# What are rows on the periodic table are known as?

The horizontal rows are called $\text{periods}$........
The horizontal rows are called $\text{periods}$........hence $\text{Periodic Table}$, and the vertical columns are called $\text{groups}$. The $\text{periods}$ and $\text{groups}$ relate to electronic structure.
Atomic size decreases across a $\text{Period}$ from left to right as we face the Table.........Why?