An element's period number equals the number of electron shells (energy levels) it has—for example, period 2 elements have 2 shells, period 3 elements have 3 shells.
How to Determine Period:
- Count the number of electron shells in the element's configuration
- This number = period number
Examples:
- Sodium (2, 8, 1): 3 shells → Period 3
- Carbon (2, 4): 2 shells → Period 2
- Calcium (2, 8, 8, 2): 4 shells → Period 4
Periodic Table Layout:
- 7 periods (horizontal rows)
- Period 1: H, He (1 shell)
- Period 2: Li through Ne (2 shells)
- Period 3: Na through Ar (3 shells)
Importants Points:
- Period = number of electron shells
- Horizontal rows on periodic table
- Easy identification: Count shells in configuration
