# Chapter 7 - Periodic Properties of the Elements - Exercises: 7.30b

$Ar$ has the same electrons and the same electron configuration as $Sc^{3+}$.

#### Work Step by Step

$Sc^{3+}$ A neutral $Sc$ atom has 21 electrons. Losing 3 electrons to become $Sc^{3+}$ cation, it now has 18 electrons. The neutral atom that has 18 electrons is $Ar$. The electron configuration of $Sc$ is $[Ar]3d^14s^2$. This shows that there 3 outermost electrons in orbitals $4s$ and $3d$ in a neutral $Sc$ atom. So, if 3 electrons are removed to make cation $Sc^{3+}$, these 3 outermost electrons would most likely the ones to be removed. Then, the electron configuration of $Sc^{3+}$ is $[Ar]$, which is exactly the electron configuration of $Ar$.

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