Which of the following actinoid elements contains no electron in "6d" subshell.

To determine which actinoid element contains no electron in the "6d" subshell, we need to refer to the periodic table. The actinoid series is located in the f-block of the periodic table, and the "6d" subshell is part of the d-block.

The actinoid series includes the elements from atomic number 89 (actinium) to atomic number 103 (lawrencium). To find the element that contains no electron in the "6d" subshell, we need to look for the element that has its highest energy electron in a different subshell.

Starting from atomic number 89, we can go through the actinoid elements and check their electron configurations. The electron configuration of an element tells us how the electrons are distributed among the different subshells.

As we move through the actinoid series, we find that the electron configuration of each element fills up the "5f" subshell before moving on to the "6d" subshell. However, when we reach atomic number 100 (fermium), we see that its electron configuration is [Rn] 5f^12 7s^2.

This means that fermium has its highest energy electron in the "7s" subshell, not the "6d" subshell. Therefore, fermium is the actinoid element that contains no electron in the "6d" subshell.