# 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?

• Its just chemical properties similarities between La and lathanides and similar initial difficulties in chemical extraction. Actinium and actinides are similar to. Ytterium and Scandium are more similar to lanthanum than aluminium which in the same group. – user2617804 Dec 18 '13 at 2:42
• Rare earth metals: Sc Y, Lanthanoids. Based one chemical similarities. @user2617804 Aluminium is not in the same group as Sc, Y! – Martin - マーチン Apr 23 '14 at 4:44
• If lanthan was not a lanthanide, they would be called the cerinides. – Stian Yttervik Jan 16 '17 at 8:34
• Because Pluto is a plutoid and the Moon is a moon, maybe? – Oscar Lanzi Apr 25 '19 at 9:52

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.

• Although lanthanoid means ‘like lanthanum’ and so should not include lanthanum, lanthanum has become included by common usage. Similarly, actinoid. The ending ‘ide’ normally indicates a negative ion, and therefore lanthanoid and actinoid are preferred to lanthanide and actinide. IUPAC Red Book (2005) IR-3.5 (p.52). The La electron configuration is commonly written as [Xe] 5d1 6s2. Still it is often referred to as f-block element, as it is partially occupied in elemental ground state. – Martin - マーチン Apr 23 '14 at 4:39
• "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." Could you elaborate on this part? Do you mean that more than one electron can occupy the $5d$ orbital (in $\ce{La}$)? Or do you mean it as a general statement, that in all inner transition elements, electrons keep transitioning between $(n-1)d$ and $(n-2)f$ orbitals? – Gerard May 15 '15 at 16:45
• @Gerard I mean that the energy associated with room temperature is sufficient to promote ground state electrons in the d-orbitals (of many atoms) into the f-orbitals. In essence, there is enough energy at room temperature so that the number of atoms with electrons found in the ground state is low. – LDC3 May 16 '15 at 1:08

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.

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.