Final answer:
Atoms of transition elements in Period 5 are characterized by an incomplete 4d subshell. This is because the electron filling order has the 4d subshell filling up after the 5s, which is filled before the 3d subshell.
Step-by-step explanation:
In general, atoms of transition elements in Period 5 are characterized by an incomplete 4d subshell. The order of electron filling for shells begins with the 1s, 2s, and continues onward, with the d subshells starting to fill after the s subshells of the same principal quantum number. However, an important point to note is that for period 5 transition metals, the 4s subshell is filled before the 3d subshell, reflecting the fact that the transition metals have their d subshells (n-1)d filled before the p subshells of the same principal quantum number n. Therefore, for period 5 transition metals, their distinguishing electron configuration feature is that they have electrons filling up the 4d subshell, as the 5s is filled prior to that, and the corresponding 5p would fill after.