Multiple Choice
Nitrogen has atomic number 7. In its ground-state electronic structure, how many occupied electron shells (principal energy levels) does a nitrogen atom have?
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9. Electrons in Atoms and the Periodic Table
Electronic Structure: Shells
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Which element contains a full second energy level (n = 2) in its ground-state electron configuration?
A
Sodium (Na)
B
Oxygen (O)
C
Neon (Ne)
D
Magnesium (Mg)
Verified step by step guidance1
Recall that the second energy level (n = 2) can hold a maximum of 8 electrons, corresponding to the 2s and 2p orbitals.
Write the ground-state electron configuration for each element to see how many electrons occupy the second energy level:
For example, Neon (Ne) has 10 electrons. Its configuration is \$1s^2 2s^2 2p^6$, which means the second energy level is fully occupied with 8 electrons.
Compare this with the other elements: Sodium (Na) has 11 electrons, configuration \$1s^2 2s^2 2p^6 3s^1\(, so the second level is full but there is an electron in the third level; Oxygen (O) has 8 electrons, configuration \)1s^2 2s^2 2p^4\(, so the second level is not full; Magnesium (Mg) has 12 electrons, configuration \)1s^2 2s^2 2p^6 3s^2$, so the second level is full but electrons also occupy the third level.
Therefore, the element with a full second energy level in its ground state is the one whose electron configuration ends with \$2s^2 2p^6$, which is Neon (Ne).
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