The valence electrons of $_{12}Mg$ are in the

Analyze the question: We need to find the orbital that holds the valence (outermost) electrons of magnesium, atomic number 12.

Concept: Electrons fill orbitals in order of increasing energy (the Aufbau principle): 1s → 2s → 2p → 3s → 3p → 4s ... Valence electrons are those in the outermost shell, which is the highest principal quantum number (n).

Solution:

Magnesium has 12 electrons:

• 1s² (2 electrons)
• 2s² (2 electrons)
• 2p⁶ (6 electrons)
• 3s² (2 electrons)

Electronic configuration: 1s² 2s² 2p⁶ 3s², or [Ne] 3s²

The outermost shell is n = 3, and it contains only the 3s orbital with 2 electrons. These 2 electrons are the valence electrons. This also explains why Mg is in Group 2 and forms Mg²⁺ by losing these two 3s electrons.

Checking the options:

• A. 3s orbital: correct, the outermost electrons are here.
• B. 2p_x: this is in an inner shell and is completely filled.
• C. 2s orbital: this is an inner shell.
• D. 1s orbital: this is the innermost shell.

Answer: A. 3s orbital

Tip: For the valence electrons of Groups 1 and 2 elements, look at the s orbital of the highest shell. The period number equals the outermost shell number, so Mg is in Period 3, which means 3s. Magnesium is in Group 2, so it has 2 valence electrons.