Modern periodic table was prepared by Bohr and other chemists. As shown in the table, modern periodic table is based on the electronic configurations of elements. Elements are arranged in the order of increasing atomic numbers in periods, and in groups the elements are placed having similar electronic configuration or having similar electrons in their outermost shells.
On the basis of electronic configuration, first shell or period completes up to He (atomic number 2). So first period contains two elements H and He.
In second shell, electrons start to fill from Li and it gets 8 electrons up to Ne (atomic number 10). So second period contains 8 elements Li, Be, B, C, N, O, F and Ne.
In third shell, electrons start to fill from Na and get 8 electrons up to Ar (atomic number 18) and contains 8 elements—Na, Mg, Al, Si, P, S, CI and Ar.
Similarly, 4th, 5th, 6th and 7th periods are completed. In this way number of elements in different periods is as follows:
In this way 114 elements are arranged in seven periods.
Description of Groups: Vertical columns are called as groups. There are 18 groups in
all.
1.Ist group contains seven elements: Hydrogen, Lithium, Sodium, Potassium, Rubidium, Caesium and Francium, i.e., H, Li, Na, K, Rb, Cs and Fr. The elements are called as alkali metals and this group is also called as alkali metal group, because the elements of Ist group easily form alkalies (strong bases) on reacting with water.
Fig. 5.2. Periodic Table of Elements - Long form
