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MCAT Chemistry Question — Calcium Electron Configuration

What is the electron configuration of Ca in Ca(OH)2?


A) 1s22s22p6


B) 1s22s22p63s2


C) 1s22s22p63s23p6


D) 1s22s22p63s23p64s2


Calcium in Ca(OH)2 exists as an ion Ca2+. The electron configuration of Ca2+ is equivalent to Argon, with orbitals up to 3p filled with electrons. Normally, Ca has the configuration of [Ar]4s2. [Ar] is shorthand for 1s22s22p63s23p6. You may see this shorthand for noble gas electron configurations on the MCAT.


A) 1s22s22p6, incorrect, This is the configuration for Neon, Na+, and Mg2+.

B) 1s22s22p63s2, incorrect, This is the configuration for Mg.

C) 1s22s22p63s23p6, correct, Ca2+ has lost its 4s1 and 4s2 electrons.

D) 1s22s22p63s23p64s2, incorrect, This is the configuration for Ca, but not Ca2+.

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