Why is there a need to classify the elements? What difficulties are likely to arise if the classification is not made?

The atomic radii of the elements of second period are given below:

(a) Arrange the elements in the decreasing order of their atomic radii, keeping the element with the largest atomic radius first. 
(b) Find out whether the arrangement is in the pattern of a period or not? 
(c) From this data, infer how the atomic size of the elements changed as one goes from left to right in a period.

By considering their position in the periodic table, which one of the following elements would you expect to have the maximum metallic character?

$\mathrm{Ga}, \mathrm{Ge}, \mathrm{As}, \mathrm{Se}, \mathrm{Br}$

The position of three elements A, B and C in the periodic table are shown below.

State whether A is a metal or non-metal?