Final answer:
The element that completes the 6s subshell is Radium (Ra).
Step-by-step explanation:
In atomic physics and quantum mechanics, the 6s subshell refers to a specific electron orbital within an atom. Subshells are subdivisions of the principal energy levels (n) and are further divided into atomic orbitals. The "6s" subshell belongs to the principal energy level 6 (n = 6) and is the s-type subshell within that level.
The element that completes the 6s subshell is Radium (Ra).
Electrons fill atomic orbitals in a specific order called the Aufbau principle. The 6s subshell is filled after the 5d subshell. Radium, with the atomic number 88, has a configuration of 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s². It fills the 6s subshell and completes it.
Therefore, the correct answer is Radium (Ra).