What is the element associated with the electron configuration [Rn] 7s² 5f¹²?

The electron configuration [Rn] 7s² 5f¹² indicates that the element has a total of 12 electrons in the 5f subshell. This places it within the actinides section of the periodic table, specifically related to the f-block elements.

Fermium, with the atomic number 100, has the electron configuration that aligns precisely with the configuration given (after radon, the next f-block filling goes into the 5f subshell). In this case, the filling of the 5f orbitals commences from 5f¹ through to 5f¹⁴, which is the range for elements such as fermium.

The other elements listed do not match this electron configuration. Nobelium, which is element 102, has an electron configuration that includes adding more electrons to the 5f subshell, while Californium and Mendelevium follow suit with configurations that indicate they have additional electrons in higher orbitals as well.

Therefore, the element that corresponds accurately to the configuration [Rn] 7s² 5f¹² is Fermium.