Mastering Chemistry - Ch. 7 Flashcards | Quizlet
In the group 3 to group 12 transition metals, the outermost s electron shell contains one or two electrons. However, in these metals, it is the d subshells that fill up going across the row. In period 4 of the table, the 3d subshell fills, and in periods 5 and 6, the 4d and 5d subshells fill, respectively.