Question

according to atomic structure what determine which element will be the first and last element of the period

Answer 1

We can determine the position of an element in a period through its atomic radius i.e, the distance between nucleus of an atom to its outermost shell.
The element having bigger atomic radius will be in the first place of a period whereas, the element with smaller atomic radius will be at the last place. Therefore, we can say that atomic radius decreases as we move left to right in a period.
The reason behind this is that, as we move from left to right in a period, number of electrons increases, then the attraction between the nucleus and electrons increases which makes the structure of an atom smaller.