Multiple choice What is the electron configuration for Ca? [Ar]4s2 [Ar]4s1 [Kr]4s2 1s2 2s2 2p64s2 Reveal answer Fill a bubble to check yourself A Correct answer Explanation Calcium (Ca) has 20 electrons. The noble gas core [Ar] accounts for 18 electrons, and the remaining 2 electrons occupy the 4s orbital, resulting in [Ar]4s2.