Lanthanoids are:
1. | 14 elements in the seventh period (atomic no. = 90 to 103) that are filling 5f sublevel. |
2. | 14 elements in the sixth period (atomic no. 58 to 71) that are filling 4f sublevel |
3. | 14 elements in the seventh period (atomic no. = 58 to 71) that are filling 4f sublevel |
4. | 14 elements in the sixth period (atomic no. 90 to 103) that are filling 4f sublevel |