State reason why
A) completion of each period is logical
B) period 2 elements are called bridge elements

1) Modern periodic table is arranged in order of increasing atomic numbers and after filling of 7 elements in the second period after lithium the eighth element sodium has same property as that of the lithium and hence placed just below lithium.Same type of periodicity is seen in other periods also. Therefore according to the similarity in the properties they are arranged in the periodic table.

2) Period 2 elements exhibit show similar properties with period 3 elements.
e.g: Li has properties similar to Mg, and Be has properties similar to Al.
Hence they are called as bridge elements.

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