Question

From the standpoint of atomic structure, what determines which element will be first and which the last in a period of the periodic table ?

Answer 1

From the standard point of atomic structure, the periodic table is based on Atomic number of an element.

Moreover, the valence electrons determines the palce in the group.

Elements having 1 valence electrons are coming under first group, the elements having 2 valence electrons in the secon group, vice versa..

If the elements have 8 valence electrons, that is the last group (18th group) in the Periodic Table (except He).