The rapid growth in chemistry and the discovery of new elements made it necessary to develop a classification of elements to help study and remember their physical and chemical properties better. One of the early attempts to develop such a classification was by Lavoisier. He classified the elements into metals and non-metals. Then Dobereiner classified the elements into groups of three called triads. The elements in a triad had similar properties and the atomic weight of the middle member was very close to the average of the other two members. Newland attempted to develop a classification of elements by arranging the elements in the increasing order of atomic masses where the every eight element had properties similar to that of the first one. According to Lothar Meyer's arrangement, the elements with similar properties occupied similar positions when their properties are plotted as a function of their atomic weights.
All the above attempts to develop a classification of elements had one major draw back, that is, they could not be applied to all the elements successfully. However, these attempts gave some important clues about the regularities among the elements and making use of these clues the Russian chemist Dmitri Mendeleev made the first most successful classification of elements.
Mendeleev's periodic table
In 1869, Mendeleev arranged the then known elements (-60) in the order of their increasing atomic masses. He then observed that the elements with similar properties recur at regular intervals. Based on this observation, Mendeleev arranged the elements in the form of a table called the periodic table ( Image 1). In this table, the elements with similar properties recur at regular intervals and fall into groups. If you think that it does resemble today's periodic table, look at the horizontal lines (Image 2 ) and then you will see that they are today's periodic table's Groups 1 - 8.
Image 1. Mendeleev;s periodic table.
Image 2. Mendeleev's periodic table compared to today's periodic table.
Advantages of Mendeleev's periodic table
- The elements are classified into groups in the periodic table. Therefore, it was possible to study and remember the properties of a large number of elements in a systematic way.
- It was possible to correct the atomic masses of some elements (e.g. Au, Be, and Pt ) based on their positions in the periodic table.
- While arranging elements in the periodic table, by intuition Mendeleev had left gaps for the undiscovered elements. Therefore, the periodic table enabled to predict the properties of some undiscovered elements (e.g. Ga and Ge)
In spite of the above advantages, Mendeleev's periodic table also had some draw backs.
Disadvantages Of Mendeleev's periodic table
- The element hydrogen was not placed correctly in the periodic table.
- Some elements were placed in the periodic table according to their similarities in properties rather in increasing order of their atomic masses. For example, Co with an atomic mass of 58.9 was placed before Ni which has an atomic mass of 58.7 in the periodic table.
- Some similar elements were grouped separately while some dissimilar elements were grouped together. For example, Cu was placed in Group I although it did not resemble the elements of this group.
- The position for landslides and actin ides were not included in the periodic table.
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