Which element is represented by the electron configuration [Rn] 7s² 5f⁹?

The electron configuration [Rn] 7s² 5f⁹ corresponds to an element in the actinide series. To understand this configuration better, let's break it down. The notation [Rn] signifies that the electron configuration starts from the nearest noble gas, radon. Following this noble gas core, the configuration indicates that two electrons are filling the 7s subshell and nine electrons are filling the 5f subshell.

The 5f subshell is where the actinide elements reside. The electrons in the 5f subshell are added progressively as you move through the actinides. As you count from the start of the 5f block (which begins at element 89, actinium), you'll find that einsteinium follows after fermium, californium, berkelium, and so forth.

Now, looking specifically at fermium and its connection to the 5f block, fermium has the atomic number 100, which corresponds to an electron configuration ending in 5f¹⁴. If we look closely and add up the electrons stemming from the configuration [Rn] 7s² 5f⁹, it totals 99, which aligns with the atomic number