Groups and Periods in Periodic Table

GROUPS AND PERIODS

(a) Groups:

"Elements with similar properties are placed in vertical columns called groups."

        There are eight groups, which are usually numbered by Roman numerals I to VIII. Each group is divided into two sub groups, designed as A and B sub-groups. The sub-groups, containing the typical or normal elements or representative elements, are labeled as A sub-groups whereas B sub-groups contain less typical elements, called transition elements and are arranged in the centre of the Periodic Table.

(b) Periods:

"The horizontal rows of the periodic table are called periods."

Period 1: 
        The period 1 contains only two elements, hydrogen and helium.

Period 2 and 3:
        The Period 2 and 3 contain eight elements each are called short periods. All the elements in these periods are typical elements and belong to A sub-groups. In these periods, every eight element resembles in properties with the first element. As lithium and beryllium in the 2nd period resemble in most of their properties with sodium and magnesium of the 3rd period respectively. Similarly boron and aluminium both show oxidation state, of +3 fluorine in 2nd period has close resemblances with chlorine of 3rd period.

Period 4 and 5:
        The period 4 and 5 are called long periods. Each long period consists of eighteen elements. Out of these, eight are typical elements belonging to A sub-groups similar to second and third periods. Whereas the other ten elements, placed in the centre of the Table belong to B sub-groups and are known as Transition Elements. In these periods the repetition of properties among the elements occur after 18 elements. As after 19K (having atomic number 19), the next element with similar properties is 37Rb.

Period 6:
        The period 6 is also a long period, which contains thirty-two elements. In this periodic there are eight typical elements, ten transition elements and a new set of fourteen elements called Lanthanides as they start after 57La. Lanthanides have remarkably similar properties and are usually shown separately at the bottom of the Periodic Table.

Period 7:
       The period 7 is incomplete so far. It contains only two normal elements 87Fr and 88Ra, seven transition elements and fourteen inner transition elements. The inner transition elements of this period are called Actinides as they follow 89Ac. The actinides are also shown at the bottom of the Periodic Table under the Lanthanides. Due to their scarcity, the inner transition elements are also called rare earth elements.
        Trans-uranic elements (elements after uranium) they are prepared in the laboratories.