How many occupied electron shells does a neutral magnesium atom have in its ground-state electronic configuration?

• A. 2
• B. 12
• C. 3
• D. 4

Answer: (C)

Verified step by step guidance

1. Recall that the number of occupied electron shells corresponds to the number of principal energy levels that contain electrons in the atom's ground state.
2. Determine the atomic number of magnesium, which is 12, meaning it has 12 electrons in a neutral atom.
3. Write the ground-state electron configuration of magnesium by filling electrons into shells in order of increasing energy: 1s, 2s, 2p, 3s, etc.
4. Recognize that the first shell (n=1) holds 2 electrons, the second shell (n=2) holds 8 electrons, and the remaining electrons go into the third shell (n=3).
5. Count the number of shells that have electrons: since electrons occupy shells 1, 2, and 3, magnesium has 3 occupied electron shells.