Further explanation
In the periodic system
The group number in the periodic system is determined by the same valence electrons (main group), while the period is determined by the number of shells
valence electrons :
- Transition metals: ns, (n-1) d, np
- Lanthanides and actinides: ns, (n-2) f, (n-1) d, np
Valence electrons for the main group: Group 1,2,13-18
Lanthanides and actinides: Group 3–16
Transition metals: Group 3–12
1. Valence electron : 4s²
Group 2
Period 4
2. Valence electron : 2s²2p²
Group 14
Period 2
3. Valence electron : 3s²3p⁴
Group 16
Period 3
4. Valence electron : 4s²3d⁸
Group 10
Period 4
5. Valence electron : 5s²5p⁶
Group 18
Period 5
6. Valence electron : 7s²
Group Lanthanides and actinides (6)
Period 7