists of elements Nitrogen(N) , Phosphorous(P), Arsenic(As), Antimony(Sb), and Bismuth(Bi). All these elements are physiologically active. Nitrogen which is present to the extent of 78.06 percent in the atmosphere of earth contributes to the control of excessive oxidation by oxygen. It takes part in the nitrogen cycle in nature and is an essential element for life . Phosphorous is another essential element for life present in the skeletal parts of animals as calcium phosphate . Both Nitrogen and phosphorous along with carbon , hydrogen and oxygen function as building block of life and also for energy transfer in plants and animals . Nitrogen compounds are used as explosives. Elemental phosphorous is toxic.

Arsenic and Antimony are stimulators but poisonous to livings . Bismuth Carbonate  was used as medicine in old medical system. Still it is used to some extent . It is important that our students study about these elements.