Question

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

Answer 1

After many unsuccessful attempts to classify elements,the modern periodic table became the first tabular structure to successfully classify various elements in accordance with their chemical properties.Not only that it also offers sufficient scope to add new elements as and when they are discovered.

Complete answer:
The periodic table which is also known as the modern periodic table of elements is a tabular structure representing all the chemical elements which are arranged by virtue of their atomic number, electron configuration, and recurring trends of chemical properties. The structure of the table shows periodic trends. The table consists of seven rows which are called periods. We generally have metals on the left and nonmetals on the right. The columns in the table are called groups, containing elements with similar chemical behaviours. Six groups have accepted names as well as assigned numbers.For example group 17 elements are known as halogens; and group 18 are the noble gases. Also displayed are four simple rectangular areas or blocks associated with the filling of different atomic orbitals.
The electronic configuration of an element will determine its position in the Modern Periodic table.The element with one valence electron is the first while the element with 8 valence electrons is placed in the 18th group of a period.

Note:
Although the modern periodic table might look perfect it has its own set of disadvantages which have been topics of great research like Position of hydrogen is not satisfactory, it can be placed either in group I or 17 group of the first period.And It’s fails to accommodate lanthanides and actinides in the main body of the periodic table and thus being kept separately at the bottom of the periodic table.