THE MODERN PERIODIC TABLE

In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass as described below. Accordingly, Mendeléev’s Periodic Law was modified and atomic number was adopted as the basis of Modern Periodic Table and the Modern Periodic Law can be stated as follows:

‘Properties of elements are a periodic function of their atomic number.’ Let us recall that the atomic number gives us the number of protons in the nucleus of an atom and this number increases by one in going from one element to the next. Elements, when arranged in order of increasing atomic number Z, lead us to the classification known as the Modern Periodic Table (Table 5.6). Prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic number.

Activity 5.3

1. How were the positions of cobalt and nickel resolved in the Modern Periodic Table?
2. How were the positions of isotopes of various elements decided in the Modern Periodic Table?
3. Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
4. Where do you think should hydrogen be placed in the Modern Periodic Table?

As we can see, the Modern Periodic Table takes care of three limitations of Mendléev’s Periodic Table. The anomalous position of hydrogen can be discussed after we see what are the bases on which the position of an element in the Modern Periodic Table depends.