CAUSE OF SIMILARITIES IN THE PROPERTIES OF ELEMENTS IN THE SAME GROUP
It has been pointed out that if the elements are arranged in order of increasing atomic numbers, the elements with similar properties are repeated after regular intervals.
The periodical repetition of elements with similar properties after certain regular intervals when the elements are arranged in order of increasing atomic number is called periodicity.
In order to understand the cause of periodicity, let us first answer the question; why do certain elements exhibit similar properties?
From the knowledge of atomic structure it may be recalled that atom has a small positively charged nucleus with electrons distributed around it. The atomic nucleus does not undergo any change during the ordinary chemical reactions. Thus, it may be assumed that the physical and chemical properties of the elements must be related to the arrangement of electrons in their atoms. Since electrons present in the inner shells do not take part in chemical combination, it must be the electrons in the outermost shell which control the properties of the atoms. Thus, if the arrangement of electrons in the outermost shell (valence shell) of the atoms is similar, their properties will also be similar. For example, the electronic configurations of alkali metal as given in Table 6.2, show the presence of one electron in their valence shells.
Table 6.2. Electronic Configurations of Some Groups
Similar behaviour of alkali metals is attributed to the similar valence shell configuration of their atoms. For example, all alkali metals react with water and liberate hydrogen gas.
2Li + 2H2 O à2LiOH + H2
2Na + 2H2O à2NaOH + H2
2K + 2H2 Oà 2KOH + H2
Similarly, if we examine the electronic configurations of other elements, we shall find that there is a repetition of the similar valence shell configuration after certain regular intervals with the regular increase of atomic number. Thus, it can be concluded that the periodic repetition of properties is due to the recurrence of similar valence shell configurations after certain regular intervals.
Thus, all the elements in a group have similar valence shell electronic configuration. The similar chemical nature of elements in a group is due to similar electron structures of their atoms. The electron structures of the first 20 elements are given in Table 6.3.
Table 6.3. Electronic Configurations of the First 20 Elements in the Periodic Table