The horizontal rows are called periods.There are seven periods in the periodic table. 

1. First period (Atomic number 1 and 2): This is the shortest period. It contains only two elements (Hydrogen and Helium). 

2. Second period (Atomic number 3 to 10): This is a short period. It contains eight elements (Lithium to Neon).

3. Third period (Atomic number 11 to 18): This is also a short period. It contains eight elements (Sodium to Argon). 

4. Fourth period (Atomic number 19 to 36): This is a long period. It contains eighteen elements (Potassium to Krypton). This includes 8 normal elements and 10 transition elements. 

5. Fifth period (Atomic number 37 to 54): This is also a long period. It contains 18 elements (Rubidium to Xenon). This includes 8 normal elements and 10 transition elements. 

6. Sixth period (Atomic number 55 to 86): This is the longest period. It contains 32 elements (Caesium to Radon). This includes 8 normal elements, 10 transition elements and 14 inner transition elements (Lanthanides). 

7. Seventh period (Atomic number 87 to 118): Like the sixth period, this period also accommodates 32 elements. Recently 4 elements have been included by IUPAC.