The missing word is "period." Each horizontal row on the periodic table is called a period. Periods represent the rows of elements arranged in order of increasing atomic number. All elements in the same period have the same number of electron shells. For example, elements in period 2 have two electron shells, while elements in period 6 have six electron shells. This arrangement helps to show recurring trends in element properties across a period, reflecting the periodic law that chemical properties vary periodically with atomic number. Thus, each horizontal row corresponds to a period on the periodic table.