The periodic table is a tool which helps us to study about 118 elements whether natural or synthetic and their compounds in a systematic way.
History of classification of elements : Till 18th century only a few elements and their compounds were known. so, it was easier to remember them. But, from the beginning of 19th century, the rate of discovery of elements and their compounds became fast and hence, the classification of elements became essential so that we can easily study about the properties of elements and their compounds.
Several chemists tried to classify elements. Some of them are as follows :
1. Doebereiner’s Triads : In 1817, a German chemist Johann Wolfgang Doebereiner presented the first classification of elements. In this classification, he prepared several groups of three elements called ‘Triads’ having similar properties. He told that the atomic weight of the middle element was the average of the rest two elements.
Ex : 1st Triads : Li(7), Na(23) & K(39) , 2nd Triad : Ca(40), Sr(87.5) & Ba(137) etc.
2. Newland’s Law of Octaves : In 1865, an English chemist John Alexander Newland gave another idea of classifying elements. He showed that when elements are arranged in the increasing order of their atomic weights each 8th element has similar properties as the 1st one. This is known as his law of octaves.
We can see that Li & Na have similar properties and so on.
3. Lothar Meyer’s Curve : In 1865, a German chemist Lothar Meyer plotted a graph between atomic volumes and atomic weights of elements. He found that elements with similar properties occupy the similar places on the curve. For example, the alkali metals lie on the top points on the curve.
4. Mendeleev’s Periodic Table : The most effective and systematic classification of elements was presented by a Russian chemist Dmitri Ivanovich Mendeleev. He proposed a law which is known as Mendeleev’s periodic law stated as follows :
The properties of elements are periodic functions of their atomic weights.
He arranged elements according to their increasing atomic weights and found that elements with similar properties are repeated after certain intervals. The key points related to his periodic table are as follows:
- There are nine vertical columns which are called groups. These groups are denoted as 0, I, II, III,…….VIII.
- Each group except 0 and VIII has been divided in two sub-groups A and B.
- There are 7 horizontal rows which are known as periods.
Importance of Mendeleev’s periodic table : The periodic table developed by Mendeleev made several contributions to the study of chemistry. Some of its important contributions are as follows :
- Systematic study of chemistry : This periodic table made the study of elements and their compounds easy and simple. One can easily know the elements with similar properties by the help of groups. Also the periodicity in the properties of elements can be understood through the arrangement of elements in groups and periods.
Updated on 31 July 2019…