Question

From the standpoint of atomic structure what determine which element will be the first which the last in period of periodic tabel

Answer 1

The number of shells in an atom determines in which period it is to be placed.
For example:
Hydrogen has only 1 shell, so it is placed in the first period.

Now sodium has 3 shells, so it is placed in the 3rd period.

And, the Number of electrons in the valence shell decides which will be the first and which will Be the last element in a period of the periodic table.