Multiple Choice
Which of the following is the correct electron configuration for phosphorus (atomic number 15) using core notation?
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10. Periodic Properties of the Elements
The Electron Configuration
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Which of the following represents the correct electron configuration for the Co(II) center found in vitamin B12?
A
[Ar] 3d^7
B
[Ar] 3d^8
C
[Ar] 3d^6 4s^1
D
[Ar] 3d^5 4s^2
Verified step by step guidance1
Identify the atomic number of cobalt (Co), which is 27, and write its ground-state electron configuration. The full configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^7 4s^2$, which can be abbreviated as $[Ar] 3d^7 4s^2$.
Understand that Co(II) means cobalt has lost two electrons, so you need to remove two electrons from the neutral cobalt configuration to find the electron configuration of Co$^{2+}$.
Recall that when transition metals form cations, electrons are removed first from the outermost shell, which is the 4s orbital, before the 3d orbitals.
Remove two electrons from the $4s^2$ orbital of the neutral cobalt configuration $[Ar] 3d^7 4s^2$, resulting in $[Ar] 3d^7$ for Co$^{2+}$.
Compare the resulting configuration $[Ar] 3d^7$ with the given options to identify the correct electron configuration for the Co(II) center.
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