What is the most common electron configuration for Chromium (Cr)?
a) [Ne] 4s23d4
b) [Ar] 4s23d4
c) [Ar] 4s13d5
d) [Ne] 3d6
[Ne] indicates filled 2s and 2p orbitals, which is incorrect for chromium. [Ar] correctly indicates filled 3s and 3p orbitals. Normally, the 4s would fill with 2 electrons next, followed by 4 electrons in the 3d orbital. However, chromium is an exception which fills 4s1 and 3d5. Copper is a similar exception which fills 4s1 and 3d10 rather than 4s2 and 3d9. These are the two main exceptions you should know for the MCAT. Note that elements in the same periods as Cr and Cu (such as Au, Ag, and Mo) similarly serve as exceptions to the usual rule.
a) [Ne] 4s23d4, incorrect, The correct orbital shorthand is [Ar].
b) [Ar] 4s23d4, incorrect, While this would normally be true, Cr and Cu are exceptions which fill their 3d orbitals to 5 and 10 before placing their last electron in 4s1.
c) [Ar] 4s13d5, correct.
d) [Ne] 3d6, incorrect, [Ar] is the correct shorthand rather than [Ne], and the correct orbital filling is shown in answer c). Note that chromium is an exception to the orbital filling guidelines.