Modern Periodic Table

Modern periodic table is based upon another law according to which atornic number is a more fundamental property of an element than atomic weight. This modern periodic law was developed by Moseley, Moseley firmly believed that atomic number is closely related to electronic configuration of an element. Moreover, it is the electronic configuration which decides majority of chemical properties of the element.

Modern Periodic Law

Modern periodic law states that physical and chemical properties of the elements are a periodic function of their atomic numbers, i.e., if the elements are arranged in order of their increasing atomic numbers, the elements with similar properties are repeated after certain regular intervals. The elements are arranged in the periodic table with their increasing atomic number in the same order in which the electrons successively occupy the energy sub-levels of higher and higher energy. An observation of the periodic table reveals that the recurrence of the periodic properties is clearly due to the recurrence of similar type of electronic configuration of elements.

A glance of the electronic configuration of IA and of zero group of elements clearly shows that elements of IA group have one electron in s sub-level of their outermost shell while the elements of the zero group have completely filled s and p sub-levels of the outer most shell.

Classification of Elements

The periodic table is a systematic arrangement of the elements in such a manner that elements possessing similar characteristics are grouped together. It has been divided into four parts viz, s-block, p-block, d-block, f-block elements

s-block Elements

Those elements in which the last electron enters the s-orbital of their respective outermost shells are called s-block elements. Since s-subshell has only one orbital which can accommodate only two electrons, therefore, there are only two groups of s-block elements. Thus, elements of group-1 (hydrogen and the alkali metals) and group-!l (alkaline earth metals) and helium comprise s-block element. Consider, for example, elements of group-1. Hydrogen has only one electron and this enters 1s orbital. The rest of the elements, i.e., Li, Na, K, Rb,Cs and Fr receive the last electron in 2s, 3s, 4s, 5s, 6s and 7s-orbitals respectively. These elements are, therefore, said to belong to s-block and have ns as the general outer shell electronic configuration.

Now consider He and the elements of group-II, i.e., Be, Mg, Ca, Sr, Ba and Ra. The last electron in these elements also enters the s-orbitals, i.e., ls, 2s, 3s, 4s, 5s, 6s and 7s-orbitals respectively. Hence these elements also belong to s-block and have ns2 as their general outer shell electronic configuration.

Thus the elements of groups-I and II, including hydrogen and helium in which the s-orbitals are beingprogressively filled in are called s-block elements. There are certain general characteristics of the s-block elements. These characteristics are,

1. These elements are highly reactive metals.

2. They have low ionization energies and are highly electropositive.

3. They are soft metals with low melting and boiling points. As an example, it has been observed that Na metal can be easily cut with a knife.

4. All are good conductors of heat and electricity,

5. Most of the metals of this block impart characteristics colours to the flame.

6. These elements are strong reducing agents.

7. These elements from ionic compounds by losing one or two valence electrons and thus show oxidation states of +1 and +2.