Multiple Choice
How many electrons can the third energy level (n = 3) hold in 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 correct electron configuration for a neutral calcium atom?
A
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2
B
1s^2 2s^2 2p^6 3s^2 3p^6 4s^1
C
1s^2 2s^2 2p^6 3s^2 3p^6 3d^2
D
1s^2 2s^2 2p^6 3s^2 3p^6 4p^2
Verified step by step guidance1
Identify the atomic number of calcium, which is 20. This means a neutral calcium atom has 20 electrons.
Recall the order in which electron orbitals are filled according to the Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, then 3d, and so on.
Fill the orbitals with electrons following this order until all 20 electrons are placed. Remember that each s orbital can hold 2 electrons, each p orbital can hold 6 electrons, and each d orbital can hold 10 electrons.
Write the electron configuration by listing the orbitals and the number of electrons in each, making sure the total adds up to 20.
Verify that the configuration matches the expected pattern for calcium, which ends with the 4s orbital fully occupied before any 3d electrons are added.
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