Answer:
Periods the horizontal rows on the periodic table.
Step-by-step explanation:
In a modern periodic table elements are arranged increasing number of their atomic numbers.
The horizontal rows known as periods , there are 7 seven periods
In his periodic table, elements lying in the same period have same number of shells around the nucleus.
The vertical columns known as groups, there total 18 groups.
In his periodic table, elements having similar properties lied in the same vertical columns or groups because all the elements had same number of valence electrons.