How many occupied electron shells (principal energy levels) does a neutral aluminum atom have in its ground-state electronic configuration?

• A. 2
• B. 3
• C. 4
• D. 13

Answer: (B)

Verified step by step guidance

1. Recall that the principal energy levels (electron shells) are designated by the principal quantum number \( n = 1, 2, 3, \ldots \), where each shell can hold a certain number of electrons.
2. Determine the atomic number of aluminum, which is 13. This means a neutral aluminum atom has 13 electrons.
3. Write the ground-state electron configuration of aluminum by filling the shells in order of increasing energy: start with \( 1s \), then \( 2s \), \( 2p \), and so on, until all 13 electrons are placed.
4. Identify the highest principal energy level that contains electrons in this configuration. The number of occupied shells corresponds to the highest \( n \) value with electrons.
5. Conclude the number of occupied electron shells by counting all principal energy levels that have electrons in the ground-state configuration of aluminum.