Why is lanthanum a lanthanide and actinium an actinide?

I do know that the lanthanides start with the element lanthanum, but why? Lanthanum doesn't even have an $f$ orbital, so why isn't it considered a transition metal? 


It's the same way with actinium, which doesn't even have any electrons in its $5f$ orbital, why isn't it also a transition metal?




According to www.webelements.com, "There is no one single or best structure for the periodic table..." 


I believe that the table was split at lanthanum since it kept the elements of the *d* orbitals together. You could argue that the table should be split with Sc, Y, La and Ac together, with the elements Ce through Lu and Th through Lr as the *f* orbital elements. The next column would have Ti, Zr, Hf and Rf.


Actually, all elements have *f* orbitals, below lanthanum they are not usually populated. Lanthanum is the first element that could have an electron in the *f* orbital in the ground state. Since the transition energy of the electron between in the *d* orbital and the *f* orbital is quite low, there should be a significant number of electrons occupying the higher energy orbital, even at room temperature.


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The thing is lanthanum and actinium were grouped in the periodic table depending on their properties, not by the orbitals they were in. Mendeleev didn't even know about the idea of orbitals when he created the periodic table. It was Bohr and other scientists which came up with the idea of orbitals, and used them according to quantum numbers and specified where an electron is.


Because of that and due to different observations, lanthanum and the other one was kept on the f- block due to its properties not because of its orbitals.
And thanks to other users who had researched about what the best idea we could come in to is that the electrons in lanthanum and actinium occupy the f orbital in their ground state (therefore giving them the properties of that of other elements in the f block.


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The word lanthanide means "like lanthanum". So it originally referred to Ce to Lu. Since La and the lanthanides share a lot of properties it became lazy practice to refer to all 15 elements as the lanthanides. That was unfortunate since it's resulted in the lanthanide contraction being associated with all 15 Ln, whereas it actually starts at Ce and finishes at Lu. This laziness has also produced countless confusion when the Ln became conflated with the f-block elements. A big mess.


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