Question

which of these elements has an electron configuration of kr5s²?
○ barium
○ calcium
○ rubidium
○ strontium

Explanation:

[Kr] represents the electron - configuration of krypton (atomic number 36). Adding 2 electrons in the 5s orbital gives an element with atomic number 38. Strontium has an atomic number of 38 and its electron - configuration is [Kr]5s². Barium has an electron - configuration of [Xe]6s², calcium has [Ar]4s², and rubidium has [Kr]5s¹.

Answer:

strontium