[Kr]

[Kaitlin Joya 1I]

For the ground-state electron configuration of rubidium, why is it [Kr]5s^1, but for silver, antimony, and iodine, it's [Kr]4d^10...?

[Emily Engelkemier 1E]

If the 4d-subshell has electrons in it (as is the case with Ag, Sb, and I, and not with Ru), its energy level (n) is one less than the s subshell. We write 4d^10 before 5s when writing the electron configurations. Ru doesn't have any 4d electrons, while Sg, Sb, and I all do.

[Kaitlin Joya 1I]

If the 4d-subshell has electrons in it (as is the case with Ag, Sb, and I, and not with Ru), its energy level (n) is one less than the s subshell. We write 4d^10 before 5s when writing the electron configurations. Ru doesn't have any 4d electrons, while Sg, Sb, and I all do.

Why doesn't Ru have any 4d electrons? How do we tell when an element doesn't have electrons in a certain subshell?

[Michelle_Duong_3H]

Hello,

When looking at elements in a row of the periodic table, the s subshell is filled before the d subshell for elements in the first two columns. It is only once the s state is occupied and an electron enters the d state that that d state is lower in energy than the s state.

Here, Rb is in the first column, so the 5s state is filled first. For silver, antimony, and iodine, the 5s orbital is filled and electrons have entered the 4d orbital. At this point, the 5s state is higher in energy than the 4d state, as expected and when writing the electron configuration, 4d will be listed before 5s.

Hope this helps!