How Do You Find the Periodic Number?

The periodic number of an element corresponds to the highest energy level (principal quantum number, n) occupied by its electrons, which is the same as the number of electron shells in its electronic configuration.

Understanding the Periodic Table

The periodic table is organized into horizontal rows called periods. The period number tells you how many electron shells an element has.

Determining the Periodic Number

Here's how to determine the periodic number of an element:

1. Write the Electronic Configuration: Determine the electronic configuration of the element. For example, Sodium (Na) has an atomic number of 11, so its electronic configuration is 1s²2s²2p⁶3s¹. A simplified version is 2, 8, 1.

2. Identify the Highest Energy Level (n): Look for the highest principal quantum number (n) in the electronic configuration. In the example of Sodium (Na), the highest n is 3 (from 3s¹).

3. Count the Number of Electron Shells: You can also determine the number of occupied electron shells. For Sodium (2, 8, 1), there are three electron shells.

4. The Period Number: The highest energy level (n) or the number of electron shells directly corresponds to the period number of the element. Therefore, Sodium (Na) is in the 3rd period.

Examples

• Hydrogen (H): 1s¹ (1 shell) - Period 1
• Oxygen (O): 1s²2s²2p⁴ (2 shells) - Period 2
• Potassium (K): 1s²2s²2p⁶3s²3p⁶4s¹ (4 shells) - Period 4

Summary

To find an element's period number, determine its electronic configuration and identify the highest principal quantum number or count its electron shells. Both values represent the period number on the periodic table.