Multiple Choice
Which of the following represents the complete ground state electron configuration for the iron (Fe) atom?
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10. Periodic Properties of the Elements
The Electron Configuration
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Which of the following is the correct electron configuration for calcium (atomic number 20)?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2
B
1s^2 2s^2 2p^6 3s^2 3p^6 4p^2
C
1s^2 2s^2 2p^6 3s^2 3p^4 4s^2
D
1s^2 2s^2 2p^6 3s^2 3p^6 3d^2
Verified step by step guidance1
Identify the atomic number of calcium, which is 20. This means calcium has 20 electrons to be placed in orbitals according to the Aufbau principle.
Recall the order of filling orbitals based on increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, then 3d.
Distribute the 20 electrons into orbitals following this order: fill 1s with 2 electrons, 2s with 2 electrons, 2p with 6 electrons, 3s with 2 electrons, 3p with 6 electrons, and then place the remaining electrons in 4s.
Note that after filling 3p orbitals (which hold 6 electrons), the next electrons go into the 4s orbital before 3d orbitals because 4s has lower energy than 3d for calcium.
Write the electron configuration by listing the orbitals and their electron counts: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$.
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