How many electrons are in the outermost shell of a rubidium atom?

• A. 0
• B. 2
• C. 3
• D. 1

Answer: (D)

Understanding how electron configuration determines valence electrons shows why rubidium has one electron in its outermost shell. Rubidium has atomic number 37, so its electrons fill up to the noble gas core [Kr], leaving just one electron in the next outermost 5s orbital. That means the outermost shell contains a single valence electron. This mirrors rubidium’s place in the alkali metals, which all have one valence electron in their outer shell. So the number of electrons in the outermost shell is one. Having zero would imply a closed outer shell, which doesn’t apply here; two would correspond to alkaline earth metals, and three would require a different electron arrangement not present in rubidium’s configuration.