Question

Elements whose atoms have three outermost sheell incomplete are called ?

Answer 1

This is only due to stability reason of the element. f-orbital is a complex orbital and have 7 sub-orbitals and can accommodate upto 14 electrons. The most stable f-orbital is when it has 7 electrons or 14 electrons in it viz. half or fully filled orbital is most stable than having any other number of electrons.

That's why when you see any atom with incomplete outermost orbitals having f-orbital before, you will find that f-orbital​must be half or fully filled.