What are horizontal rows in a periodic table called? 

The horizontal rows in the periodic table are called periods. In the modern periodic table, there are 7 periods. Period 1 has only two elements, while periods 2 and 3 have 8 elements. Periods 4 and 5 have 18 elements. Periods 6 and 7 have 32 elements.

The vertical columns are referred to as groups. The elements in a group have the same number of valence electrons.