Which of the following elements has its valence electrons in the s-orbital?

Analyze the question: We need the element whose valence (outermost) electrons sit only in an s-orbital, with no p electrons in the outer shell.

Concept: Write the electron configuration of each element. The valence electrons are those in the highest energy level (highest principal quantum number n).

Solution:

• Aluminum (Z = 13): 1s² 2s² 2p⁶ 3s² 3p¹. Valence electrons are in 3s and 3p. ✗
• Phosphorus (Z = 15): 1s² 2s² 2p⁶ 3s² 3p³. Valence electrons are in 3s and 3p. ✗
• Sodium (Z = 11): 1s² 2s² 2p⁶ 3s¹. Its single valence electron is in the 3s orbital only. ✓
• Carbon (Z = 6): 1s² 2s² 2p². Valence electrons are in 2s and 2p. ✗

Sodium is the only element listed that is in Group 1 (an s-block element), so its valence electron is entirely in an s-orbital.

Answer: C. Sodium.

Tip: Use the periodic table blocks. Groups 1 and 2 are the s-block, so their valence electrons are only in s-orbitals. Groups 13 to 18 are the p-block, so their valence electrons include p-orbitals. Aluminum, phosphorus, and carbon are all p-block elements.