Which element has the electron configuration [Rn] 5f⁴?

The electron configuration [Rn] 5f⁴ indicates that we are looking for an element that has four electrons in the 5f subshell, after the radon core ([Rn]). The [Rn] core represents the completed electron shells up to radon, which is element 86.

Moving into the actinide series, the 5f subshell is filled with elements starting from actinium (Ac) and continuing through to lawrencium (Lr). The element that corresponds to the configuration [Rn] 5f⁴ is plutonium (Pu), which is element 94.

In this configuration, the "5f" refers to the specific subshell where the electrons are being added, and the "4" indicates that there are four electrons in that particular subshell. Each successive actinide element adds one more electron into the 5f subshell. Therefore, looking at the sequence of actinide elements, and knowing that uranium (U) has the configuration [Rn] 5f², americium (Am) has [Rn] 5f⁷, and neptunium (Np) is even earlier in the series with [Rn] 5f³,