Question

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

Explanation:

1. Recall the electron configuration rules and noble gas configurations. Krypton (\(Kr\)) has an atomic number of 36, representing the electron configuration up to the 4p⁶ level.
2. The given configuration is \([Kr]5s^{2}\), so we calculate the total number of electrons: atomic number of \(Kr\) (36) + 2 (from \(5s^{2}\)) = 38.
3. Check the atomic numbers of the options:

• Barium (Ba) has an atomic number of 56, electron configuration \([Xe]6s^{2}\).
• Calcium (Ca) has an atomic number of 20, electron configuration \([Ar]4s^{2}\).
• Rubidium (Rb) has an atomic number of 37, electron configuration \([Kr]5s^{1}\).
• Strontium (Sr) has an atomic number of 38, and its electron configuration is \([Kr]5s^{2}\), which matches the given configuration.

Answer:

strontium