elements are classified into rows and columns in the periodic table.
Elements are arranged in the increasing order of the atomic number
elements with same number of valence electrons fall into the same column. these columns are called groups.
elements having the same amount of outermost energy shells fall into the same row. These rows are called periods.
therefore elements where the highest energy electron is in the second energy shell belongs to period 2
and elements with highest energy electron is the third energy shell belongs to period 3 and so on.
Therefore rows represent the periods of elements
answer is an elements period is its row in the periodic table
