[Lemos]

I'm doing a chemistry challenge in my school and so far no one has gotten the right answer.
The challenge is based on the periodic table, and one of the questions is:
"What is the name of the element that contains one electron in the orbital 5f?"

My teacher said it was Actinium, but it doesn't seems to be the right answer and my personal research on the internet seems to agree aswell, as I couldn't find any element with "5f¹" and the Actinium has the following "true"configuration:
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d1

I've read that the Aufbau principle can have some deviances but I couldn't understand it very well, and  I've realized, by looking through some periodic tables and other sites, that there is no element with an orbital 5f1, starting only by 5f² with the element Protactinium.

So, am I missing something here?
Thanks in advance! 

[Hunter2]

Normally it should be Thorium. But the 5f jumps into the 6d.

[Lemos]

So, would that make the question "wrong"?

[Jasim]

If you consider the question purely in terms of the Aufbau principle and the layout of the periodic table, then Ac would be the answer. But reality doesn't follow the Aufbau principle strictly, especially with regards to single electrons and the d and f orbitals. There is no element with a single electron in the f orbital, unless you excite an electron in an atom such as Actinium (maybe also Thorium or other elements, though Actinium would require the least amount of energy). So the real correct answer would be an excited Actinium atom if you want what would be the least energy required.

The reason the Aufbau principle isn't followed strictly is because electrons will ALWAYS go to where they need the least amount of energy to go....Remember this, especially if you go on in chemistry, I would argue that it is one of the most fundamental and important principles in all of chemistry. And in the case of Actinium and Thorium, it requires less energy for the electrons to fill the first two spots in 6d rather than move into the f orbital. The reasons behind this involve quantum mechanics and physical chemistry, and is well beyond my ability to explain thoroughly or concisely in a single forum post...It may also be a bit beyond your teacher's knowledge - tell him/her to take it from a chemist and look it up.

The same holds true for Lanthanum and Cerium (which is a particularly interesting one). The Aufbau principle also doesn't quite hold when considering elements that fall near the beginning, middle, or end of some orbitals - interesting effects are sometimes seen in those regions. Elements that fall into these spots can have a lower energy state in which an electron is gained or lost - this can alter how an element fills it's orbitals in a manner similar to how the d and f orbitals can alter how elements fill their orbitals.

[Lemos]

Thanks for the beautiful answer!
Let's see if I can get the question right now!

I don't know if I'm allowed, but since I'm on this topic already, could you give me an insight on what exactly it means to ask the type of an element?

For example, what type of element is Lanthanum?
The answer would be a lanthanide, or a transition metal, maybe both?
Thanks again. 

[Jasim]

Elements can be categorized by a variety of traits. You can refer to elements in terms of their metallic behavior - metals, metalloids, transition metalds, alkyl metals, nonmetals....You can talk about elements in terms of their electronegativity some elements being electropositive, others being electronegative, and more or less so to varying degrees....You can refer to elements based on the concept of hardness/softness (see: http://en.wikipedia.org/wiki/HSAB_theory )....You can refer to elements based on their rarity...You can refer to elements based upon what orbital makes up their valence shell such as lanthanides (4f) and actinides (5f).

When we talk about transition metals, that term is usually reserved for those metals for which the d shell constitutes the valence. I would refer to Lanthanum as a rare earth or lanthanide.

These various classifications are not strictly defined, lutetium is a good example. Furthermore, some of these terms and classifications may not necessarily be useful for describing the chemistry of the elements, though usually it is the case. Depending on the type and nature of the chemistry or even the type of chemist you talk to, different terms may be used to describe elements. Physical chemists use slightly different terminology than organic chemists, because their focus on the same kind of chemical reaction will be different - but the chemistry itself doesn't change.