Multiple Choice
Which of the following is the correct electron configuration for barium (Ba)?
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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 arsenic (atomic number 33)?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5
B
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^5 4p^6
C
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3
D
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^1
Verified step by step guidance1
Identify the atomic number of arsenic, which is 33. This means arsenic has 33 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, 3d, 4p, and so on.
Write the electron configuration by filling orbitals in order until you reach 33 electrons: start with 1s^2 (2 electrons), then 2s^2 (2 electrons), 2p^6 (6 electrons), 3s^2 (2 electrons), 3p^6 (6 electrons), 4s^2 (2 electrons), 3d^{10} (10 electrons), and finally place the remaining electrons in 4p orbitals.
Calculate how many electrons remain after filling up to 3d^{10}: 2 + 2 + 6 + 2 + 6 + 2 + 10 = 30 electrons, so 3 electrons remain to be placed in the 4p orbital.
Conclude that the correct electron configuration for arsenic is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3$, since the 4p orbital holds the last 3 electrons.
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