Yes, I have.
No, I have not
They are the only elements on the periodic table that have an electron configuration that ends in "f". These are the f block elements.
They all are radioactive; these are the radioactive elements.
They all have unfilled inner shells, leading to unpredictable chemical behavior. These may be though of as the unpredictable elements.
They all have been discovered in the last fifty years; these are the recently discovered elements
Because they are all so unstable that they are treated separately.
To give the periodic table a more convenient rectangular shape. If they were placed in chronological order it would make the periodic table much wider.
By doing this the valence pattern is maintained. Looking down any column. For example ytterbium (Yb) has six valence electrons, and so is placed in the same column as oxygen.
Purely for historical reasons. They were all discovered recently and so are placed there because there is nowhere else to place them- all other positions are occupied.
1s^2 2s^2 2p^6 3s^2 3d^10 3p^6 4s^2 4p^6
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 4d^10 4p^6
1s^2 2s^2 2p^4 3s^2 3p^6 4s^2 3d^10 4p^4
None of the above