[Rn] 5f¹¹ 7s²

Here's how to break it down:

* [Rn] represents the electron configuration of the previous noble gas, radon (Rn), which has 86 electrons.

* 5f¹¹ indicates that the 5f subshell of einsteinium contains 11 electrons.

* 7s² indicates that the 7s subshell contains 2 electrons.

Einsteinium has an atomic number of 99, so adding the electrons from radon (86) to the electrons in the 5f and 7s subshells (11 + 2 = 13) gives us a total of 99 electrons, which is the correct number for einsteinium.