Modern Periodic Table
The modern periodic table is a tabular arrangement of the chemical elements, organized by their atomic number, electron configuration, and recurring chemical properties. It is divided into periods and groups.

Periods in the Periodic Table
Each horizontal row in the periodic table is called a period. There are a total of 7 periods in the modern periodic table. The periods are numbered 1 to 7, starting from the top row.

Explanation
1. The first period (Period 1) consists of only two elements, hydrogen and helium, as they have the simplest atomic structures.
2. The second period (Period 2) contains elements with one additional electron shell compared to Period 1.
3. As we move down the periodic table, each subsequent period adds an additional electron shell, resulting in the increase of atomic size and the number of electrons.
4. The seventh period (Period 7) is the longest period and contains the largest number of elements. It is still being filled with newly discovered elements.
5. The properties of elements within the same period change gradually as we move from left to right due to the periodic trends in atomic structure.

Conclusion
In conclusion, the modern periodic table consists of 7 periods, each representing a different energy level or electron shell. Understanding the organization of elements into periods is essential for predicting their properties and behavior in chemical reactions.