In1869, he stated this observation in the following form which is known as Mendeleev’s Periodic Law.
A periodic function is the one which repeats itself after a certain interval. According to the periodic law : The chemical and physical properties of elements are a periodic function of their atomic masses.
A tabular arrangement of the elements based on the periodic law is called periodic table.
Mendeleev believed that atomic mass of elements was the most fundamental property and arranged them in its increasing order in horizontal rows till he encountered an element which had properties similar to the first element.
He placed this element below the first element and thus started the second row of elements. Proceeding in this manner he could arrange all the known elements according to their properties and thus created the first periodic table.
Main features of Mendeleev’s periodic table
1. The horizontal rows present in the periodic table are called periods. You can see that there are seven periods in the periodic table. These are numbered from 1 to 7
2. Properties of elements in a particular period show regular gradation (i.e. increase or decrease) from left to right.
3. The vertical columns present in it are called groups. You must have noticed that these are nine in number and are numbered from I to VIII and Zero
4. Groups I to VII are subdivided into A and B subgroups. Groups Zero and VIII don’t have any subgroups.
5. All the elements in a particular group are chemically similar in nature. They show regular gradation in their physical properties and chemical reactivities.
After learning about the main features we shall now learn about the main merits of Mendeleev’s periodic table. Merits of Mendeleev’s periodic classification
1. Classification of all elements : Mendeleev’s was the first classification which successfully included all the elements.
2. Prediction of new elements: Mendeleev’s periodic table had some blank spaces in it. These vacant spaces were for elements that were yet to be discovered.
For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka–boron,Eka–aluminium and Eka–silicon, respectively. The properties of Eka–Aluminium predicted by Mendeléev and those of the element, gallium which was discovered later and replaced Eka aluminium, are listed as follows
Although it was a very successful attempt but it also had some defects in it. Now we shall discuss thedefects in this classification.
Defects in Mendeleev’s periodic classification: In spite of being a historic achievement Mendeleev’s periodic table had some defects in it. The following were the main defects in it:
1. Position of hydrogen: Hydrogen resembles alkali metals (forms H+ ion just like Na+ ions) as well as halogens ( forms H- ion similar to Cl- ion).Therefore, it could neither be placed with alkali metals(group I ) nor with halogens (group VII ).
2. Position of isotopes: Different isotopes of same elements have different atomic masses, therefore, each one of them should be given a different position in the periodic table. On the other hand, because they are chemically similar, they had to be given same position.
3. Anomalous pairs of elements: At certain places, an element of higher atomic mass has been placed before an element of lower atomic mass. For example, Argon (39.91) is placed before potassium (39.1)
CHECK YOUR PROGRESS IN LEARNING
1. Elements A, B and C constitute a Dobereiner’s triad. What is the relationship in their atomic masses?
2. How many elements were included in the arrangement given by Newland?
3. Which property of atoms was used by Mendeleev to classify the elements?
4. How many groups were originally proposed by Mendeleev in his periodic table?
5. Where in the periodic table are chemically similar elements placed, in a group or in a period?
6. Mendeleev’s periodic table had some blank spaces in it. What do they signify?
7. What name was given to the element whose properties were similar to the element eka-aluminium
predicted by Mendeleev?
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