Multiple Choice
In the context of electron configuration, what is the maximum number of electrons that can occupy the third shell (n = 3) of an atom?
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10. Periodic Properties of the Elements
The Electron Configuration
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Which of the following is the complete electron configuration for calcium (atomic number 20)?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4p^2
B
1s^2 2s^2 2p^6 3s^2 3p^6 3d^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 4s^2
Verified step by step guidance1
Identify the atomic number of calcium, which is 20. This means calcium has 20 electrons to place in its electron configuration.
Recall the order of filling electron orbitals based on the Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, then 3d, and so on.
Start filling the orbitals with electrons according to their capacities: 1s can hold 2, 2s can hold 2, 2p can hold 6, 3s can hold 2, 3p can hold 6, and 4s can hold 2 electrons.
Sum the electrons as you fill each orbital: 2 (1s) + 2 (2s) + 6 (2p) + 2 (3s) + 6 (3p) + 2 (4s) = 20 electrons, which matches calcium's atomic number.
Compare the given options to this configuration. The correct electron configuration for calcium is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$, because it correctly accounts for all 20 electrons in the proper order.
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