How Many Periods Are in the Periodic Table? A Clear Guide to Its Structure
The periodic table has 7 periods. A period is a horizontal row of elements, numbered from top to bottom. For example, sodium (Na) is in Period 3, the third row of the table.
A period number generally indicates the highest principal energy level occupied by an element’s electrons. The table arranges elements by increasing atomic number, and elements in the same period share the same number of occupied electron shells. Their chemical properties change across a row and repeat in patterns in later rows. Encyclopaedia Britannica
One example: sodium in Period 3
Sodium is in Period 3 and Group 1. Its electron configuration ends in 3s, which identifies the third principal energy level. You can also find its period by locating sodium in the table and counting down to the third row.
The International Union of Pure and Applied Chemistry’s periodic table uses the same seven-period layout. IUPAC
Why the periods have different lengths
Periods do not all contain the same number of elements. Their lengths follow the order in which electron subshells fill:
| Periods | Number of elements in each period |
|---|---|
| 1 | 2 |
| 2 and 3 | 8 |
| 4 and 5 | 18 |
| 6 and 7 | 32 |
The first period contains hydrogen and helium. Periods 2 and 3 are shorter because they fill the s and p subshells. Periods 4 and 5 include the d subshell, while periods 6 and 7 include the f subshell as well. The lanthanides belong to Period 6, and the actinides belong to Period 7, even though they are often shown in separate rows beneath the main table.
Key Takeaways
- The periodic table has 7 periods, arranged as horizontal rows.
- A period number is generally linked to the highest principal energy level occupied by an element’s electrons.
- Period lengths are 2, 8, 8, 18, 18, 32 and 32 elements.
- Sodium is in Period 3; its electron configuration ends in 3s.
