Dear student,

There are 14 elements counting from atomic number 58 (Cerium) to 71(Lutetium) and they are called as Lanthanides. And the another 14 elements counting from atomic number 90 (Thorium) to 103(Lawrencium) are called Actinides.

These 28 elements are arranged in extra place that is below the Main Periodic Table, i.e, separated from s, p, or d block elements and the elements in these two series are collectively called as f-block elements .

For f block elements the outermost three shells are incomplete.