Multiple Choice
Which of the following represents the correct electron configuration for a neutral atom of iron (Fe, atomic number 26)?
10. Periodic Properties of the Elements
The Electron Configuration
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Which of the following represents the correct electron configuration for a neutral atom of cobalt (atomic number 27)?
A
1s^2 2s^2 2p^6 3s^2 3p^6 3d^9 4s^2
B
1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^2
C
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^7
D
1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^8
Verified step by step guidance1
Identify the atomic number of cobalt, which is 27. This means a neutral cobalt atom has 27 electrons to arrange in its electron configuration.
Recall the order in which electron orbitals are filled according to the Aufbau principle: $1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d$.
Fill the orbitals in order, making sure to add electrons up to the total of 27. Start with $1s^2$, then $2s^2$, $2p^6$, $3s^2$, $3p^6$, which accounts for 18 electrons so far.
Next, fill the $4s$ orbital before the $3d$ orbital because $4s$ is lower in energy when empty or partially filled. Add $2$ electrons to $4s$, bringing the total to 20 electrons.
Finally, add the remaining 7 electrons to the $3d$ orbital, resulting in $3d^7$. This completes the electron configuration for cobalt as $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^7$.
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