Which of these elements has an electron configuration of [Kr]5s2?

Strontium has atomic number 38, so its electron configuration is [Kr]5s²: the krypton core accounts for 36 electrons, and the two remaining electrons fill the 5s orbital. Therefore, the correct choice is strontium. Calcium has [Ar]4s², rubidium has [Kr]5s¹, and barium has [Xe]6s².

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