Chemistry: The Central Science (13th Edition)

Published by Prentice Hall
ISBN 10: 0321910419
ISBN 13: 978-0-32191-041-7

Chapter 7 - Periodic Properties of the Elements - Exercises - Page 292: 7.30c


$Cr$ has the same number of electrons as $Fe^{2+}$, but does not have the same electron configurations. No other neutral atoms have both the same number of electrons and the same electron configurations as $Fe^{2+}$.

Work Step by Step

$Fe^{2+}$ A neutral $Fe$ atom has 26 electrons. Losing 2 electrons to become $Fe^{2+}$ cation, it now has 24 electrons. The neutral atom that has 24 electrons is $Cr$. The electron configuration of $Fe$ is $[Ar]3d^64s^2$. Now we need to remove 2 electrons so that $Fe$ would become $Fe^{2+}$, and 2 electrons to be removed are 2 electrons that fill the $4s$ orbital (when electrons need to be removed to create ions, we would remove the electrons of the orbitals with the highest prinicipal quantum number, in the case, 4). Then, the electron configuration of $Fe^{2+}$ is $[Ar]3d^6$. Unfortunately, the electron configuration of $Cr$ is $[Ar]3d^54s^1$. So, the electron configurations are different.
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